Thermal Mass

Masses of high thermal capacity materials heat up more slowly and release heat over a longer time. A cast iron frying pan takes a while to heat up, but releases a nice even heat to the cooking food, and stays warm even when off the burner.

Materials with high thermal capacity have low thermal resistance. When heat is applied on one side of the material, it moves fairly quickly to the cooler side until a stable condition is reached, at which time the process slows down.

Brick, earth, stone, plaster, metals, and concrete all have high thermal capacity. Fabrics have low thermal capacity. Thin partitions of low thermal capacity materials heat and cool rapidly, so the temperature fluctuates dramatically; a tin shack can get very hot in the sun and very cold at night. Insulating materials have low thermal capacity since they are not designed to hold heat; they prevent heat from passing through them by incorporating lots of air spaces between their thin fibers.

Massive constructions of materials with high thermal capacity heat up slowly, store heat, and release it slowly. Think of how a brick or stone fireplace works. The effect is to even out the otherwise rapid heat rise and fall of temperatures as the fire flares and dies. Masses of masonry or water can store heat from solar collectors to be released at night or on cloudy days.

A portion of a room's operative temperature can be composed of radiant energy stored in thermal mass. This allows changes in the room's air temperature to be evened out over time. When the air temperature of a room normally kept at 21 °C (70°F) is allowed to drift down to 10°C (50°F) for the night, the room's operative temperature gradually follows down to 10°C as well.

If the air temperature is rapidly brought back up to 21 °C, the operative temperature rises back up more gradually. The resulting lag in heating or cooling depends upon the amount of mass in the room, and its ability to give off or take on heat. The effect may take a few hours to work, or even more. This thermal lag can help moderate changes and is useful in passive solar design, but may also mean that the room won't heat back up to the desired level fast enough. Heating or cooling the room's air temperature more than the usual amount can compensate for this slow change in temperature during warm-up or cool-down periods.

High thermal mass materials can be an integral part of the building envelope, or may be incorporated into the furnishings of the space. For maximum benefit, they must be within the insulated part of the building. The building's envelope will store heat if it has a large amount of mass. This will delay the transmission of heat to the interior, resulting in a thermal lag that can last for several hours or even for days; the greater the mass, the longer the delay. Where thermal mass is used inappropriately, excessively high temperatures or cooling loads may result on sunny days, or insufficient storage may occur overnight.

The choice of whether or not to use high quantities of thermal storage mass depends on the climate, site, interior design conditions, and operating pattern of the building. High thermal mass is appropriate when outdoor temperatures swing widely above and below the desired interior temperature. Low thermal mass is a better choice when the outside temperature remains consistently above or below the desired temperature.

Heavy mud or stone buildings with high thermal mass work well in hot desert climates with extreme changes in temperature from day to night (Fig. 16-1). The hot daytime outdoor air heats the exterior face of the wall, and migrates slowly through the wall or roof toward the interior. Before much of the heat gets to the interior, the sun sets and the air cools off outside. The radiation of heat from the ground outside to the sky cools the outdoor air below the warmer temperature of the building exterior, and the warm building surfaces are then cooled by convection and radiation. The result is a building interior that is cooler than its surroundings by day, and warmer by night.

In a hot damp climate with high night temperatures, a building with low thermal capacity works best. The building envelope reflects away solar heat and reacts quickly to cooling breezes and brief reductions in air temperatures. By elevating the building above the ground on wooden poles to catch breezes, using light

Figure 16-1 Taos Pueblo, New Mexico, around 1880.

thatch for the roof, and making the walls from open screens of wood or reeds, the cooling breeze keeps heat from being retained in the building (Fig. 16-2).

In a cold climate, a building that is occupied only occasionally (like a ski lodge) should have low thermal capacity and high thermal resistance. This will help the building to warm up quickly and cool quickly after occupancy, with no stored heat wasted on an empty interior. A well-insulated frame coupled with a woodpaneled interior is a good combination.

The high thermal capacity of soil ensures that basement walls and walls banked with earth stay fairly constant in temperature, usually around 13°C to 15°C (mid-fifties in degrees Fahrenheit) year-round. Earth-

Figure 16-2 Treehouse in Buyay, Mount Clarence, New Guinea.
Figure 16-3 Dugout home near McCook, Nebraska, 1890s.

bound walls are not exposed to extreme air temperatures in cold weather (Fig. 16-3). They should be insulated to thermal resistance values similar to the aboveground portions of the building. Burying horizontal sheets of foamed plastic insulation just below the soil's surface can minimize frost penetration into the ground adjacent to the building.

Figure 16-4 Thermal conductivity.

Figure 16-4 Thermal conductivity.

tact than highly textured ones, resulting in better conduction of heat and a cooler feeling.

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