Search results
Results from the WOW.Com Content Network
A perfect roof would absorb no heat in the summer and lose no heat in the winter. To do this it would need a very high SRI to eliminate all radiative heat gains in summer and losses in winter. High SRI roofs act as a radiant barrier , providing a thermos-bottle effect.
Through-pass absorbers have the most surface area which enables relatively high conductive heat transfer rates, but significant pressure drop can require greater fan power, and deterioration of certain absorber material after many years of solar radiation exposure can additionally create problems with air quality and performance.
Although ordinary black paint has high solar absorption, it also has high thermal emissivity, and thus it is not a selective surface. Typical values for a selective surface might be 0.90 solar absorption and 0.10 thermal emissivity, but can range from 0.8/0.3 for paints on metal to 0.96/0.05 for commercial surfaces.
A typical unvented thermal storage wall consists of a south facing masonry or concrete wall with a dark, heat-absorbing material on the exterior surface and faced with a single or double layer of glass. High transmission glass maximizes solar gains to the mass wall.
Different roof materials absorb more or less heat. A higher roof albedo , or the whiter a roof, the higher its solar reflectance and heat emittance, which can reduce energy use and costs. Cool roofs combine high solar reflectance with high infrared emittance , thereby simultaneously reducing heat gain from the sun and increasing heat removal ...
The hot roof material then radiates its gained heat energy onto the cooler attic surfaces, including the air ducts and the attic floor. A radiant barrier reduces the radiant heat transfer from the underside of the roof to the other surfaces in the attic. Some studies show that radiant barriers can reduce cooling costs 5% to 10% when used in a ...
Hence heat becomes trapped and builds up in the air space between the high heat capacity thermal mass and the glazing that faces the sun. [15] Another phenomenon that plays a role in the Trombe wall's operation is the time lag caused by the heat capacity of the materials. Since Trombe walls are quite thick and made of high heat capacity ...
Radiant barriers prevent radiant heat transfer equally in both directions. However, heat flow to and from surfaces also occurs via convection, which in some geometries is different in different directions. Reflective aluminum foil is the most common material used as a radiant barrier. It has no significant mass to absorb and retain heat.