Heat transfer considerations in architecture

Kathleen Perks

In order to analyze energy loss from a building, three modes of heat transfer must be considered. These modes are radiation, conduction, and air infiltration.

In studying the total heat transfer from a building, extreme conditions are of most concern. Since loss of energy is driven primarily by the magnitude of a temperature gradient, the coldest possible environment is chosen for analysis so as to achieve results for the maximum heat flow. Hence, winter temperatures for a given building location are used.

Because houses are not air tight, heat transfer caused by air infiltration is considered. This air flow rate is quantified by examining the amount of air that leaves the building per hour(ft^3/hr). There are two key components of heat loss due to air infiltration that are the driving forces for which air enters or leaves a building. The first component is motivated by a temperature difference between inside and outside environments. The value for the air exchange rate increases linearly as temperature outside increases.
The second component is driven by
differences between internal and external absolute humidity. Absolute humidity is quantified as the number of grains of humidity per pound of air and is highly dependent on air temperature.

It is interesting to note that the infiltration rate for a pressurized building is zero. Although this may be a costly process, pressurization could be a valid alternative for eliminating heat transfer due to air exchange.

Heat loss due to conduction and radiation involves a temperature difference, information about the geometry of the building, and data regarding the characteristics of the material that the building is made of. In addition, both conduction and radiation require separate calculations for energy losses for each component of the building (walls, floor, roof, etc.).

Heat that is emitted from the exterior surface of the building to the sky is considered to be radiative. Since radiation is a surface phenomenon that originates from the emission of matter, both the surface area and the emissivity of the material must be obtained for each component of the building. Once radiative heat losses of all components are calculated, they are added togethe to get the total energy loss due to radiation.

Conduction is the transmission of heat through a medium of specific characteristics. Again, in analyzing a building that medium may be a wall, roof, window or floor. Each of these components, in turn, will most likely be a composite of many different materials. All materials are assigned a resistance (R) value which takes into account the thickness of the material and the ease by which heat passes through it. R values were derived by experimentation. Conductive heat loss is inversely related to R. Hence, the greater the insulating properties of the material, the larger the R value, and the less heat transmitted through the material. For each component of the building, all the materials from which it is composed are taken into account by adding up their R values. Again, surface area and temperature gradients come into play as key aspects of this mode of heat loss. For conduction, interior and exterior temperatures produce the necessary temperature gradient.

When heat fluxes of air infiltration, conduction, and radiation are calculated, they are added up to yield the total energy loss of the building. From this information an appropriate heating system can be designed and installed, or a building can be revised so as to cut down the amount of heat loss.