Passive Solar Cooling, Part 2.pdf

Part II — Applied Construction

Cliff Mossberg

I n 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.

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.

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.

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.

Figure 1: Seasonal Variation of the Solar Path

For Barton Creek, Belize, approximately 17

North Latitude

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.

Solar noon,

approx.

December 21,

shortest day

Solar noon,

approx.

June 21,

longest day

Sunset

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.

50 °

Sunset

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

Sunrise

Home Power #83 • June / July 2001

Cooling

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

at the angle of sun that will

completely shade the wall

Tilt of the Earth's axis in relation to the sun is 23

27'

Sun's

position at

equinox

D 3

Projected line of the eave’s

shadow on the south wall at

noon, on the shortest day

of the year (winter solstice)

(rounded off to 23

for our purposes)

Position of the sun

at northern extreme of travel

on summer solstice

C 1

Angle of the sun in the northern sky

on the day of summer solstice

D 2

6 °

D 1

C 2

17 °

C 3

Position of the sun in

the sky at noon on

winter solstice,

lowest seasonal travel

6 °

Lines D 1 & D 2 are parallel

3:1 slope

12"

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.

2' 0"

3' 0"

5' 2"

North porch is

shaded from the

sun at its most

extreme northern

position

Base of wall

All angles are drawn from this point

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.

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.

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 C 1 . If C 2 is

drawn at the exact same angle as C 1 , but touching the

edge of the roof overhang on the north wall, the lower

extension of C 2 will indicate the path of the sun’s rays

on the north side of this building.

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 D 2

as a line parallel to D 1 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 D 2

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.

In this case, the sun will not ever touch the base of the

north wall. C 3 is the position the sun would have to

travel to for it to begin to heat the base of the wall. C 3 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.

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

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 D 1 , the angle of the

sun’s rays at its extreme southern sky position. It is

immediately obvious that D 1 does not touch both the

Home Power #83 • June / July 2001

Cooling

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.

A wall of decorative concrete block allows ventilation

and provides shade, transmitting only muted light.

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.

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.

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.

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.

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.

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 upscale-

style Belizian house. It will take a substantial foundation

to support such a screen. Such massive architecture is

not necessary.

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.

Louvered “jalousie” windows are coated with a reflective surface to block sun.

They also readily facilitate ventilation.

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.

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.

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.

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.

Figure 3: Cross Section of a Roof in the Tropics

"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

Reflective mylar radiant

barrier with air space

on both sides

Single layer of 15 lb. roofing felt

on top of 1/2 inch plywood

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.

1 x 2 inch wood spacer

1 x 4 inch nailer on

each side of rafter

Fiberglass batt

insulation

Wall framing

Three 1 x 8 inch

boards laminated into

a continuous rafter

1/2 inch hardwood

plywood finished on

one side

Home Power #83 • June / July 2001

1 x 4 inch purlins on

2 foot centers

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.

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.

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.

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.

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.

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.

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.

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.

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.

Materials such as glass wool—fiberglass—and several

types of closed-cell foams are non-toxic and non-

irritating 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.

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.

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

Stability in Insulation Materials

Many insulating materials are available that do this

Home Power #83 • June / July 2001

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