Though the climate of Belize is hot and humid, residents can use various passive techniques to create a cooler, more comfortable living environment

84 Home Power #82 • April / May 2001
uch of my early adult life was
spent homesteading in the
Alaskan bush. Winters are the
predominant force there, and like most
others in a northern or temperate zone
climate, my main concern was keeping
my living space warm. My location
colored my entire world view. It never
occurred to me that in some places in
the world, the problem was to stay cool.
Most of the residents of the United States have similar
problems of perception. Because of this necessary
emphasis on heating, there is not a lot of information
available on alternative methods of cooling. In 1980, I
was fortunate enough to “retire” to the Central American
country of Belize where I routinely encountered
temperatures in the eighties and nineties, and humidity
in the upper 30 percent of its range. 95°F (35°C) at 95
percent humidity will quickly draw your attention to the
need to cool down.
In the United States and most other industrial nations,
cooling is dealt with by refrigeration. Air conditioners are
predominantly powered by electricity, which is usually
produced by burning fossil fuels. Affluence allows us to
condition our living space using an expensive fuel of
convenience. Most “third world” nations only allow this
luxury to the very well-off. Where grid power is available
in Belize, it costs 25 cents per kilowatt-hour. This is far
too expensive for the average person to use for cooling
on a regular basis.
Cooling for the Humid Tropics
Over the years, I’ve studied the problem of low energy
input cooling in the tropics worldwide. There are two
very different environments that demand solutions to
the cooling problem. Hot, arid landscapes may require
cooling as much as hot, humid areas, but the principles
used to address the two problems are quite specific.
In this series of articles, I will try to pass on what I have
learned about using sun, wind, and the basic principles
of heat transfer to create a comfortable living
environment. I am specifically targeting the humid
tropics, but many of the principles I will discuss are
relevant to arid areas as well. I will emphasize passive
techniques here—things that can be done without using
any technically derived energy to move heat, or
techniques using devices to control heat flow
automatically.
This will be a multi-part article. In the first part, I cover
the basic principles of heat transference, and try to
explain how they interact and what type of effects they
produce. Later, I will discuss materials and
environmental factors. Also, I will specifically apply the
basic principles to building design and construction.
Heat Fundamentals
Heat is the motion of molecules in a substance. The
hotter the temperature, the more energetic the motion
becomes. There is no such thing as “cold”—there is
only more or less heat. Cold is our own subjective
reaction to a condition of too little heat for the body to
be in its comfort zone.
This is an important concept because there is no one
perfect temperature at which we are all comfortable.
The human comfort zone depends on several factors,
Cliff Mossberg
©2001 Cliff Mossberg
Though the climate of Belize is hot and humid, residents can use various passive techniques
to create a cooler, more comfortable living environment.
Part I — Basic Principles
85 Home Power #82 • April / May 2001
Cooling
not least of which is the human acclimatization to the
specific environment we live in.
While temperature is proportional to the energy of
vibration in molecules of a substance, heat quantity is a
measure of the numbers of these molecules and the
temperature at which they are vibrating. A large pan of
boiling water has more heat in it than a small one does,
even though they are at the same temperature.
As matter heats up, the molecules move farther apart—
they expand. Thus for the same volume of matter, there
are fewer molecules if the material is hotter. This means
that the same volume of our hypothetical material
weighs less per unit volume when it is hot and more
when it is cold and dense. This is true of solids, liquids,
and gasses that are unconfined.
Three Modes of Heat Transfer
There are three ways that heat can be transferred
between a source and a receiver body. They are
radiation, conduction, and convection.
They all accomplish the task of imparting heat energy to
a receiver body, and they do so in proportion to the
difference in temperature between the sending source
and the receiving body (called “delta t” and written
“Δt”—Δ means “the change in”). The higher the
difference in temperature between a heat source and a
heat receiver, the faster heat will flow into the receiver
and the faster its temperature will rise.
Radiation
When we talk about the electromagnetic spectrum, all
we’re talking about is “radio” waves—waves of
magnetic energy that can propagate through a vacuum
in space, thus transferring energy from the sun, stars,
and galaxies to our earth. We are familiar with AM radio
and the higher frequencies of FM radio and TV, but the
radio spectrum contains many other waves of much
higher frequencies. Visible light is a series of radio
waves that our bodies can detect directly.
Other frequencies such as infrared (lower in frequency
than visible light), ultraviolet (above the frequency of
visible light), and x-rays (very, very high frequency) are
undetectable by the
human eye. Yet these
frequencies transfer
energy as surely as the
visible light frequencies,
and we are affected
directly by them. Infrared
radiation from the sun
produces the feeling of
heat on our skin when the
sun’s rays hit us.
Ultraviolet radiation causes sunburn, and x-rays can kill
or mutilate our body’s cells.
Infrared radiation is the vehicle of heat transference that
is most important to life on earth. It is heat radiation
transmitted directly to the earth by the sun. It is one of
the principles that allows a woodstove or a bonfire to
radiate heat that warms at a distance.
Visible wavelengths can be converted to infrared
radiation when they fall on an absorptive surface, such
as a roof or a photovoltaic panel. The energy in these
light waves is absorbed by the surface, causing
heating. This heating in turn causes re-radiation from
the absorber as heat, or infrared light. This is the
reason hot water collector panels are self limiting in
their efficiency. The collector panel heats up the water
until the water re-radiates as much energy back to the
sky as it takes in. At this point, there is no further gain in
collection of radiant energy possible.
A roof heats up in the sun’s rays until it re-radiates
infrared heat energy down into the house as well as out
Window
Radiant Heat:
Energy in wavelengths that
travel through through glass,
air, and even a vacuum
Sunlight
Becomes heat when
energy excites
molecules of object
in its path
Methods of Heat Transmission
Transmission Transmission Direction of Heat
Method Mechanism Medium Movement
Radiation Electromagnetic Vacuum or transparent Any direction, line
radiant energy medium of sight from source
Conduction Molecule to molecule Any substantial Any direction into
mechanical transference material in contact material in contact
Convection Physical relocation of Usually movement Usually upward,
a heated substance of a heated fluid unless forced
Heat Transmission through Radiation
86 Home Power #82 • April / May 2001
Cooling
into the air. If the ceiling has no barrier to radiant
energy, this radiation will heat up the ceiling surface,
which in turn will re-radiate the heat directly into the
living area of the structure. Radiant energy is the
principle vehicle for moving heat in a downward
direction into a structure.
Effects of Solar Incidence
There are several factors that affect the ability of a
surface to absorb or radiate infrared energy, and one of
the most important is the angle at which the radiation
hits the absorbing surface, known as the angle of
incidence. If you want to absorb energy at the
maximum efficiency, radiation should fall on a collection
surface that is exactly perpendicular to that radiation.
The diagram above shows a variety of panel angles in
relation to the sun's rays. When the panel is
perpendicular to the sun's rays, the most energy is
intercepted. When the panel is set at 45 degrees to the
sun's rays, only about 70 percent of the available
energy is captured.
Absorption & Reflectance
Another factor that affects the amount of radiation
converted to thermal energy on a hypothetical earth
“panel” is the color and texture of the surface. This is so
fundamental to our experience that the concept is
understood intuitively. Dark surfaces absorb heat and
energy, while light surfaces reflect them. Rough
surfaces absorb energy, while smooth surfaces reflect
it. What is not so intuitive is that colors and textures that
absorb energy well, also radiate energy well.
Reflective metallic foils take advantage of this. They are
actually conductors, but when specifically engineered
into buildings to control radiant energy, they are as
much as 95 percent effective at blocking radiant energy
absorption. They are also very resistant to re-radiating
absorbed energy.
To be this effective, a radiant barrier must be installed
with an air space on one or both sides of the material.
Its mirror surface will then reflect any infrared energy
rather than absorbing it and conducting it as heat.
Conduction
Conduction is the most intuitively understood mode of
heat flow. For conduction to occur, materials must be in
contact with each other. For example, imagine a copper
bar one foot long, two inches wide, and half an inch
thick (30 x 5 x 1.3 cm)—a rather substantial piece of
copper. If we support this bar, and place a candle or a
Bunsen burner under one end, the bar will slowly heat
up from one end to the other. Soon the whole bar will
be too hot to touch. Heat is being transmitted by
conduction throughout the bar.
0° (parallel) to
sun’s rays:
No rays
intercepted
45° to sun’s rays:
71% of rays
intercepted
15° to sun’s rays:
26% of rays intercepted
60° to sun’s rays:
87% of rays
intercepted
75° to sun’s rays:
97% of rays
intercepted
30° to sun’s rays:
50% of rays intercepted
90°
45°
Panel width
71% of
panel width
Sun
90° (perpendicular) to
sun’s rays:
100% of rays intercepted
Solar Incidence at Various Angles Absorbtance Characteristics
for Common Building Materials
Asphalt Shingles
Surface Solar Absorptance
Dark 95%
White 75%
Rough Wood
Dark 95%
White 60%
Smooth Wood
Dark 90%
White 50%
Glazed or Enameled Surfaces
Dark 87%
White 37%
Stucco
Dark 90%
White 50%
Unpainted Brick
Dark 85%
White 65%
Concrete Block
Dark 95%
Unpainted 77%
White 55%
87 Home Power #82 • April / May 2001
Cooling
What is happening here is that the heat source is
exciting the molecules in the copper to vibrate more
enthusiastically, becoming more and more energetic as
the temperature increases. As these copper molecules
pick up physical motion from the heat energy, they
continuously “bump” into the molecules next door.
This physical disturbance imparts energy to the
adjacent molecules, causing them to increase their
vibrational energy—they warm up. Heating progresses
down the bar, away from the heat source, until the
whole bar has reached a state of equilibrium based on
the amount of heat supplied by the source.
Conductive Heat Flow
Radiant energy is one of the loss factors that draws
heat from the bar. Another factor that allows the bar to
lose heat is conduction to the medium surrounding it.
This is a loss by physical contact with the fluid—air—
surrounding the bar.
Different materials will move heat at different rates.
Based on these rates, materials are classified as
“insulators” if they retard the flow of heat, or
“conductors” if they facilitate the movement of heat.
These are far from absolute definitions. Most insulators
are designed to retard heat flow in conduction, but there
are some exceptions such as metallic foil radiant
barriers.
Air can be either an insulator or a conductor. For
example, air is used as an insulator to slow down the
transmission of heat in homes. It is the “dead” air space
in fiberglass batt insulation that does the work. But air is
also a cheap and relatively effective conductor of heat
in electric motors, vehicle cooling systems, and many
other applications. So while it is important to
understand how material properties affect heat flow,
you should also realize that these properties can be
applied in many ways to achieve an engineering goal.
Boundary Layer
In conduction, heat flows through a substance because
of tangible physical interaction between molecules.
These same forces allow heat to flow between any
substances that are in contact with one another. The
boundary where one substance stops and another
begins (between the copper bar and the air, for
example) is known as the interface. Heat flow across an
interface can be complicated by factors that are not
obvious. The first of these factors is the variable rate of
conduction by different materials. The second factor is
the mobility that a fluid has, which results in convective
flow.
Conductive heat flow is impeded when a fluid such as
air is in contact with a heated surface such as a wall.
This impediment is caused when a layer of stagnant air
is changed in temperature and density by heat moving
across the interface. The air in the layer next to the wall
Conductive Heat:
Excited (hotter) molecules heat the molecules in contact with them
Heat Source
Warmer Air
Boundary Layer
Boundary Layer
Wall Colder Air
th
(indoor
ambient
temperature)
tc
(outdoor
ambient
temperature)
tw
(wall
temp.)
Δt = th - tc
Colder Temperature Warmer
Thermal Conduction
Conduction through a Boundary Layer
Materials and their Conductivity
Material Conductivity (Conductance)*
Copper 220.000
Aluminum 122.000
Steel 25.000
Concrete 0.600
Water 0.350
Brick, red 0.270
Rubber, soft 0.100
Wood, pine 0.070
Corkboard 0.025
Rock wool 0.023
Air 0.014
Vacuum 0.000
*BTU per hour per sq. ft. per degree per foot thickness
88 Home Power #82 • April / May 2001
Cooling
will heat up more that the air some distance away.
When this situation exists, the change in temperature
(Δt) between the warm wall and the warm layer of air is
reduced. This cuts back on heat flow.
The existence of the boundary layer and its removal is
the essence of “wind chill.” This is when it feels colder
than the real ambient temperature because of the extra
heat loss when the wind blows away the boundary layer
around our bodies. This is undesirable when we are
trying to keep warm, but very desirable when we are
trying to cool down.
The conductivity of any material can be measured and
quantified so that the relative qualities that make it an
insulator or conductor can be examined in absolute
terms. The conductivity table lists some materials and
their conductivity. Even without knowing how to use the
“soup” of units with which these materials are labeled, it
is obvious that copper has a very high conductance
value (220), while air is very low (0.014).
Convection
In its most generic form, convection involves the
movement of heat by transporting some hot substance.
Convective heat movement is usually associated with
the movement of fluids. There are two common forms
of convection—”forced” and “free.” In forced convection,
power is used to move a heated fluid from the source of
heat to the heat destination. Vehicle radiator type
cooling systems and hot water or hot air home heating
systems are common examples of this.
Since we are interested in heat flow that occurs without
any energy input from us, we will be concentrating on
free convection to move our heat. Free unpowered
convection happens due to the difference in density or
molecular concentration per unit volume that occurs
when a fluid is heated.
Molecular Density & Weight
The same volume of material weighs less per unit
volume when it is hot and more when it is cold. Thus a
“cold” (less hot) fluid packs more matter into the same
volume than the same amount and type of fluid when it
is heated.
The practical result of this change in density is that a
hot fluid, being lighter, will “float” on a colder fluid.
Conversely, a cold fluid will move downward under the
pull of gravity until it finds the lowest level possible.
These are dynamic processes. The fluid actually
physically flows from one position to the next as its
thermal status changes. Such flow results in the
movement of heat.
If you put your hand over a heated stove burner, you
can feel air rising off the burner. A hot air balloon
depends on the change in density between the hot air
inside the balloon and the cooler air outside it to rise
into the sky. On a warm summer day, the lake you swim
in will have a warm layer at the top and cooler water
underneath. These are all examples of fluid movement
caused by a change in density that causes convective
heat to rise.
Convection is the movement of the heat rather than the
movement of the fluid. But the two are inexorably
intertwined, so much so that we also call the fluid
movement convective flow.
Stratification & the Greenhouse Effect
Hot air flows up; cold air flows down. This causes
several familiar effects such as stratification. The warm
water on the lake surface in the example above is a
case of stratification. Water in the lake is heated by
sunlight and rises to the top level, where it cannot go up
any farther. Here it forms a layer. It gives off some of
the sun-induced heat to the air above it, becomes more
dense, and eventually sinks again.
Depending on the amount of solar energy available, this
convection loop will stabilize so that approximately the
same amount of water is constantly heated, rises, gives
off its heat, and sinks back into the cold depths. Thus
solar heat is moved from the lake to the air.
The conversion of visible light energy into re-radiated
radiant energy contributes to what is called the
“greenhouse effect.” That’s the label for the tendency of
heat to build up in a greenhouse so that the air inside is
much warmer than ambient outside temperature.
Warmer Air
Colder Air
Cold
Window
Updraft Downdraft
Heat
Source
Thermal Convection in a Fluid
89 Home Power #82 • April / May 2001
Cooling
This happens because glass that is transparent to
visible light waves impedes the re-radiation of infrared
wavelengths. The trapped radiation heats the structure,
fixtures, and air inside the building. This heated air is
trapped inside the greenhouse by the glass (probably
causing stratification), and cannot move the heat away
by convection.
Chimney Effect & Boundary Layer Disturbance
Convection directly affects the comfort of our living
space, and even the clothes that keep us warm. It also
affects the boundary layer, which is made up of
stagnant air that acts like an insulator.
If the Δt between the ambient air and the boundary
layer is anything but zero, the boundary layer will
attempt to rise or fall of its own accord, inducing
convective heat flow. This can be the boundary layer
around our own warm bodies on a cool day, chilled air
flowing down a cold windowpane to create floor drafts
in a dwelling, or heat rising off the inside of a solar
heated wall.
Another convective phenomena commonly
encountered is the “chimney effect.” In most furnaces,
exhaust gasses exit the combustion process under the
influence of convection. The heated gasses are lighter
than the ambient air, so they rise up the chimney,
pulling air into the furnace or stove through cracks or
through a controlled draft regulator. The hotter the flue
gasses and the longer the chimney (within limits
imposed by conductive heat loss), the faster the gasses
will exit, so the stronger the gas column flowing up the
chimney will be. Most stoves and furnaces would simply
not work if this convective flow was not possible.
This chimney effect is not limited to chimney flues. It
can be used in a building as a tool to move hot air out
of the living space. The rising hot air can be supplied by
solar energy. The resultant air movement is used to
induce whole house ventilation where it might otherwise
be difficult to achieve passively.
Wind as a Heat Mover
Under the right circumstances, warm lake water will
heat the cooler air above it, inducing another fluid
convection cell in the air. This air is heated, rises, cools,
and circulates back down to the surface to be heated
again. This process is much the same as the drafts
settling off a cold window. It is much greater in volume,
and we call this movement wind. Anything that can
affect the heating of the air mass is important.
Wind is our ally. We have limited ourself by definition to
creating comfort passively in our living environment. We
have cut ourselves off (or been cut off by
circumstances) from the use of highly concentrated
fossil fuel derived energy. Yet to move heat around to
our advantage, it takes energy—sometimes large
amounts of it. Wind is the one source of energy readily
available to us that can do this job.
The differences in reflectance of the earth’s surface is
important to heat absorption wherever we are. Black
basalt rock will absorb more solar energy than light
silica sand. A farmer’s pasture will absorb less heat
energy than the concrete streets and building walls in a
city. This brings us back to the basics of material,
surface texture, and color.
We don’t usually think of something like a parking lot
affecting natural breezes. Yet such a man-made feature
can have a vast effect on the microclimate that we are
subjected to in our living spaces. A large black parking
lot will absorb a lot of solar energy. This solar energy
will be transmitted into the soil through conduction, reradiated
into the surrounding environment as radiant
heat, and will heat the air above it, which can then rise
convectivly.
This convective flow may induce local breezes where
there would be none, or it may disrupt natural wind flow.
The radiant energy will distribute itself outward from its
source to all the surrounding areas adjacent to the lot,
causing local heating and possibly destroying any
benefits a locally induced breeze might produce.
Conductive heating of soil will create a reservoir of heat
that will continue to radiate to the surroundings long
after the ambient air temperature should have become
naturally cooler. All three factors as well as terrain and
vegetative cover are interactive and each affects the
other.
Humidity & Evaporation
No discussion of wind and weather would be
comprehensive without understanding the role of
humidity and evaporation. Wind and weather are
formed as part of a large heat cycle driven by solar
energy. One of the principle forces acting on this cycle
is the addition or subtraction of heat through
evaporation.
It takes one BTU (British thermal unit) to raise the
temperature of one pound of water from 211 to 212°F
(99.4 to 100°C), but 970.4 BTUs are needed to turn it to
steam at 212°F. Those 970.4 BTUs are known as latent
heat, measured under standard conditions of one
atmosphere of pressure at sea level.
Water does not have to boil to absorb this latent heat. It
will slowly evaporate at room temperature, requiring the
same latent heat. Evaporation requires heat, and this
heat, coming from surroundings, cools the environment
considerably. The heat taken in or given off as this
process occurs creates a very complicated thermal
dance in everything from deserts to hurricanes.
90 Home Power #82 • April / May 2001
Cooling
Unless it is artificially dried, air contains water vapor
suspended in molecule-sized droplets. The amount of
water air can hold is determined by its temperature and
density. Hot air can hold more moisture than cold air.
So that there will be some common point of reference
when talking about air moisture or humidity, figures for
the water content are given as “relative humidity.”
Relative humidity measurements are given in the
percent of moisture that air holds relative to its
maximum possible moisture content at a given
temperature. The range runs from 0 percent for
absolutely dry air to 100 percent for air that holds as
much moisture as is physically possible. This is known
as the saturation point. Anything greater than 100
percent relative humidity will lead to free water
condensing out of the air as mist, fog, clouds, rain, or
snow.
The amount of moisture that air can absorb under any
condition is dependent on temperature and the amount
of moisture it already contains. Thus air measuring 70
percent humidity can only absorb the equivalent of the
remaining 30 percent moisture capacity. The lower the
air humidity, the more potential moisture the air can still
absorb.
The more moisture that can still be absorbed, the more
potential there is for heat removal through evaporation.
By evaporating moisture into the air as humidity, cooling
can be produced. And the more moisture that can be
absorbed, the more efficiently you can cool with
evaporation. Humidity bears directly on the creation of
the human comfort zone, since the body depends on
evaporation through perspiration to rid it of excess heat.
Vegetative Cover
The black surface of an asphalt parking lot is a very
good absorber of thermal energy. The dark green
surface of vegetation is also a good absorber of thermal
energy, yet the plants cool their microenvironment. How
can this be?
Plants are designed to effectively trap solar energy. But
instead of absorbing light and producing heat, they
produce plant sugars through photosynthesis. Much of
this solar energy has no chance to be turned into
excess heat. It is directed to the plants’ needs instead.
Because of this, the use of green foliage to block
sunlight striking a building is very effective. The
advantage of such shade is obvious when it comes
from trees, but the use of vining plants on trellises
covering roofs and walls also works effectively to lower
temperatures.
One of the products plant leaves give off is water vapor,
a vegetative “breath” that is transpired from pores in the
leaves. Transpiration is the process of taking in gasses
(mostly CO2) and sunlight, and giving off oxygen and
water vapor. This evaporating water absorbs heat from
the leaves and the surrounding air, cooling the local
microclimate. The combination of transpiration and
evaporation is called “evapotranspiration.”
Local Breezes
Transpiration can also play a significant role in local
breeze generation. The figure on the facing page is a
scale cross section of the Barton Creek valley where I
lived in Belize. The east side of the valley and the
adjacent hill was cleared for pasture when the original
settlers moved in. It is covered with low bushes and a
dense fern covering that is locally called “tiger bush.” It
faces squarely into the afternoon sun, and the rate of
vegetative transpiration is poor.
The west side of the valley was too steep to be cleared,
so it is mostly covered with undisturbed jungle canopy.
Direct morning sun hits this slope and is cooled by the
vegetation, but late in the afternoon when the east
slope is hottest, this west slope is taking the indirect
(non perpendicular) sun’s rays and is cooled still further.
Air is heated on the east slope and rises, while it is
cooled on the west slope by the tree canopy and sinks
down into the valley.
The net result of this differential movement is a strong
afternoon breeze that blows straight across the valley in
the hot dry season, contrary to the direction of the
prevailing Caribbean trade winds. The existence of
such a wind is completely counterintuitive, but very
much appreciated because it is much more local and
intense than the prevailing breeze. This illustrates how
much significance local and regional factors, both
natural and man-made, can have on ventilation and
heat flow.
Terrain such as hills or mountains can act as deflectors
to re-route prevailing winds, either creating a wind
shadow or augmenting wind velocity. Up to a point,
when you are trying to get cool, more is better, so
astute selection of a house site with an emphasis on
maximizing (or minimizing) local wind is important. A
good site for wind will provide the energy needed to
deal with uncomfortable temperatures either passively
or actively.
Human Heat Physiology
In spite of any surplus heat from the environment, the
body must maintain an internal temperature very close
to 98.6°F (37°C). There are a great many mechanisms
that we have evolved to effect this precise temperature
regulation. All three mechanisms for transferring heat
are at work—radiation, conduction, and convection. In
addition to those three, the body also uses
91 Home Power #82 • April / May 2001
Cooling
perspiration—shedding excess heat through the latent
heat of evaporation.
The high relative humidity typically encountered in the
humid tropics (around 95–98%), will severely interfere
with the ability to lose heat through evaporation. The air
is already saturated and cannot hold more moisture.
This is the great difference between a hot humid
environment and a hot arid environment.
Where the humidity is low, the body has the cooling
mechanism of evaporation at its disposal and the low
air moisture considerably increases the efficiency of the
process. This makes the designer’s job much easier in
such an environment.
Since I am targeting passive cooling in a hot humid
climate, my emphasis will be on techniques for the
humid tropics and subtropics. For those readers who
are fortunate enough to have dry desert conditions as
their design criteria, I direct you to two books by the
Egyptian architect Hassan Fathy. These books are
superb, clear, well illustrated, and relatively nontechnical.
Acclimatization
When I moved from Alaska to Belize in 1980, I was
adapted to the subarctic environment of interior Alaska.
Winter temperatures plunged to -60°F (-51°C) routinely,
while summers were “oppressively hot” at 80°F (27°C).
I could work in shirtsleeves at 35 to 40°F (1.6–4.4°C)
and be comfortable.
In five years in Belize, the coldest temperature I ever
encountered was around 55°F (13°C). The typical high
temperatures were 75 to 80°F (24–27°C) in winter, 90
to 95°F (32–35°C) in the wet season, and 95 to 108°F
(35–42°C) in the hot, dry season. Getting used to these
temperatures so that my body could regulate itself was
difficult. I acclimated about 80 percent in the first year,
and by the end of year two, I was 90 to 95 percent
acclimated. I never reached 100 percent in the five
years I lived there full time.
If you live in Phoenix, Arizona where the temperatures
go to 125°F (52°C) in August, and you are used to a
72°F (22°C) air-conditioned environment, you will never
acclimate to the heat because you are not forced into it.
But if you are out in the heat as it gradually increases
over the spring and summer, you will find yourself
growing accustomed to an environment that would have
seemed impossibly hostile before. If you are acclimated
to the local climate, whether hot or cold, it will take
much less energy input to remain in the comfort zone
under adverse conditions.
The Comfort Zone
The comfort zone is defined as those combinations of
conditions of humidity, temperature, and air motion
under which 80 percent of the population experiences a
feeling of thermal comfort. In temperate zones, this is
from 68 to 80°F (20–27°C), and 20 to 80 percent
humidity.
House Site
Barton Creek:
Elevation 246 feet (75 m)
Afternoon
Sun
600 500 400 300 200 100 0 100 200 300 400 500 600 700 800 900 1000 1100 1200
1500 1000 500 500 1000 1500 2000 2500 3000 3500
0
200
400
600
800
0
100
200
300
Elevation 640 feet
(195 m)
Elevation 787 feet
(240 m)
Elevation 902 feet
(275 m)
Tree Covered Slope:
Cools air
Shrub Covered Slope:
Heats air
Prevailing Winds
Elevation in Feet
Elevation in Meters
Distance in Meters
Distance in Feet
Downdraft:
Cooling air
Updraft:
Heating air
Localized Winds in the Barton Creek Valley of Belize
92 Home Power #82 • April / May 2001
Cooling
Different conditions can redefine this zone of comfort.
Air motion or breeze can extend it to almost 98 percent
humidity and 90°F (32°C). Evaporative cooling can
extend the highest comfort temperature up to 105°F
(41°C) at lower humidities. High thermal mass (such as
rock or concrete) acts like a thermal flywheel, remaining
cool into the day, and warmer at night than ambient air.
Thermal mass alone can extend the comfort zone up to
95°F (35°C), while thermal mass cooled by nighttime
ventilation can extend this zone all the way up to 110°F
(43°C). Combinations of techniques are even more
effective.
Evaporation (Perspiration) & Air Motion
At higher humidity and temperature, most of the excess
body heat is lost through perspiration. Air motion can
increase the boundaries of the comfort zone up to 98
percent humidity. This boundary would be 80 percent in
still air.
Research with a large sample of people shows that
comfort can be maintained at 100 percent humidity and
82°F (28°C), if air velocity across the skin is maintained
at around 300 feet per minute. This is the approximate
velocity of a good ceiling fan on high speed. At lower
humidities (50 percent or less), temperatures of around
90°F (32°C) are comfortable at this velocity. Because of
this relationship, the designer’s goal is to create or
preserve air velocity in the dwelling whenever possible.
A breeze blowing against our bodies removes heat
through two mechanisms—convection and latent heat
transfer. When convection occurs, the skin heats the air
and this heated air is carried away by the breeze. With
latent heat transfer, perspiration evaporates, soaking up
heat from the skin in the process. Moving air aids the
process of evaporation at higher humidities, as well as
removing the boundary layer on the skin. This dead air
layer acts as an insulator to block thermal transfer from
the skin to the air.
The boundary layer also blocks evaporative transfer
from the skin to the air. This layer heats up and reduces
the Δt between the skin and the air, slowing down heat
exchange. It also absorbs moisture from the skin, but is
unable to immediately pass this on to the surrounding
air. The boundary layer thus rises in humidity, reducing
the difference in humidity between the skin and the air.
This slows down skin evaporation and the exchange of
heat to the air. Air movement shifts this boundary layer
of warm, moist air, allowing the skin to come in contact
with drier, cooler air that can cool more efficiently.
Summary
In Part 1, I’ve taken a look at the basic principles
governing the movement of heat, and tried to give you a
feel for the way these forces interact with the
environment. We’ve looked at comfort, and found that
the experience of thermal comfort is largely subjective
to the individual.
In the next article, I will move from the general to the
specific. I’ll try to apply these principles of thermal
design to the goal of creating a comfortable, passively
cooled house in the Barton Creek valley of tropical
Belize.
Access
Cliff Mossberg, PO Box 16, Kasilof, AK 99610
907-262-6098 • attara@gci.net
Resources for Further Study:
Building for the Caribbean Basin and Latin America;
Energy-Efficient Building Strategies for Hot, Humid
Climates, Kenneth Sheinkopf, 1989, Solar Energy
Research and Education Foundation, 4733 Bethesda
Ave. #608, Bethesda, MD 20814 • 301-951-3231
Fax: 301-654-7832 • plowenth@seia.org
www.seia.org
Air Conditioning: Home and Commercial, Edwin P.
Anderson and Roland E. Palmquist, Theodore Audel &
Co., a division of Howard W. Sams & Co., Inc.,
Indianapolis, Indiana, 1978. Any library should have a
comparable book on air conditioning that will treat this
subject thoroughly.
Architecture For the Poor, 1973; and Natural Energy
and Vernacular Architecture, Principles and Examples
with Reference to Hot Arid Climates, 1986, Hassan
Fathy, both published by The University of Chicago
Press, Chicago. These books can be hard to find. I was
able to locate them through my regional inter-library
loan program and have them brought to my local library.
66 Home Power #83 • June / July 2001
In Part 1 of this series (see HP82,
page 84), I outlined the three modes
of heat transfer, and tried to integrate
this knowledge with the factors of
human comfort. This information is
basic to any design or construction
geared towards minimizing the
discomfort of living in the tropics.
In Part 2, I would like to show some of the design
techniques for dealing with the effects of heat and
humidity in a dwelling located in what we know as “the
humid tropics.” This label differentiates this climate from
that of a hot, arid, desert type of environment. The
desert might ultimately be hotter than the conditions
found in the humid tropics. But the low humidity found
in the desert makes it practical to use some techniques
of dealing with the heat that we cannot use in more
humid locations.
Solar Incidence as a Design Element
The sun is the primary engine of heat gain in a tropical
dwelling. It is not usually ambient air temperature that
causes heat discomfort, but the radiant energy of
sunlight, either directly or re-radiated in long wave
infrared. The first line of defense against heat buildup in
a building is to minimize the surfaces that sunlight can
fall on.
It is obvious that the building’s roof is going to be the
main absorber of solar energy. If the roof is designed to
block heat flow down into the dwelling, and made large
enough to cover and shade the walls, the builder should
be successful at reducing unwanted heat. This simple
concept is more difficult to accomplish that it seems at
first.
If the sun was always in the high-noon position, the job
would be simple, but it’s not. In the morning, it starts out
shining low in the eastern sky. It can heat up a
building’s walls for many hours before it rises high
enough for the roof’s shadow to shield the east wall
from radiant energy. In the afternoon, the sinking sun
has the same effect on the western wall.
Orientation for Minimum Incidence
Something can be done at the design stage to reduce
this wall heating. The very first effective step is to
design and orient the structure on the building site so
that the areas of the east and west walls are minimized.
Long, unshaded walls on the east and west sides of
a building can significantly contribute to the heating
problem.
This problem is not as severe on the north
and south walls. The sun will be lower in the
southern sky in winter when wall heating is
not as big a problem. But the sun will
never be as low in the southern sky as it
is near sunrise and sunset in the east
and west, so engineering roof
overhangs to block the southern sun
is much easier.
Roof Overhangs
In Figure 2, angle A represents
directly overhead. Angle B
has its pivot point at the base
of the south wall. It is plotted
at the local angle of north latitude. At
that angle, the sun would appear
Part II — Applied Construction Cliff Mossberg
©2001 Cliff Mossberg
S
N
E
W
50° 84°
Sunrise
Sunrise
Sunset
Sunset
Solar noon,
approx.
December 21,
shortest day
Solar noon,
approx.
June 21,
longest day
For Barton Creek, Belize, approximately 17° North Latitude
17° North
Figure 1: Seasonal Variation of the Solar Path
67 Home Power #83 • June / July 2001
Cooling
directly overhead at the equator on the days of the solar
equinox. A is easy to find—it is straight up. B is easy to
compute graphically once local latitude is known. Once
you have B, you have a baseline.
If we swing an angle north 23° from B, we will have the
northernmost angle of the sun’s travel in the sky in
Belize. In this case, it is an angle of 6° north of vertical,
or 84° vertical declination from level ground, pointing
north (Figure 1). I have labeled this line C1. If C2 is
drawn at the exact same angle as C1, but touching the
edge of the roof overhang on the north wall, the lower
extension of C2 will indicate the path of the sun’s rays
on the north side of this building.
In this case, the sun will not ever touch the base of the
north wall. C3 is the position the sun would have to
travel to for it to begin to heat the base of the wall. C3 is
an imaginary angle, since the sun is never that far down
in the northern sky at this time of day and this location
in Belize. This shows us that a standard 2 foot (0.6 m)
overhang on the north edge of the roof is sufficient to
shade this north wall at all times of the year at this
location.
Returning to our baseline B, we need to turn another
23° angle, south from B this time, just as we turned
north before. This will produce line D1, the angle of the
sun’s rays at its extreme southern sky position. It is
immediately obvious that D1 does not touch both the
base of the south wall and the edge of the roof
overhang. We know from this that the roof overhang is
insufficient, even at 3 feet (0.9 m), to completely shade
the south wall.
The south roof overhang would have to be extended all
the way out to 5 feet 2 inches (1.6 m) to completely
shade the wall. This large overhang would be
structurally weak in high winds, and would also hang
down far enough to block the view out of windows on
the south wall. A compromise between 100 percent
shade, vision, and structural rigidity will be necessary.
There are at least two possible solutions to this need for
compromise. In Figure 2, I have chosen to construct D2
as a line parallel to D1 but moved over enough so that it
touches the south roof overhang. If it is extended down
to intersect the wall, the lower projection of D2
represents the limit of the south wall shading. Above the
intersection with the wall will be shaded; below will see
direct sun at this time of the year. The line of shade
appears here to be sufficient to keep the sun’s rays out
of the window openings.
Vegetation
Trees and shrubs that shade the structure are one
approach to blocking sunlight. From a practical
standpoint, it is difficult and extremely expensive to add
mature trees of any size to a building design. The usual
procedure is to plant smaller ones and tolerate the sun
Figure 2: Angles of the Sun and Cast Shadows
Local latittude: 17° 10'
(rounded off to 17°)
Sun directly overhead
two times a year Projected line of the eave’s
shadow on the south wall at
noon, on the shortest day
of the year (winter solstice)
Position of the sun in
the sky at noon on
winter solstice,
lowest seasonal travel
Lines D1 & D2 are parallel
2' 0"
Position of the sun
at northern extreme of travel
on summer solstice
Sun's
position at
equinox
Maximum amount of south wall exposed to
winter sun. This exposure is a compromise to
allow for a reasonable (3 ft.) roof overhang,
rather than the 5 ft. 2 inches necessary to
shade the entire wall. A 3 ft. overhang will
block sun from shining in the windows at noon
during the warmest time of the year.
40°
North porch is
shaded from the
sun at its most
extreme northern
position
Angle of the sun in the northern sky
on the day of summer solstice
6°
16°
A
B
C2
C1
D3
17°
Projected line of the eave’s shadow
at the angle of sun that will
completely shade the wall
6°
40°
3' 0"
5' 2"
N
S
Base of wall
All angles are drawn from this point
4"
12"
3:1 slope
Tilt of the Earth's axis in relation to the sun is 23° 27'
(rounded off to 23° for our purposes)
23°
23°
D2
D1
C3
68 Home Power #83 • June / July 2001
Cooling
until the smaller trees are big enough to produce shade.
Unfortunately this can take ten years or longer. Where
possible, keep what you have.
Vining plants are a good alternative to trees, with one
serious caveat. One of the goals of a tropical house
design is the exclusion of termites from the wooden
parts of the structure. This can be done by building
elevated columns with termite collars on top. Any
vegetation planted on the ground and close enough to
the structure to touch it will provide a path for termites
to circumvent the exclusion features of the design.
Without the termite problem, it would be effective to use
a trellis on the east and west walls. Vining plants such
as passion fruit can intercept the sunshine and put it to
good use growing flowers or edibles.
Wall Shading with Architectural Elements
It is possible to use architectural elements to moderate
direct sun on the walls. Properly designed architectural
screens can be made to block and modulate sunlight to
good advantage. The photo above illustrates the use of
such a screen, here composed of simple decorative
concrete blocks placed together into a pleasing texture.
This very effectively opens up a whole wall to air and
muted sunlight.
This screen can conceal wooden or metal louvers fitted
with insect screens. These can be opened for the warm
dry weather, but closed for storms. In this design, the
concrete screen is integrated as part of an upscalestyle
Belizian house. It will take a substantial foundation
to support such a screen. Such massive architecture is
not necessary.
Hassan Fathy describes a traditional
screen used throughout the Middle
East that is made up of round turned
spindles arranged into a rectangular
grid. It is known as a mashrabiya.
The same term is used to describe
vertical louvered blinds that can be
adjusted to shade an entire wall.
Both of these devices allow
conditioned light to enter the
building for illumination, while
blocking the strong exterior sunlight.
The harsh contrast of the sun
beating on the outside of the screen
blocks outsiders from seeing
through the screen to the inside. But
it allows someone on the inside to
easily see out into the bright
exterior.
Window Shading Devices
There are two problems to deal with
if you wind up with sunshine on your outer walls. There
is the re-radiation of the solar energy into the interior
from the walls. I’ll deal with that next. But first I want to
deal more thoroughly with the problem of solar energy
directly heating the interior space through the window
openings. Where this is a problem, the windows
themselves can be constructed to block the sun’s rays
through reflective glass coatings and through the use of
solar screens.
Jalousie windows are commonly used in the tropics.
They use single panes of glass to form the louvers.
These single panes have virtually no insulation value. In
contrast, double and triple pane argon-filled glass used
in the colder regions are designed primarily to block
conductive and radiant heat flow outward, not to
facilitate natural ventilation inward. They would be
valuable in an air conditioned house.
While air conditioning has a role in tropical cooling, it is
not going to be a factor in our passive design focus. We
want to foster good air circulation and a design that
excludes solar radiation. Jalousie windows glazed with
glass that uses reflective films can do this.
Glass can be made with a permanent reflective coating
deposited on one face. This is conventionally either
bronze or aluminum in color. This coated glass can
block up to 80 percent of the heat energy in incoming
sunshine. Films that can be applied to uncoated glass
are also available for this purpose, and provide
approximately the same excellent result. The downside
to reflective coatings is a reduction in the amount of
visible light entering a house for general illumination.
A wall of decorative concrete block allows ventilation
and provides shade, transmitting only muted light.
69 Home Power #83 • June / July 2001
Cooling
For a radiant barrier to be effective, it
must have an air space on one or
both sides. Aluminum is a very good
conductor of heat. Without this air
space, the foil would simply move
heat from whatever substance is on
one side of it to whatever is on the
other. It would do this very efficiently.
When it is installed with an adjacent
air space, the air (which is a good
insulator for heat transfer in the
conduction mode) blocks conduction
of heat from the foil, while the poor
emissivity of the foil blocks heat
transfer through the process of
radiation.
Roof Design & Radiant Barriers
The roof is the most critical heat
blocking device in your arsenal. It
can operate passively, blocking
radiant energy from moving
downward into the house using a
radiant barrier. It restricts conductive flow of heat
through the roofing materials. And it can be designed to
use thermal convective flow to carry off air heated by
the roofing.
The roof design I prefer is actually two roofs
sandwiched together. The upper roof blocks wind and
rain. It also contains convective air channels (see
Figure 3) between spacers over the structural joists.
These cavities form ducts so that air heated by the hot
roofing can rise and exhaust at the high point through
thermal convection. Below these vent channels is a
layer of radiant barrier material. This barrier blocks the
heat that is radiated by the metal roofing, keeping it out
of the dwelling.
Solar screens that go on the outside of the windows in
place of conventional insect screens are also very
effective, reducing the incoming heat energy by up to
60 percent. Using both of these strategies produces a
tropical window that is extremely effective at blocking
invading radiant energy, while still providing excellent
ventilation. The cost is higher than uncoated glass and
normal screening, but it is worth the money.
There are many traditional methods available for
blocking solar heat from infiltrating the inside of a house
through the window openings. As a general rule,
external devices such as awnings, louvers, and roll
shades are more effective than inside devices such as
venetian blinds and roll shades. The efficiency of each
device is a function of its material, color, and texture.
Radiant Barriers
The material of choice for blocking both visible light and
infrared is a shiny sheet of polished metal. Aluminum
foil is one of the best materials, reflecting up to 95
percent of both wavelengths. This foil is a very good
conductor of heat energy, but it is a very poor radiator of
radiant heat energy. It has a maximum emission
inversely proportional to its reflectance.
In English, that means that a highly polished aluminum
foil might only re-radiate 5 percent of the radiant heat
energy falling on it. It is an ideal blocker of radiant
energy. Used in this way, these foils are known as
radiant barriers. Under peak sunshine conditions, a
radiant barrier can reduce heat inflow by as much as 40
percent or more.
Louvered “jalousie” windows are coated with a reflective surface to block sun.
They also readily facilitate ventilation.
"Galalume"
corrugated aluminum
plated steel roofing
2 x 3 inch wood spacer creating a
ventilation chamber between eaves
and roof ridge. Heat buildup between
inner and outer roofs rises and
exhausts at ridge
1 x 4 inch purlins on
2 foot centers
Reflective mylar radiant
barrier with air space
on both sides
Wall framing
Single layer of 15 lb. roofing felt
on top of 1/2 inch plywood
1/2 inch hardwood
plywood finished on
one side
Fiberglass batt
insulation Three 1 x 8 inch
boards laminated into
a continuous rafter
1 x 4 inch nailer on
each side of rafter
1 x 2 inch wood spacer
Figure 3: Cross Section of a Roof in the Tropics
70 Home Power #83 • June / July 2001
Cooling
The lower sandwich contains the structure as well as
fiberglass batt insulation to block conductive heat
flowing downward into the living area. As mentioned in
Part I, radiant heating is the principal mode of heat flow
downward. Conductive heat does not move as readily
downward through materials.
Where the roof is built of standard sheet roofing over
rafters, installing the radiant barrier is quite simple. It
can be tacked to the underside of the rafters, above the
ceiling joists. For this use, radiant barrier is available in
several different designs.
Radiant Barrier in the Walls
Walls can also easily incorporate a radiant barrier.
Where double-wall construction is used, the barrier
material can be installed on the inside with the foil
material facing the outer wall. In areas where insulation
is to be used in the wall, more care must be taken so
that there is an air space between the insulation and
the barrier material.
One method of utilizing the radiant barrier material
requires that it be installed on the outside of the
sheathing. Spacers are then nailed over the barrier
material, and a second, vented skin is installed on the
outside. Vents at the top and bottom of this second
building skin form a solar chimney, allowing heated air
to exhaust from the wall by convection. This tactic
works with either open single-wall construction or
insulated double-wall construction.
Building Insulation
Many materials have been developed to do the job of
holding air as an insulator. From sawdust, thatch, and
straw, to high tech materials such as aero-gells and
ceramic foams, all materials have pros and cons. The
first materials I’ve mentioned are organic, and subject
to biological degradation. The second two are
ridiculously expensive for home use. Good home
insulating materials should be cheap, effective, and
stable.
The ideal building insulation is nothing. The
nothingness of the vacuum in space is a case in point.
Heat flow due to conduction or convection simply
cannot occur in a vacuum because it depends on the
interaction between molecules of a substance to move
the heat. No substance equals no heat movement. But
a vacuum is not easily maintained.
Among commonly available materials, air is a very good
insulator. It is cheap and efficient, but air has a
tendency not to stay in one place when it is heated. We
need to stop convective air movements by trapping it.
Stability in Insulation Materials
Many insulating materials are available that do this
successfully. Sawdust is one of the earliest and
cheapest insulators. One of the great drawbacks of
using sawdust is that it can absorb water from rain or
moisture in the air, or even from the building interior.
Water absorption will degrade the insulation value, and
may lead to bacterial, fungal, or insect damage.
Sawdust is also subject to settling. Even the
mechanical vibrations a building may be subject to can
cause settling of the sawdust, opening up large cavities
above the insulating material where convective heat
flow can occur. A good insulator must be more than
efficient; it must be stable too, maintaining its original
volume and material properties.
Insulation Toxicity
To be a stable building insulator, a material must
contain as much air as possible, trapped in a matrix of
inert material. Rock wool is one of the oldest
commercial insulators available in batt form. It is still
used around heating systems where resistance to flame
or high heat is desirable.
Rock wool is manufactured from inert materials that
have been heated and spun out into fine fibers. It is
then fabricated into batts containing innumerable small
air spaces. It is a brittle material with friable fibers that
can break down easily during handling. These fibers
can be a severe irritant to the human body, both to the
lungs and to the skin.
So besides being stable, a good building insulator
should be benign to the people who must install it and
live around it. Asbestos is the classic example of the
perfect insulation material that is also supremely toxic.
Materials such as glass wool—fiberglass—and several
types of closed-cell foams are non-toxic and nonirritating
to a greater or lesser degree. Fiberglass is less
benign than other materials, but not nearly as irritating
as rock wool.
Fire Retardant Qualities
Another material that is common in the residential
building trades is cellulose insulation. This is
manufactured out of ground-up paper, frequently
newspaper. It has fire retardant added, and sometimes
materials to make it resistant to insect damage.
Cellulose is a very efficient, non-toxic insulator, but it
has a tendency to settle in vertical cavities, just as
sawdust does. Because of this, it is primarily used as
loose fill above ceilings. If it is kept dry, it works very
well.
Foam boards and foamed-in-place urethanes are
excellent insulators, but they do not like heat. Under
high heat conditions, they can produce toxic gases that
are lethal. Under sustained heat conditions such as
R-Values of Common Building Materials
Material R-Value
Insulation
Polyurethane, per inch 7.00
Polystyrene, extruded (blue board), per inch 5.00
Polystyrene (bead board), per inch 3.85
Rock wool, per inch 3.45
Fiberglass batt, per inch 3.35
Masonry
Concrete blocks, 8 inches 1.11
Brick, common, 4 inch 0.80
Concrete blocks, 4 inches 0.71
Stucco, 1 inch 0.20
Concrete, per inch 0.08
Siding
Wood bevel siding, 3/4 inch 1.05
Wood shingles 0.87
Wood bevel siding, 1/2 inch 0.81
Aluminum siding 0.61
Roofing
Wood shingles 0.94
Asphalt shingles 0.44
Felt paper, 12 lb. 0.06
Wall Covering
Insulation board sheathing 1.32
Cement board, 1/4 inch 0.94
Gypsum board (drywall), 5/8 inch 0.56
Gypsum board (drywall), 1/2 inch 0.45
Windows
Sealed double glazing 1.92
Single thickness glazing 0.91
Wood
Common construction softwoods, 3-1/2 inches 4.35
Common construction softwoods, 1-1/2 inches 1.89
Common construction softwoods, 3/4 inch 0.94
Plywood, construction grade, 3/4 inch 0.93
Maple, oak, or tropical hardwoods, 1 inch 0.91
Particleboard, 5/8 inch 0.82
Plywood, construction grade, 5/8 inch 0.78
Hardwood finished floor, 3/4 inch 0.68
Plywood, construction grade, 1/2 inch 0.62
Plywood, construction grade, 1/4 inch 0.31
Tempered hardboard, 1/4 inch 0.31
Regular hardboard, 1/4 inch 0.25
71 Home Power #83 • June / July 2001
Cooling
those found under a tropical roof, they can break down
and outgas, losing their closed-cell foam structure, and
seriously degrading their insulation ability.
The material I prefer for insulation in the tropics is glass
wool, most commonly known as fiberglass. It is
available in the U.S. in either batts or loose fill that can
be blown into place. Fiberglass is similar to rock wool in
its physical construction. Since it is “spun” out of fine
strands of real glass, it is inert to heat, resistant to
airborne moisture in the form of high humidity, and is a
very effective insulation. It is slightly more physically
irritating to handle than some other insulations, but new
materials are better than aged materials in this respect.
Shipping Cost
Since all of the commonly accepted thermal insulations
are light and bulky, they are expensive to ship long
distances. The cost of shipping this type of product is
based on its volume rather than its weight. That can be
substantial.
Fiberglass suffers from the same drawback that other
insulating materials do. It is difficult to obtain in Belize
and other tropical areas because it frequently must be
shipped in from more developed nations. Many nations
place a high customs duty on imported goods such as
these.
Where it is available, two-component urethane foam
insulation is very convenient because the resin to
manufacture it can be shipped by the barrel, in
concentrated form. With modest equipment, the twopart
resin can be combined and applied directly. It will
then expand in place. Keep in mind that this foam does
not like high heat.
How Much Insulation?
Some people define “R” values as “resistance” to the
flow of heat. This is a good way to think of R-values.
R-values can be added together, and they are a directly
proportional measure of heat resistance. The chart at
right lists common building materials, including
insulation materials, and their associated R-values.
Where there is little difference between inside and
ambient temperatures, and where air movement
through natural ventilation is the goal, uninsulated walls
and floors are acceptable. In a temperate climate,
where winter heat and summer air conditioning
expense is an important factor, a well-insulated house
envelope is required. R-values in the floors, walls, and
ceilings are specified by the location of the house in
specific climate zones.
The type of energy used to heat or cool a building
affects recommendations too, with higher R-values
specified for electric heat than for fossil fuels, for
72 Home Power #83 • June / July 2001
Cooling
example. Additionally, fiberglass batts are only available
in certain thicknesses, so recommendations usually
adhere to what is available. 3-1/2 inch (9 cm) thick batts
are rated R-11, 5-1/2 inches (14 cm) at R-19, etc.
Dead Air Spaces Used as Thermal Blocks
There are other ways of designing to resist heat flow
from solar-heated walls besides radiant barriers and
insulated surfaces. In new construction, during the
design phase, it’s necessary to be aware of potential
heating problems. It is cost effective to design cabinets,
closets, garages, or other unoccupied or infrequently
occupied spaces along those walls that are sources of
interior heating due to exterior solar radiation. This
practice creates a double wall with an interior dead air
space to resist heat moving across that space into the
living environment.
Mass Used as a Thermal Flywheel
From the adobe pueblos of the southwest Indians to the
rock walls in the ancient stone city of Great Zimbabwe,
many indigenous forms of architecture have taken
advantage of the thermal storage inherent in large
mass. This mass can store heat, and can also even out
the temperature fluctuations in a hostile living
environment. The modern equivalent of these classic
examples is the Trombe wall.
Using mass to mitigate temperature swings in a
dwelling only works well where the temperature
differential between the mass and the tempering heat
source is fairly large. If you try to adapt the Trombe wall
or other less passive applications of thermal mass
storage to cooling in the humid tropics, you are limited
by environmental factors.
Solar-driven temperatures inside a poorly designed
building can go up to 125°F (52°C) in the heat of the
day. This gives you a nice temperature differential to
drive heat exchange, but such a gain is never desirable!
But you do need a significant temperature difference to
move much heat from the hot interior mass to the
cooler outside nighttime air. In a passively cooled house
in the humid tropics, there is no concentrated source of
“cold” that can drive such a cooling heat flow the way
there is with solar heating.
Convection
In Passive Cooling—Part 1, I covered the theory of
convection, the movement of heat carried by the flow of
a fluid such as air or water. Here I will try to explain how
the designer or builder can use the building envelope to
force convective flow to occur passively—without any
input of energy other than what is applied to the fluid
through natural influences.
I should say here that I do not personally subscribe to
the need for entirely passive designs. Where the energy
is available or where you can create it efficiently, there
are good arguments for the use of active designs. Low
voltage DC ceiling fans are a good example.
The key to good design is the word “efficiently.” Both
passive and active cooling systems can be designed
that are so expensive to install that it could well be
more efficient, all things considered, to run a generator
and an air conditioner. So when I talk about efficiency of
design, I am factoring in the overall cost of the design,
not just operating costs.
Chimney Effect Ventilation
Hot air is less dense and therefore lighter than cool air.
It rises or floats on the heavier, cooler air. As with all
forms of heat flow, “hot” and “cold” are qualities that are
relative to the temperature of a human body—98.6°F
(37°C). There is no absolute quantity known as “hot” or
“cold.”
The important consideration is the difference in
temperature between one heat source and another, not
whether it is hot or cold. This concept is known
technically as Δt (delta t), shorthand for the change in,
or the difference in temperature.
Δt governs all things thermal, including radiation of
energy from one hot body to another, conduction
through a substance, or how easily hot air will float on
cooler air. If Δt is high, hot air is more buoyant and will
rise faster. If Δt is small, there is less tendency for a
heated mass of air to move upwards. I am using air
here as a familiar example, but technically, any fluid
from air to water to molten metals will support
convective heat flow.
Solar chimneys are structures designed to heat air with
solar energy. This heated air then rises in a duct, just as
furnace-heated air in a stovepipe rises. Under most
conditions, stand-alone solar chimneys cannot justify
their cost with their performance. Solar-enhanced
ventilators (roof panels that are designed into new
construction) may have a slightly better cost/benefit
ratio, but as a general rule, their performance is
disappointing. They are especially ill-suited to the humid
tropics.
Roof Venting
In Part 1, I described heat buildup in the attic air space
under a hot roof, and I showed how this buildup
transfers heat to the ceiling and then down into the
living space. If we return to that example, we can now
discuss the role convection will play.
In the example above, the hot roof will attain
temperatures of around 140°F (60°C) maximum. In the
living space, the desirable temperature is around 72°F
(22°C). There is a Δt between the roof heat source and
73 Home Power #83 • June / July 2001
Cooling
the ceiling of 68°F (20°C). That is
sizeable.
Suppose now that we open the roof
up and allow the hot air, which has
risen to the highest point of the roof,
to keep rising and escape? This air
removal technique is known as roof
venting, and it is highly
recommended for any enclosed roof
or attic space.
Of course, for air to flow out of a
cavity, there must be provision for
replacement air to flow in. The hot air flows out, creating
a very slight vacuum, which draws cooler air in from
some other place, usually around the roof eaves or
gables.
As this replaces the hot air with much cooler air, the Δt
between the attic air space and the ceiling membrane is
considerably reduced. The Δt between the roof and the
attic air is increased, allowing more heat to transfer
from the roof surface to the attic air, which is vented
outside to the ambient air. This reduces the roof
temperature. Clearly, convection can be useful.
Whole House Venting
The type of convective heat removal described above is
not just useful in attics and roofs. It is also useful for
whole house ventilating under certain conditions. The
point of whole house ventilation is to completely change
the air inside the living envelope periodically.
Large fans are typically used for whole house
ventilation in hot climates. These are installed in the
ceiling, and thermostatically controlled to respond to
overheating of the living space. This is typical for
houses without refrigeration-type air conditioning that
encounter seasonal high temperatures. For our
purposes, we must try to accomplish the same end
goal, but without the fan. (When practical, a whole
house system is an excellent application for a solarpowered
fan.)
Whether you employ a fan or rely only on convection for
whole house ventilation, it is desirable to achieve about
twenty air changes per hour, or 0.33 air changes each
minute. The volume of the structure can be found by
multiplying the floor area by the wall height. For the
house in Figure 4, it works out to about 6,850 cubic feet
(194 m3). So the resulting airflow desired is around
2,260 cubic feet (64 m3) per minute (0.33 x 6,850 =
2,260.5).
Disadvantages of Convection Alone
In Figure 4, the outer wall of the house is around 8 feet
(2.4 m) tall, while the roof over the clerestory windows
in the center is over 14 feet (4.3 m) tall. The interior of
this house has a cathedral ceiling that rises to a high
point above the clerestory. Hot air can flow out at this
high point to drive whole house venting. Calculating the
ventilating airflow under the best of conditions gives
about 300 cfm—not very good! Here we are assuming
no wind augmentation, just the induced circulation due
to hot air rising and exhausting.
The reality is that we are not going to be able to
ventilate this dwelling without the help of solar energy.
Either we will need it to run an active fan system, or at a
minimum to heat up the building so there is differential
temperature gain that can be put to work moving air.
But the last thing you want to do is introduce hot air just
to get rid of the hot air! Convective cooling alone is not
possible in this house under these rainy-season
conditions. During the dry season, some air exchange
is possible using convection.
Buried Cooling Tubes
Another idea that frequently creeps into conversations
about passive cooling is the use of earth tubes as air
intakes for solar chimney driven ventilation. The
principle here is that pipes are buried in the cooler earth
to draw air into the structure. The intake air cools down
to earth temperature as it is drawn in, cooling the
building.
Where a source of forced ventilation is available, such
as an electrically driven blower, this can be made to
work. Even then, there are potential problems with
moisture build-up in the tubes, which can lead to
introducing mold and mildew into the structure. Without
using a powered blower to force air through the cooling
tubes, non-circulation or even reverse circulation
(pulling heated air into the structure from a hot source)
is a possibility.
It is important to remember that stack-effect ventilation
requires that the average temperature in the air column
be higher than the cooler surrounding air. If the air
column is 85°F (29°C) in the dwelling, 100°F (38°C) in
Prevailing
Wind
A low pressure area is developed
as wind passes over the top edge
of the clerestory roof.
This vacuum draws hot inside
air, which has risen into the
clerestory, out of the building.
High pressure on this side forces air
through windows and also up into
cooling ducts under the roofing.
Figure 4:Ventilation Paths
74 Home Power #83 • June / July 2001
Cooling
the stack 10 feet (3 m) above, and 70°F (21°C) 10 feet
below, down inside the cooling tubes, we have an
average column temperature of 85°F. Ambient air
temperatures outside would have to be lower than 85°F
for upward movement of the air column to occur.
Wind Used for Ventilation
Wind is a form of convective air movement driven by
the sun. It is a concentrated form of energy. Every time
you double wind velocity, you increase wind energy
eight times, because wind energy is a cubic function of
velocity.
Wind will act on a building, whether we intend it to or
not. Contrary winds can and do drive heated air
backwards in solar ventilating ducts. They can allow
cold air infiltration into a heated
building envelope, and they
generally do unexpected things in a
structure not well thought out to
resist wind dynamics.
Where a reliable breeze is available,
you can use it to good advantage to
drive air exchange through the
envelope of a building. A
considerable amount of information
is available about how wind interacts
with the planes and curves of a
building structure.
The U.S. Federal Emergency
Management Administration (FEMA)
has thoroughly explored the
dynamics of wind/structure
interaction, seeking a better
understanding of hurricane damage
to buildings. Figure 5 and 6 are
taken from FEMA course material,
and illustrate the envelope dynamics
of a building very well. This
information is basic to
understanding how the forces
developed by wind can be used to
foster local area and whole house
ventilation.
Wind blowing against the walls and
roof of a building is forced along the
planes of the surfaces. When it
reaches the limit of a surface—the
corner of the wall or the edge or
peak of the roof—it continues to
blow in the direction in which it has
been flowing. This is a property of
the inertia of the mass of the air in
the wind current.
As it passes the edge of a building panel, wind does not
turn the corner and follow the building planes. Instead,
it lifts away from those flat sides, creating an area of
lower pressure just past the edge. Technically, it makes
a transition from smooth laminar flow along the panel to
turbulent flow away from the second panel.
Air Flow Around Walls
Figure 5 illustrates wind flow as if we were looking
down from above on the floor plan of a rectangular
building. On the left is a pictorial schematic of the path
of the wind flow. On the right is a schematic diagram of
the vector forces of pressure and vacuum induced by
the wind pattern on the left. Arrows pointing inward at
the wall represent pressure. Arrows pointing outward,
Wind
Flow
Building envelope
closed
Wind forces on
outside of building
envelope only
A C
D
B
Wind
Flow
Building envelope
opened on
windward side
Wind forces
penetrating building
produce inside
pressure on wall
equal to dynamic
wind pressure
A C
D
B
Wind
Flow
Building envelope
opened on
leeward side
Wind forces create
slight vacuum
on lee of building;
negative pressure
is transferred to
inside walls
A C
D
B
I-a I-b
II-a II-b
III-a III-b
Figure 5: Wind-Induced Pressure Vectors
Under Different Conditions of Building Ventillation
75 Home Power #83 • June / July 2001
Cooling
away from the wall, represent vacuum. The curved lines
are a rough representation of a graph of the
pressure/vacuum forces, showing how they vary in
different locations.
Illustrations I-a and I-b in Figure 5 show a building with
sealed walls and roof. In this theoretical illustration,
there are no paths for pressure to be transmitted into
the envelope. Every time a building design creates an
impediment to smooth airflow, it will induce a high
pressure area. And every time airflow is forced over a
hard edge with nothing behind it, a modest vacuum will
be created.
Figure 5, illustrations II-a and II-b show a building with a
breach in its windward wall. This can be a door,
window, or just siding torn off under high wind forces.
There are no other openings in the walls, so pressure
builds up inside the building until it exactly equals the
dynamic force of the wind entering the windward wall.
The air is actually compressed somewhat, causing a
rise in the static pressure against all of the inside walls.
Once the inside static pressure and the dynamic
wind pressure equalize, it is just like a balloon
that’s been blown up. No more air can blow into
the building because it is balanced by the force
pushing out by pressure of compression. As II-b
illustrates, the pressure inside this building is
exactly equal to the highest pressure developed
on the windward wall. This is because the wall
opening is located in the area of highest
pressure.
If the wall opening were moved over to an area
with less pressure (near the corner), that lesser
pressure would be what is transmitted to the
inside of the building. Wall C is subject not only
to the force developed by the mild vacuum
pulling on the outside, but also to the force of
the static pressure inside. These forces add up.
In hurricane winds, a building can explode under
these forces.
The designer should be sensitive to the areas of
wind-induced high and low pressure in a
structure. Maximum interior air flow value can
be achieved by allowing pressure into a building
envelope at points of highest dynamic wind
pressure. Conversely, air can be drawn from
inside a building most efficiently by strategic
placement of exit venting at points where the
wind has developed negative pressure.
Combining both strategies gives a very effective
push/pull effect. It is desirable to have the
outflow openings larger than the inflow. A six to
one ratio of outflow to inflow area is optimum.
Air Flow Over Roofs
Figure 6 shows buildings in cross section to illustrate
the dynamics of airflow over different roofs. The flat roof
and the low-pitched gable roof are subject to negative
forces trying to lift up on them. Roofs with pitches over
40° do not have sufficiently sharp eaves to cause the
flowing air to pull away from the roof as it moves along.
Consequently, the windward side of the roof receives a
substantial impact from the direct wind. So there are
positive pressures on one side of this building, including
the roof, and negative pressures on the downwind side.
These are the notes of the tune we want to play. Now
we must put the notes together into a melody. If you
open a building up to ventilation on the windward side
only, you will have no ventilation. The inside and
outside pressures cancel each other out, and that’s
that. This illustrates that you must have both an inlet
and an exit for air to flow. Air must flow out of the
envelope as fast as it can flow in or there will be
pressure build-up that will restrict inflow.
Wind
Flow
Flat Roof
A C
D
B
I-a I-b
Floor
Wind
Flow
Gabled roof
less than 40°
A C
D
B
II-a II-b
Floor
Wind
Flow
Gabled roof
greater than 40°
A C
D
B
III-a III-b
Floor
Figure 6: Wind-Induced Pressure Vectors
Over Different Roof Configuartions
76 Home Power #83 • June / July 2001
Cooling
Direct Action on Building
Figure 7 illustrates some specific building treatments
that are effective in fostering passive building
ventilation. The important concept here is that even
under moderate wind loads, there are pressure
differentials on the outside walls and roof of the
structure. If the designer takes advantage of these, it is
possible to induce significant forced ventilation
circulation through a structure. Under the right
conditions, this ventilation will equal or exceed what you
could obtain with an electric fan.
Conclusion
In Passive Cooling Part 1—Basic Principles, I described
the three basic mechanisms of heat transfer. I also
related the transfer of heat to the sensation of comfort
that people seek.
In Passive Cooling Part 2—Applied Construction, I have
tried to relate the basic information in Part I to the world
of wood, concrete, and glass. Here, to a limited degree,
I have shown specific building techniques for thwarting
the penetration of heat into a living environment. I’ve
covered the principles and pitfalls of using natural
forces to create that comfort envelope we seek in order
to keep the effects of excessive heat at bay.
I have also tried to list other, more extensive sources of
information on the subject of passive cooling, for the
enthusiastic reader. I hope this has been of some help
to people trying to live comfortably in the humid tropics.
It can be done.
Access
Cliff Mossberg, PO Box 16, Kasilof, AK 99610
attara@gci.net
Resources for further study:
Architecture For the Poor, 1973, and Natural Energy
and Vernacular Architecture, Principles and Examples
with Reference to Hot Arid Climates, 1986, by Hassan
Wind
Flow
Roof overhang creates
pressure under
windward eaves, and
vacuum above
overhang.
Wind
Flow
Cupola on roof
ridge has movable
openings on both
sides to control
variable winds.
Wind
Flow
Clerestory windows
work well when winds
are constant.
Wind
Flow
Wind creates localized
areas of high pressure
where its flow is restricted
by the structure.
Wind
Flow
Placement of walls
and windows can
control air flow
and venting.
Wind
Flow
Vegetation can
be used to
induce high
and low
pressures
when building
design cannot.
Elevation Models
Floor Plan Models
Figure 7: Strategies for Passively Inducing Ventilation
Vent windows placed high on the downwind wall allow
negative pressure to extract hot air from inside.
77 Home Power #83 • June / July 2001
Cooling
Fathy, both published by The University of Chicago
Press, Chicago. These books can be hard to find. I was
able to locate them through my regional inter-library
loan program.
Building for the Caribbean Basin and Latin America;
Energy-Efficient Building Strategies for Hot, Humid
Climates, Kenneth Sheinkopf, 1989, Solar Energy
Research and Education Foundation, 4733 Bethesda
Ave., #608, Bethesda, MD 20814 • 301-951-3231
Fax: 301-654-7832 • plowenth@seia.org
www.seia.org
Radiant Barriers: A Question and Answer Primer, by
Ingrid Melody • Florida Solar Energy Center
www.fsec.ucf.edu/Pubs/EnergyNotes/En-15.htm
Low Energy Cooling; A Guide to the Practical
Application of Passive Cooling and Cooling Energy
Conservation Measures, Donald W. Abrams, Van
Nostrand Reinhold Company, New York, 1986
Apparently no longer in print, but check with used
bookstores and inter-library loan.
Passive Cooling, 1989, edited by Jeffrey Cook, US$55
from The MIT Press, 5 Cambridge Center, Cambridge,
MA 02142 • 800-356-0343 or 617-625-8569
Fax: 6l7-625-6660 • mitpress-orders@mit.edu
http://mitpress.mit.edu
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