This is why radiant heat transfer can be limited by the use of radiant barriers, which have shiny foil surfaces. Dark, rough surfaces will generally absorb radiant heat and become warm, while smooth, shiny surfaces will reflect the radiation. The earth receives all its solar heat through radiation. Radiant heat transfer occurs when electromagnetic waves travel in a straight line through a vacuum or air between surfaces that are at different temperatures, heating any object in its path. What is radiant heat transfer and why are radiant barriers effective? Although the radiant barrier may be somewhat effective in retaining heat within a cold-climate home, it may also block winter radiant heat gain in the attic. Radiant barriers are most effective in blocking summer radiant heat gain and saving air-conditioning costs. Properties to look for are high reflectivity and very low emittance (the energy radiated from the material’s surface), which enable it to reflect the radiant heat energy and give off (emit) very little. Some radiant barriers have a reflective surface on both sides so either side can face the open air space. If a single reflective surface is used alone and faces an open space, such as an attic, it is called a radiant barrier (sometimes called radiant heat barrier). Reflective insulation placed in walls or on the attic floor must be perforated to allow water vapor to pass through it. Reflective systems are usually located between roof rafters, floor joists, or wall studs. The resistance to heat flow depends on the direction of heat flow with this type of insulation most effective in reducing downward heat flow and requiring an air space next to the reflective side.
Reflective insulation systems are usually made from aluminum foils with a variety of backings such as roof sheathing, kraft paper, plastic film, cardboard, etc.
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