When the sun heats a roof, it's primarily the sun's radiant energy that makes the roof hot. A large portion of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates its gained heat energy onto the cooler attic surfaces, including the air ducts and the attic floor. Radiant barrier / radiant foil barrier insulation reduces the radiant heat transfer from the underside of the roof to the other surfaces in the attic.
A radiant barrier's performance is determined by three factors:
- Emissivity (or emittance) – the ratio of the radiant energy (heat) leaving (being emitted by) a surface to that of a black body at the same temperature and with the same area. It's expressed as a number a between 0 and 1. The higher the number, the greater the emitted radiation.
- Reflectivity (or reflectance) – a measure of how much radiant heat is reflected by a material. It's also expressed as a number between 0 and 1 (sometimes it is given as a percentage between 0 and 100%). The higher the number, the greater the reflectivity.
- The angle the incident radiation strikes the surface—a right angle (perpendicular) usually works best.
Five key benefits of installing a radiant barrier
- Reduced heating and cooling costs (lower utility bills)
- Increased comfort levels
- Increases building's energy efficiency
- Keeps heat out during the summer
- Keeps heat in during the winter
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