Understanding Solar Radiation in Buildings - Simplified Physics
This entry was posted on February 9, 2016.
In summer, considerable radiation from the sun is absorbed by buildings, because the outer surfaces of most roofs and walls have high absorptivity.
Much of this radiant energy absorbed from the outside turns into heat, which flows by conduction through the solid material to the cooler inside surfaces of the roof and wall. The radiation inward from those inside surfaces, across buildings spaces, of invisible infra-red heat rays, is considerable, because these surfaces usually have over 90% emissivity.
Black, non-metallic surfaces such as asphalt, slate, paint, paper, etc. are poor insulators against heat rays in either case, outside or inside; with 85% to 98% absorptivity for solar radiation, and 90% to 98% in enclosed spaces.
Aluminum paint or gilt paint as it is formerly known, while it lasts, has a radiant heat absorption and emissivity of 40% to 65% in enclosed spaces. It performs better outdoors, with only 30% to 50% absorption for direct solar radiation. (1955 ASHAE Guide, page 95)
White paint, whitewash, white tile, white brick and plaster perform well outdoors, with absorptivity for solar radiation of only 30% to 50%. But they perform very poorly in enclosed spaces, no better than black paint, with an absorptivity and emissivity of 85% to 95%. (1955 ASHAE Guide, page 95)
Brick of any color, and concrete, in enclosed spaces, have an absorptivity and emissivity of 85% to 95%. But for solar radiation outdoors, red brick and concrete have an absorptivity of 65% to 80%; while yellow and buff brick have 50% to 70% absorptivity.
One reason for this difference between indoor and outdoor absorptivity and emissivity is that in addition to the infra-red heat ray rays of frequencies or wave lengths corresponding to those found at ordinary building air space temperatures; the sun emits heat rays of frequencies or wave lengths corresponding to a broad range of other temperatures. The sun also emits other rays, visible and invisible. All of these rays, with heat rays predominating, engender a certain amount of heat when they strike a surface and are absorbed, for example by the outer surface of the roof or wall of a building.
There is also sky or diffuse radiation from the surrounding atmosphere, also from buildings, trees, etc.
Enclosed building spaces are usually without light. Everything is dark and colorless. Radiation between surfaces in such space is limited largely to invisible, infra-red heat rays.