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Chapter 3
Steady Heat Conduction
Special Topic: Heat Transfer Through the Walls and Roofs
3134C
The
R
value of a wall is the thermal resistance of the wall per unit surface area. It is the same as
the unit thermal resistance of the wall. It is the inverse of the
U
factor of the wall,
R
= 1/
U
.
3135C
The effective emissivity for a planeparallel air space is the “equivalent” emissivity of one surface
for use in the relation
that results in the same rate of radiation heat transfer
between the two surfaces across the air space.
It is determined from
)
(
4
1
4
2
effective
rad
T
T
A
Q
s
−
=
σε
&
11
1
1
2
εε
ε
effective
1
=+−
where
ε
1
and
ε
2
are the emissivities of the surfaces of the air space.
When the effective emissivity is
known, the radiation heat transfer through the air space is determined from the
relation above.
&
Q
rad
3136C
The unit thermal resistances (
R
value) of both 40mm and 90mm vertical air spaces are given to
be the same, which implies that more than doubling the thickness of air space in a wall has no effect on
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Resistance, Mass, Heat

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