66 Home Power #83 • June / July 2001

In Part 1 of this series (see HP82,page 84), I outlined the three modesof heat transfer, and tried to integrate

this knowledge with the factors ofhuman comfort. This information isbasic to any design or constructiongeared towards minimizing thediscomfort of living in the tropics.In Part 2, I would like to show some of the designtechniques for dealing with the effects of heat andhumidity in a dwelling located in what we know as “thehumid tropics.” This label differentiates this climate fromthat of a hot, arid, desert type of environment. Thedesert might ultimately be hotter than the conditionsfound in the humid tropics. But the low humidity foundin the desert makes it practical to use some techniquesof dealing with the heat that we cannot use in morehumid locations.

Solar Incidence as a Design ElementThe sun is the primary engine of heat gain in a tropicaldwelling. It is not usually ambient air temperature thatcauses heat discomfort, but the radiant energy ofsunlight, either directly or re-radiated in long waveinfrared. The first line of defense against heat buildup ina building is to minimize the surfaces that sunlight canfall on.

It is obvious that the building’s roof is going to be themain absorber of solar energy. If the roof is designed toblock heat flow down into the dwelling, and made largeenough to cover and shade the walls, the builder shouldbe successful at reducing unwanted heat. This simpleconcept is more difficult to accomplish that it seems atfirst.

If the sun was always in the high-noon position, the jobwould be simple, but it’s not. In the morning, it starts outshining low in the eastern sky. It can heat up abuilding’s walls for many hours before it rises highenough for the roof’s shadow to shield the east wallfrom radiant energy. In the afternoon, the sinking sunhas the same effect on the western wall.

Orientation for Minimum IncidenceSomething can be done at the design stage to reducethis wall heating. The very first effective step is todesign and orient the structure on the building site sothat the areas of the east and west walls are minimized.

Long, unshaded walls on the east and west sides ofa building can significantly contribute to the heating

problem.

This problem is not as severe on the northand south walls. The sun will be lower in the

southern sky in winter when wall heating isnot as big a problem. But the sun willnever be as low in the southern sky as itis near sunrise and sunset in the east

and west, so engineering roofoverhangs to block the southern sunis much easier.

Roof OverhangsIn Figure 2, angle A represents

directly overhead. Angle Bhas its pivot point at the baseof the south wall. It is plotted

at the local angle of north latitude. Atthat angle, the sun would appear

Part II — Applied Construction Cliff Mossberg©2001 Cliff Mossberg

S

NE

W

84°50°

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

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directly overhead at the equator on the days of the solarequinox. A is easy to find—it is straight up. B is easy tocompute graphically once local latitude is known. Onceyou have B, you have a baseline.

If we swing an angle north 23° from B, we will have thenorthernmost angle of the sun’s travel in the sky inBelize. In this case, it is an angle of 6° north of vertical,or 84° vertical declination from level ground, pointingnorth (Figure 1). I have labeled this line C1. If C2 isdrawn at the exact same angle as C1, but touching theedge of the roof overhang on the north wall, the lowerextension of C2 will indicate the path of the sun’s rayson the north side of this building.

In this case, the sun will not ever touch the base of thenorth wall. C3 is the position the sun would have totravel to for it to begin to heat the base of the wall. C3 isan imaginary angle, since the sun is never that far downin the northern sky at this time of day and this locationin Belize. This shows us that a standard 2 foot (0.6 m)overhang on the north edge of the roof is sufficient toshade this north wall at all times of the year at thislocation.

Returning to our baseline B, we need to turn another23° angle, south from B this time, just as we turnednorth before. This will produce line D1, the angle of thesun’s rays at its extreme southern sky position. It isimmediately obvious that D1 does not touch both the

base of the south wall and the edge of the roofoverhang. We know from this that the roof overhang isinsufficient, even at 3 feet (0.9 m), to completely shadethe south wall.

The south roof overhang would have to be extended allthe way out to 5 feet 2 inches (1.6 m) to completelyshade the wall. This large overhang would bestructurally weak in high winds, and would also hangdown far enough to block the view out of windows onthe south wall. A compromise between 100 percentshade, vision, and structural rigidity will be necessary.

There are at least two possible solutions to this need forcompromise. In Figure 2, I have chosen to construct D2as a line parallel to D1 but moved over enough so that ittouches the south roof overhang. If it is extended downto intersect the wall, the lower projection of D2represents the limit of the south wall shading. Above theintersection with the wall will be shaded; below will seedirect sun at this time of the year. The line of shadeappears here to be sufficient to keep the sun’s rays outof the window openings.

VegetationTrees and shrubs that shade the structure are oneapproach to blocking sunlight. From a practicalstandpoint, it is difficult and extremely expensive to addmature trees of any size to a building design. The usualprocedure is to plant smaller ones and tolerate the sun

Figure 2: Angles of the Sun and Cast ShadowsLocal latittude: 17° 10'

(rounded off to 17°)Sun directly overhead

two times a year Projected line of the eave’sshadow on the south wall atnoon, on the shortest day

of the year (winter solstice)

Position of the sun inthe sky at noon on

winter solstice,lowest seasonal travel

Lines D1 & D2 are parallel

2' 0"

Position of the sunat northern extreme of travel

on summer solstice

Sun'sposition atequinox

Maximum amount of south wall exposed towinter sun. This exposure is a compromise toallow for a reasonable (3 ft.) roof overhang,rather than the 5 ft. 2 inches necessary toshade the entire wall. A 3 ft. overhang willblock sun from shining in the windows at noonduring the warmest time of the year.

40°

North porch isshaded from thesun at its most

extreme northernposition

Angle of the sun in the northern skyon the day of summer solstice

6°

16°

AB

C2

C1

D3

17°

Projected line of the eave’s shadowat the angle of sun that willcompletely shade the wall

6°40°

3' 0"

5' 2"N

S

Base of wallAll 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. Wherepossible, keep what you have.

Vining plants are a good alternative to trees, with oneserious caveat. One of the goals of a tropical housedesign is the exclusion of termites from the woodenparts of the structure. This can be done by buildingelevated columns with termite collars on top. Anyvegetation planted on the ground and close enough tothe structure to touch it will provide a path for termitesto circumvent the exclusion features of the design.Without the termite problem, it would be effective to usea trellis on the east and west walls. Vining plants suchas passion fruit can intercept the sunshine and put it togood use growing flowers or edibles.

Wall Shading with Architectural ElementsIt is possible to use architectural elements to moderatedirect sun on the walls. Properly designed architecturalscreens can be made to block and modulate sunlight togood advantage. The photo above illustrates the use ofsuch a screen, here composed of simple decorativeconcrete blocks placed together into a pleasing texture.This very effectively opens up a whole wall to air andmuted sunlight.

This screen can conceal wooden or metal louvers fittedwith insect screens. These can be opened for the warmdry weather, but closed for storms. In this design, theconcrete screen is integrated as part of an upscale-style Belizian house. It will take a substantial foundationto support such a screen. Such massive architecture isnot necessary.

Hassan Fathy describes a traditionalscreen used throughout the MiddleEast that is made up of round turnedspindles arranged into a rectangulargrid. It is known as a mashrabiya.The same term is used to describevertical louvered blinds that can beadjusted to shade an entire wall.

Both of these devices allowconditioned light to enter thebuilding for i l lumination, whileblocking the strong exterior sunlight.The harsh contrast of the sunbeating on the outside of the screenblocks outsiders from seeingthrough the screen to the inside. Butit allows someone on the inside toeasily see out into the brightexterior.

Window Shading DevicesThere are two problems to deal with

if you wind up with sunshine on your outer walls. Thereis the re-radiation of the solar energy into the interiorfrom the walls. I’ll deal with that next. But first I want todeal more thoroughly with the problem of solar energydirectly heating the interior space through the windowopenings. Where this is a problem, the windowsthemselves can be constructed to block the sun’s raysthrough reflective glass coatings and through the use ofsolar 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. Incontrast, double and triple pane argon-filled glass usedin the colder regions are designed primarily to blockconductive and radiant heat flow outward, not tofacilitate natural ventilation inward. They would bevaluable in an air conditioned house.

While air conditioning has a role in tropical cooling, it isnot going to be a factor in our passive design focus. Wewant to foster good air circulation and a design thatexcludes solar radiation. Jalousie windows glazed withglass that uses reflective films can do this.

Glass can be made with a permanent reflective coatingdeposited on one face. This is conventionally eitherbronze or aluminum in color. This coated glass canblock up to 80 percent of the heat energy in incomingsunshine. Films that can be applied to uncoated glassare also available for this purpose, and provideapproximately the same excellent result. The downsideto reflective coatings is a reduction in the amount ofvisible light entering a house for general illumination.

A wall of decorative concrete block allows ventilation and provides shade, transmitting only muted light.

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For a radiant barrier to be effective, itmust have an air space on one orboth sides. Aluminum is a very goodconductor of heat. Without this airspace, the foil would simply moveheat from whatever substance is onone side of it to whatever is on theother. It would do this very efficiently.When it is installed with an adjacentair space, the air (which is a goodinsulator for heat transfer in theconduction mode) blocks conductionof heat from the foil, while the pooremissivity of the foil blocks heattransfer through the process ofradiation.

Roof Design & Radiant BarriersThe roof is the most critical heatblocking device in your arsenal. Itcan operate passively, blockingradiant energy from movingdownward into the house using a

radiant barrier. It restricts conductive flow of heatthrough the roofing materials. And it can be designed touse thermal convective flow to carry off air heated bythe roofing.

The roof design I prefer is actually two roofssandwiched together. The upper roof blocks wind andrain. It also contains convective air channels (seeFigure 3) between spacers over the structural joists.These cavities form ducts so that air heated by the hotroofing can rise and exhaust at the high point throughthermal convection. Below these vent channels is alayer of radiant barrier material. This barrier blocks theheat that is radiated by the metal roofing, keeping it outof the dwelling.

Solar screens that go on the outside of the windows inplace of conventional insect screens are also veryeffective, reducing the incoming heat energy by up to60 percent. Using both of these strategies produces atropical window that is extremely effective at blockinginvading radiant energy, while still providing excellentventilation. The cost is higher than uncoated glass andnormal screening, but it is worth the money.

There are many traditional methods available forblocking solar heat from infiltrating the inside of a housethrough the window openings. As a general rule,external devices such as awnings, louvers, and rollshades are more effective than inside devices such asvenetian blinds and roll shades. The efficiency of eachdevice is a function of its material, color, and texture.

Radiant BarriersThe material of choice for blocking both visible light andinfrared is a shiny sheet of polished metal. Aluminumfoil is one of the best materials, reflecting up to 95percent of both wavelengths. This foil is a very goodconductor of heat energy, but it is a very poor radiator ofradiant heat energy. It has a maximum emissioninversely proportional to its reflectance.

In English, that means that a highly polished aluminumfoil might only re-radiate 5 percent of the radiant heatenergy falling on it. It is an ideal blocker of radiantenergy. Used in this way, these foils are known asradiant barriers. Under peak sunshine conditions, aradiant barrier can reduce heat inflow by as much as 40percent or more.

Louvered “jalousie” windows are coated with a reflective surface to block sun.They also readily facilitate ventilation.

"Galalume"corrugated aluminumplated steel roofing

2 x 3 inch wood spacer creating aventilation chamber between eavesand roof ridge. Heat buildup between

inner and outer roofs rises andexhausts at ridge

1 x 4 inch purlins on2 foot centers

Reflective mylar radiantbarrier with air space

on both sides

Wall framing

Single layer of 15 lb. roofing felton top of 1/2 inch plywood

1/2 inch hardwoodplywood finished on

one side

Fiberglass battinsulation Three 1 x 8 inch

boards laminated intoa continuous rafter

1 x 4 inch nailer oneach side of rafter

1 x 2 inch wood spacer

Figure 3: Cross Section of a Roof in the Tropics

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The lower sandwich contains the structure as well asfiberglass batt insulation to block conductive heatflowing downward into the living area. As mentioned inPart I, radiant heating is the principal mode of heat flowdownward. Conductive heat does not move as readilydownward through materials.

Where the roof is built of standard sheet roofing overrafters, installing the radiant barrier is quite simple. Itcan be tacked to the underside of the rafters, above theceiling joists. For this use, radiant barrier is available inseveral different designs.

Radiant Barrier in the WallsWalls can also easily incorporate a radiant barrier.Where double-wall construction is used, the barriermaterial can be installed on the inside with the foilmaterial facing the outer wall. In areas where insulationis to be used in the wall, more care must be taken sothat there is an air space between the insulation andthe barrier material.

One method of utilizing the radiant barrier materialrequires that it be installed on the outside of thesheathing. Spacers are then nailed over the barriermaterial, and a second, vented skin is installed on theoutside. Vents at the top and bottom of this secondbuilding skin form a solar chimney, allowing heated airto exhaust from the wall by convection. This tacticworks with either open single-wall construction orinsulated double-wall construction.

Building InsulationMany materials have been developed to do the job ofholding air as an insulator. From sawdust, thatch, andstraw, to high tech materials such as aero-gells andceramic foams, all materials have pros and cons. Thefirst materials I’ve mentioned are organic, and subjectto biological degradation. The second two areridiculously expensive for home use. Good homeinsulating materials should be cheap, effective, andstable.

The ideal building insulation is nothing. Thenothingness of the vacuum in space is a case in point.Heat flow due to conduction or convection simplycannot occur in a vacuum because it depends on theinteraction between molecules of a substance to movethe heat. No substance equals no heat movement. Buta vacuum is not easily maintained.

Among commonly available materials, air is a very goodinsulator. It is cheap and efficient, but air has atendency not to stay in one place when it is heated. Weneed to stop convective air movements by trapping it.

Stability in Insulation MaterialsMany insulating materials are available that do this

successfully. Sawdust is one of the earliest andcheapest insulators. One of the great drawbacks ofusing sawdust is that it can absorb water from rain ormoisture in the air, or even from the building interior.Water absorption will degrade the insulation value, andmay lead to bacterial, fungal, or insect damage.

Sawdust is also subject to settl ing. Even themechanical vibrations a building may be subject to cancause settling of the sawdust, opening up large cavitiesabove the insulating material where convective heatflow can occur. A good insulator must be more thanefficient; it must be stable too, maintaining its originalvolume and material properties.

Insulation ToxicityTo be a stable building insulator, a material mustcontain as much air as possible, trapped in a matrix ofinert material. Rock wool is one of the oldestcommercial insulators available in batt form. It is stillused around heating systems where resistance to flameor high heat is desirable.

Rock wool is manufactured from inert materials thathave been heated and spun out into fine fibers. It isthen fabricated into batts containing innumerable smallair spaces. It is a brittle material with friable fibers thatcan break down easily during handling. These fiberscan be a severe irritant to the human body, both to thelungs and to the skin.

So besides being stable, a good building insulatorshould be benign to the people who must install it andlive around it. Asbestos is the classic example of theperfect insulation material that is also supremely toxic.

Materials such as glass wool—fiberglass—and severaltypes of closed-cell foams are non-toxic and non-irritating to a greater or lesser degree. Fiberglass is lessbenign than other materials, but not nearly as irritatingas rock wool.

Fire Retardant QualitiesAnother material that is common in the residentialbuilding trades is cellulose insulation. This ismanufactured out of ground-up paper, frequentlynewspaper. It has fire retardant added, and sometimesmaterials to make it resistant to insect damage.Cellulose is a very efficient, non-toxic insulator, but ithas a tendency to settle in vertical cavities, just assawdust does. Because of this, it is primarily used asloose fill above ceilings. If it is kept dry, it works verywell.

Foam boards and foamed-in-place urethanes areexcellent insulators, but they do not like heat. Underhigh heat conditions, they can produce toxic gases thatare lethal. Under sustained heat conditions such as

R-Values of Common Building Materials

Material R-ValueInsulationPolyurethane, per inch 7.00Polystyrene, extruded (blue board), per inch 5.00Polystyrene (bead board), per inch 3.85Rock wool, per inch 3.45Fiberglass batt, per inch 3.35

MasonryConcrete blocks, 8 inches 1.11Brick, common, 4 inch 0.80Concrete blocks, 4 inches 0.71Stucco, 1 inch 0.20Concrete, per inch 0.08

SidingWood bevel siding, 3/4 inch 1.05Wood shingles 0.87Wood bevel siding, 1/2 inch 0.81Aluminum siding 0.61

RoofingWood shingles 0.94Asphalt shingles 0.44Felt paper, 12 lb. 0.06

Wall CoveringInsulation board sheathing 1.32Cement board, 1/4 inch 0.94Gypsum board (drywall), 5/8 inch 0.56Gypsum board (drywall), 1/2 inch 0.45

WindowsSealed double glazing 1.92Single thickness glazing 0.91

Wood

Common construction softwoods, 3-1/2 inches 4.35Common construction softwoods, 1-1/2 inches 1.89Common construction softwoods, 3/4 inch 0.94Plywood, construction grade, 3/4 inch 0.93Maple, oak, or tropical hardwoods, 1 inch 0.91Particleboard, 5/8 inch 0.82Plywood, construction grade, 5/8 inch 0.78Hardwood finished floor, 3/4 inch 0.68Plywood, construction grade, 1/2 inch 0.62Plywood, construction grade, 1/4 inch 0.31Tempered hardboard, 1/4 inch 0.31Regular hardboard, 1/4 inch 0.25

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those found under a tropical roof, they can break downand outgas, losing their closed-cell foam structure, andseriously degrading their insulation ability.

The material I prefer for insulation in the tropics is glasswool, most commonly known as fiberglass. It isavailable in the U.S. in either batts or loose fill that canbe blown into place. Fiberglass is similar to rock wool inits physical construction. Since it is “spun” out of finestrands of real glass, it is inert to heat, resistant toairborne moisture in the form of high humidity, and is avery effective insulation. It is slightly more physicallyirritating to handle than some other insulations, but newmaterials are better than aged materials in this respect.

Shipping CostSince all of the commonly accepted thermal insulationsare light and bulky, they are expensive to ship longdistances. The cost of shipping this type of product isbased on its volume rather than its weight. That can besubstantial.

Fiberglass suffers from the same drawback that otherinsulating materials do. It is difficult to obtain in Belizeand other tropical areas because it frequently must beshipped in from more developed nations. Many nationsplace a high customs duty on imported goods such asthese.

Where it is available, two-component urethane foaminsulation is very convenient because the resin tomanufacture it can be shipped by the barrel, inconcentrated form. With modest equipment, the two-part resin can be combined and applied directly. It willthen expand in place. Keep in mind that this foam doesnot like high heat.

How Much Insulation?Some people define “R” values as “resistance” to theflow of heat. This is a good way to think of R-values. R-values can be added together, and they are a directlyproportional measure of heat resistance. The chart atright l ists common building materials, includinginsulation materials, and their associated R-values.

Where there is little difference between inside andambient temperatures, and where air movementthrough natural ventilation is the goal, uninsulated wallsand floors are acceptable. In a temperate climate,where winter heat and summer air conditioningexpense is an important factor, a well-insulated houseenvelope is required. R-values in the floors, walls, andceilings are specified by the location of the house inspecific climate zones.

The type of energy used to heat or cool a buildingaffects recommendations too, with higher R-valuesspecified for electric heat than for fossil fuels, for

72 Home Power #83 • June / July 2001

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example. Additionally, fiberglass batts are only availablein certain thicknesses, so recommendations usuallyadhere to what is available. 3-1/2 inch (9 cm) thick battsare rated R-11, 5-1/2 inches (14 cm) at R-19, etc.

Dead Air Spaces Used as Thermal BlocksThere are other ways of designing to resist heat flowfrom solar-heated walls besides radiant barriers andinsulated surfaces. In new construction, during thedesign phase, it’s necessary to be aware of potentialheating problems. It is cost effective to design cabinets,closets, garages, or other unoccupied or infrequentlyoccupied spaces along those walls that are sources ofinterior heating due to exterior solar radiation. Thispractice creates a double wall with an interior dead airspace to resist heat moving across that space into theliving environment.

Mass Used as a Thermal FlywheelFrom the adobe pueblos of the southwest Indians to therock walls in the ancient stone city of Great Zimbabwe,many indigenous forms of architecture have takenadvantage of the thermal storage inherent in largemass. This mass can store heat, and can also even outthe temperature fluctuations in a hostile l ivingenvironment. The modern equivalent of these classicexamples is the Trombe wall.

Using mass to mitigate temperature swings in adwelling only works well where the temperaturedifferential between the mass and the tempering heatsource is fairly large. If you try to adapt the Trombe wallor other less passive applications of thermal massstorage to cooling in the humid tropics, you are limitedby environmental factors.

Solar-driven temperatures inside a poorly designedbuilding can go up to 125°F (52°C) in the heat of theday. This gives you a nice temperature differential todrive heat exchange, but such a gain is never desirable!But you do need a significant temperature difference tomove much heat from the hot interior mass to thecooler outside nighttime air. In a passively cooled housein the humid tropics, there is no concentrated source of“cold” that can drive such a cooling heat flow the waythere is with solar heating.

ConvectionIn Passive Cooling—Part 1, I covered the theory ofconvection, the movement of heat carried by the flow ofa fluid such as air or water. Here I will try to explain howthe designer or builder can use the building envelope toforce convective flow to occur passively—without anyinput of energy other than what is applied to the fluidthrough natural influences.

I should say here that I do not personally subscribe tothe need for entirely passive designs. Where the energy

is available or where you can create it efficiently, thereare good arguments for the use of active designs. Lowvoltage DC ceiling fans are a good example.

The key to good design is the word “efficiently.” Bothpassive and active cooling systems can be designedthat are so expensive to install that it could well bemore efficient, all things considered, to run a generatorand an air conditioner. So when I talk about efficiency ofdesign, I am factoring in the overall cost of the design,not just operating costs.

Chimney Effect VentilationHot air is less dense and therefore lighter than cool air.It rises or floats on the heavier, cooler air. As with allforms of heat flow, “hot” and “cold” are qualities that arerelative 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 intemperature between one heat source and another, notwhether it is hot or cold. This concept is knowntechnically as ∆t (delta t), shorthand for the change in,or the difference in temperature.

∆t governs all things thermal, including radiation ofenergy from one hot body to another, conductionthrough a substance, or how easily hot air will float oncooler air. If ∆t is high, hot air is more buoyant and willrise faster. If ∆t is small, there is less tendency for aheated mass of air to move upwards. I am using airhere as a familiar example, but technically, any fluidfrom air to water to molten metals will supportconvective heat flow.

Solar chimneys are structures designed to heat air withsolar energy. This heated air then rises in a duct, just asfurnace-heated air in a stovepipe rises. Under mostconditions, stand-alone solar chimneys cannot justifytheir cost with their performance. Solar-enhancedventilators (roof panels that are designed into newconstruction) may have a slightly better cost/benefitratio, but as a general rule, their performance isdisappointing. They are especially ill-suited to the humidtropics.

Roof VentingIn Part 1, I described heat buildup in the attic air spaceunder a hot roof, and I showed how this builduptransfers heat to the ceiling and then down into theliving space. If we return to that example, we can nowdiscuss the role convection will play.

In the example above, the hot roof will attaintemperatures of around 140°F (60°C) maximum. In theliving space, the desirable temperature is around 72°F(22°C). There is a ∆t between the roof heat source and

73Home Power #83 • June / July 2001

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the ceiling of 68°F (20°C). That issizeable.

Suppose now that we open the roofup and allow the hot air, which hasrisen to the highest point of the roof,to keep rising and escape? This airremoval technique is known as roofventing, and it is highlyrecommended for any enclosed roofor attic space.

Of course, for air to flow out of acavity, there must be provision forreplacement air to flow in. The hot air flows out, creatinga very slight vacuum, which draws cooler air in fromsome other place, usually around the roof eaves orgables.

As this replaces the hot air with much cooler air, the ∆tbetween the attic air space and the ceiling membrane isconsiderably reduced. The ∆t between the roof and theattic air is increased, allowing more heat to transferfrom the roof surface to the attic air, which is ventedoutside to the ambient air. This reduces the rooftemperature. Clearly, convection can be useful.

Whole House VentingThe type of convective heat removal described above isnot just useful in attics and roofs. It is also useful forwhole house ventilating under certain conditions. Thepoint of whole house ventilation is to completely changethe air inside the living envelope periodically.

Large fans are typically used for whole houseventilation in hot climates. These are installed in theceiling, and thermostatically controlled to respond tooverheating of the living space. This is typical forhouses without refrigeration-type air conditioning thatencounter seasonal high temperatures. For ourpurposes, we must try to accomplish the same endgoal, but without the fan. (When practical, a wholehouse system is an excellent application for a solar-powered fan.)

Whether you employ a fan or rely only on convection forwhole house ventilation, it is desirable to achieve abouttwenty air changes per hour, or 0.33 air changes eachminute. The volume of the structure can be found bymultiplying the floor area by the wall height. For thehouse in Figure 4, it works out to about 6,850 cubic feet(194 m3). So the resulting airflow desired is around2,260 cubic feet (64 m3) per minute (0.33 x 6,850 =2,260.5).

Disadvantages of Convection AloneIn 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 ofthis house has a cathedral ceiling that rises to a highpoint above the clerestory. Hot air can flow out at thishigh point to drive whole house venting. Calculating theventilating airflow under the best of conditions givesabout 300 cfm—not very good! Here we are assumingno wind augmentation, just the induced circulation dueto hot air rising and exhausting.

The reality is that we are not going to be able toventilate this dwelling without the help of solar energy.Either we will need it to run an active fan system, or at aminimum to heat up the building so there is differentialtemperature gain that can be put to work moving air.But the last thing you want to do is introduce hot air justto get rid of the hot air! Convective cooling alone is notpossible in this house under these rainy-seasonconditions. During the dry season, some air exchangeis possible using convection.

Buried Cooling TubesAnother idea that frequently creeps into conversationsabout passive cooling is the use of earth tubes as airintakes for solar chimney driven ventilation. Theprinciple here is that pipes are buried in the cooler earthto draw air into the structure. The intake air cools downto earth temperature as it is drawn in, cooling thebuilding.

Where a source of forced ventilation is available, suchas an electrically driven blower, this can be made towork. Even then, there are potential problems withmoisture build-up in the tubes, which can lead tointroducing mold and mildew into the structure. Withoutusing a powered blower to force air through the coolingtubes, 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 ventilationrequires that the average temperature in the air columnbe higher than the cooler surrounding air. If the aircolumn is 85°F (29°C) in the dwelling, 100°F (38°C) in

PrevailingWind

A low pressure area is developedas wind passes over the top edge

of the clerestory roof.This vacuum draws hot insideair, which has risen into the

clerestory, out of the building.

High pressure on this side forces airthrough windows and also up intocooling ducts under the roofing.

Figure 4: Ventilation Paths

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the stack 10 feet (3 m) above, and 70°F (21°C) 10 feetbelow, down inside the cooling tubes, we have anaverage column temperature of 85°F. Ambient airtemperatures outside would have to be lower than 85°Ffor upward movement of the air column to occur.

Wind Used for VentilationWind is a form of convective air movement driven bythe sun. It is a concentrated form of energy. Every timeyou double wind velocity, you increase wind energyeight times, because wind energy is a cubic function ofvelocity.

Wind will act on a building, whether we intend it to ornot. Contrary winds can and do drive heated airbackwards in solar ventilating ducts. They can allowcold air infiltration into a heatedbuilding envelope, and theygenerally do unexpected things in astructure not well thought out toresist wind dynamics.

Where a reliable breeze is available,you can use it to good advantage todrive air exchange through theenvelope of a building. Aconsiderable amount of informationis available about how wind interactswith the planes and curves of abuilding structure.

The U.S. Federal EmergencyManagement Administration (FEMA)has thoroughly explored thedynamics of wind/structureinteraction, seeking a betterunderstanding of hurricane damageto buildings. Figure 5 and 6 aretaken from FEMA course material,and illustrate the envelope dynamicsof a building very well. Thisinformation is basic tounderstanding how the forcesdeveloped by wind can be used tofoster local area and whole houseventilation.

Wind blowing against the walls androof of a building is forced along theplanes of the surfaces. When itreaches the limit of a surface—thecorner of the wall or the edge orpeak of the roof—it continues toblow in the direction in which it hasbeen flowing. This is a property ofthe inertia of the mass of the air inthe wind current.

As it passes the edge of a building panel, wind does notturn the corner and follow the building planes. Instead,it lifts away from those flat sides, creating an area oflower pressure just past the edge. Technically, it makesa transition from smooth laminar flow along the panel toturbulent flow away from the second panel.

Air Flow Around WallsFigure 5 illustrates wind flow as if we were lookingdown from above on the floor plan of a rectangularbuilding. On the left is a pictorial schematic of the pathof the wind flow. On the right is a schematic diagram ofthe vector forces of pressure and vacuum induced bythe wind pattern on the left. Arrows pointing inward atthe wall represent pressure. Arrows pointing outward,

WindFlow

Building envelopeclosed

Wind forces onoutside of building

envelope onlyA C

D

B

WindFlow

Building envelopeopened on

windward side

Wind forcespenetrating building

produce insidepressure on wallequal to dynamic

wind pressure

A C

D

B

WindFlow

Building envelopeopened on

leeward side

Wind forces createslight vacuum

on lee of building;negative pressureis 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

75Home Power #83 • June / July 2001

Cooling

away from the wall, represent vacuum. The curved linesare a rough representation of a graph of thepressure/vacuum forces, showing how they vary indifferent locations.

Illustrations I-a and I-b in Figure 5 show a building withsealed walls and roof. In this theoretical illustration,there are no paths for pressure to be transmitted intothe envelope. Every time a building design creates animpediment to smooth airflow, it will induce a highpressure area. And every time airflow is forced over ahard edge with nothing behind it, a modest vacuum willbe created.

Figure 5, illustrations II-a and II-b show a building with abreach 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 pressurebuilds up inside the building until it exactly equals thedynamic force of the wind entering the windward wall.The air is actually compressed somewhat, causing arise in the static pressure against all of the inside walls.

Once the inside static pressure and the dynamicwind pressure equalize, it is just like a balloonthat’s been blown up. No more air can blow intothe building because it is balanced by the forcepushing out by pressure of compression. As II-billustrates, the pressure inside this building isexactly equal to the highest pressure developedon the windward wall. This is because the wallopening is located in the area of highestpressure.

If the wall opening were moved over to an areawith less pressure (near the corner), that lesserpressure would be what is transmitted to theinside of the building. Wall C is subject not onlyto the force developed by the mild vacuumpulling on the outside, but also to the force ofthe static pressure inside. These forces add up.In hurricane winds, a building can explode underthese forces.

The designer should be sensitive to the areas ofwind-induced high and low pressure in astructure. Maximum interior air flow value canbe achieved by allowing pressure into a buildingenvelope at points of highest dynamic windpressure. Conversely, air can be drawn frominside a building most efficiently by strategicplacement of exit venting at points where thewind has developed negative pressure.Combining both strategies gives a very effectivepush/pull effect. It is desirable to have theoutflow openings larger than the inflow. A six toone ratio of outflow to inflow area is optimum.

Air Flow Over RoofsFigure 6 shows buildings in cross section to illustratethe dynamics of airflow over different roofs. The flat roofand the low-pitched gable roof are subject to negativeforces trying to lift up on them. Roofs with pitches over40° do not have sufficiently sharp eaves to cause theflowing air to pull away from the roof as it moves along.Consequently, the windward side of the roof receives asubstantial impact from the direct wind. So there arepositive pressures on one side of this building, includingthe roof, and negative pressures on the downwind side.

These are the notes of the tune we want to play. Nowwe must put the notes together into a melody. If youopen a building up to ventilation on the windward sideonly, you will have no ventilation. The inside andoutside pressures cancel each other out, and that’sthat. This illustrates that you must have both an inletand an exit for air to flow. Air must flow out of theenvelope as fast as it can flow in or there will bepressure build-up that will restrict inflow.

WindFlow

Flat Roof

A C

D

B

I-a I-b

Floor

WindFlow

Gabled roofless than 40°

A C

D

B

II-a II-b

Floor

WindFlow

Gabled roofgreater 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 BuildingFigure 7 illustrates some specific building treatmentsthat are effective in fostering passive buildingventilation. The important concept here is that evenunder moderate wind loads, there are pressuredifferentials on the outside walls and roof of thestructure. If the designer takes advantage of these, it is

possible to induce significant forced ventilationcirculation through a structure. Under the rightconditions, this ventilation will equal or exceed what youcould obtain with an electric fan.

ConclusionIn Passive Cooling Part 1—Basic Principles, I describedthe three basic mechanisms of heat transfer. I alsorelated the transfer of heat to the sensation of comfortthat people seek.

In Passive Cooling Part 2—Applied Construction, I havetried to relate the basic information in Part I to the worldof wood, concrete, and glass. Here, to a limited degree,I have shown specific building techniques for thwartingthe penetration of heat into a living environment. I’vecovered the principles and pitfalls of using naturalforces to create that comfort envelope we seek in orderto keep the effects of excessive heat at bay.

I have also tried to list other, more extensive sources ofinformation on the subject of passive cooling, for theenthusiastic reader. I hope this has been of some helpto people trying to live comfortably in the humid tropics.It can be done.

AccessCliff Mossberg, PO Box 16, Kasilof, AK 99610attara@gci.net

Resources for further study:Architecture For the Poor, 1973, and Natural Energyand Vernacular Architecture, Principles and Exampleswith Reference to Hot Arid Climates, 1986, by Hassan

WindFlow

Roof overhang createspressure under

windward eaves, andvacuum above

overhang.

WindFlow

Cupola on roofridge has movableopenings on bothsides to controlvariable winds.

WindFlow

Clerestory windowswork well when winds

are constant.

WindFlow

Wind creates localizedareas of high pressure

where its flow is restrictedby the structure.

WindFlow

Placement of wallsand windows can

control air flowand venting.

WindFlow

Vegetation canbe used toinduce high

and lowpressures

when buildingdesign cannot.

Elevation Models

Floor Plan Models

Figure 7: Strategies for Passively Inducing Ventilation

Vent windows placed high on the downwind wall allownegative pressure to extract hot air from inside.

77Home Power #83 • June / July 2001

Cooling

Fathy, both published by The University of ChicagoPress, Chicago. These books can be hard to find. I wasable to locate them through my regional inter-libraryloan program.

Building for the Caribbean Basin and Latin America;Energy-Efficient Building Strategies for Hot, HumidClimates, Kenneth Sheinkopf, 1989, Solar EnergyResearch and Education Foundation, 4733 BethesdaAve., #608, Bethesda, MD 20814 • 301-951-3231Fax: 301-654-7832 • plowenth@seia.orgwww.seia.org

Radiant Barriers: A Question and Answer Primer, byIngrid Melody • Florida Solar Energy Centerwww.fsec.ucf.edu/Pubs/EnergyNotes/En-15.htm

Low Energy Cooling; A Guide to the PracticalApplication of Passive Cooling and Cooling EnergyConservation Measures, Donald W. Abrams, VanNostrand Reinhold Company, New York, 1986Apparently no longer in print, but check with usedbookstores and inter-library loan.

Passive Cooling, 1989, edited by Jeffrey Cook, US$55from The MIT Press, 5 Cambridge Center, Cambridge,MA 02142 • 800-356-0343 or 617-625-8569Fax: 6l7-625-6660 • mitpress-orders@mit.eduhttp://mitpress.mit.edu

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