Surface Resistance^
With heated building, there is a temperature difference between outdoor air and the exterior wall
surface, and similarly, between room air and the interior wall surfaces. The difference in temperature is
due to a thin film of air that adheres to the wall. The rate of heat flow between the wall’s surface and
adjacent air is called the surface air film conductance, expressed as f1 (inside air film) and f0 (outside air
film).
Any air movement (wind) against the skin of a building strips away this warmed air film and replaces I
with cooler air, thus increasing the rate of heat loss from the building. Therefore, air movement
decreases the surface resistance. The Table below list values of f and R for natural convection (no wind)
and for wind velocities of 5 to 30 mph. When calculating winter heat loss, use the R value for a 15 mph.
wind unless the building is well protected from winter winds.
SURFACE FILM CONDUCTANCE AND RESISTANCE FOR VERTICAL SURFACES
Wind velocity
(mph)
Surface Film Conductance
(f)
Surface Film Resistance *
(R)
0 (natural convection)
1.46
0.68
5
3.20
0.31
71/2
4.00
0.25
10
4.60
0.22
15
6.00
0.17
20
7.30
0.14
25
8.60
0.12
30
10.00
0.10
NOTE: *R =1/f; units are hr.-sq. ft.-˚F/Btu.
SOURCE: Clifford, Strock and Richard L. Koral, Handbook of Air Conditioning, Heating and Ventilating
^Mazria, Edward, The Passive Solar Energy Book, Expanded Professional Edition