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