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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 At (delta t), shorthand for the change in, or the difference in temperature.

At 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 At is high, hot air is more buoyant and will rise faster. If At 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 At between the roof heat source and 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 wind 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 At between the attic air space and the ceiling membrane is considerably reduced. The At 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 solar-powered 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

Figure 4: Ventilation Paths

High pressure on this side forces air through windows and also up into cooling ducts under the roofing

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

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.

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,

Figure 5: Wind-Induced Pressure Vectors Under Different Conditions of Building Ventillation

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.

Wind Flow

Figure 5: Wind-Induced Pressure Vectors Under Different Conditions of Building Ventillation

Wind Flow

Wind forces on outside of building envelope only

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