C haptEr 9 design Environments and systems
314
cold outside environment, and vice versa during the summer. Transfer through a building surface depends on convection on the inner
side of a surface, conductance through the surface, and convection
on the outer side of surface. Walls or roofs in a building typically
do not consist of just one layer; they usually have multiple layers,
with each of these layers having a different thickness and a different
thermal conductivity. Instead of considering all these layers separately, it is common practice to use just one value that will take into
account all of them. This single value, which combines convective
heat transfer at both sides of the wall and the conductance through
each layer, is called the overall heat transfer coefficient, or U-value. It
can be calculated and it has units of W/(m
2 K). The U-value is not a
physical measure; it is used for convenience and easier calculations
of heat transfers. The bigger the U-value, the larger the amount of
heat that is transferred. If the U-value for a wall is known, it is easy
to calculate the amount of heat loss or heat gain by multiplying the
U-value by the temperature difference and by the area of the wall.
Thermal resistance, or R-value, is the inverse of the U-value (units are
m
2
K/W). The R-value tells us how good are the insulating properties
of an overall building assembly. Hence, higher R-values mean better
insulating properties.
Normal design practice generally includes a determination of
heating and cooling loads from external or internal sources, an
exact description of the geometry of the building or space as well
as associated material distributions, and the subsequent development or use of thermal modeling approaches that analytically
predict the characteristics of the thermal environment expected
to be present. Design parameters are varied to achieve prespecified temperature and relative humidity levels and other criteria
related to human comfort. Some simple kinds of heat-transfer
characteristics can be done by hand calculation—for example, the
conductive heat loss through a wall can be found by simple considerations involving the temperature differential present among
wall faces, the thermal conductivity coefficient of the wall material, and the thickness of the wall—but most analysis algorithms
that have the robustness and breadth needed to completely
predict the thermal characteristics of an environment are based
on extensive energy balance and thermal modeling approaches
that are quite involved and hence normally done within a computer environment.
Thermal loads may come from many sources. For buildings, the
external environment provides a primary load source. Obviously,
designing a thermal environment in a building in a hot-humid
314
cold outside environment, and vice versa during the summer. Transfer through a building surface depends on convection on the inner
side of a surface, conductance through the surface, and convection
on the outer side of surface. Walls or roofs in a building typically
do not consist of just one layer; they usually have multiple layers,
with each of these layers having a different thickness and a different
thermal conductivity. Instead of considering all these layers separately, it is common practice to use just one value that will take into
account all of them. This single value, which combines convective
heat transfer at both sides of the wall and the conductance through
each layer, is called the overall heat transfer coefficient, or U-value. It
can be calculated and it has units of W/(m
2 K). The U-value is not a
physical measure; it is used for convenience and easier calculations
of heat transfers. The bigger the U-value, the larger the amount of
heat that is transferred. If the U-value for a wall is known, it is easy
to calculate the amount of heat loss or heat gain by multiplying the
U-value by the temperature difference and by the area of the wall.
Thermal resistance, or R-value, is the inverse of the U-value (units are
m
2
K/W). The R-value tells us how good are the insulating properties
of an overall building assembly. Hence, higher R-values mean better
insulating properties.
Normal design practice generally includes a determination of
heating and cooling loads from external or internal sources, an
exact description of the geometry of the building or space as well
as associated material distributions, and the subsequent development or use of thermal modeling approaches that analytically
predict the characteristics of the thermal environment expected
to be present. Design parameters are varied to achieve prespecified temperature and relative humidity levels and other criteria
related to human comfort. Some simple kinds of heat-transfer
characteristics can be done by hand calculation—for example, the
conductive heat loss through a wall can be found by simple considerations involving the temperature differential present among
wall faces, the thermal conductivity coefficient of the wall material, and the thickness of the wall—but most analysis algorithms
that have the robustness and breadth needed to completely
predict the thermal characteristics of an environment are based
on extensive energy balance and thermal modeling approaches
that are quite involved and hence normally done within a computer environment.
Thermal loads may come from many sources. For buildings, the
external environment provides a primary load source. Obviously,
designing a thermal environment in a building in a hot-humid