RADIANT BARRIER
How They Work
Heat travels from a warm area to a cool area by a combination of conduction, convection, and radiation. Heat flows by conduction from a hotter location within a material or assembly to a colder location. Heat transfer by convection occurs when a liquid or gas is heated by a surface, becomes less dense, and rises (natural convection), or when a moving stream of air absorbs heat from a warmer surface (forced convection). Radiant heat travels in a straight line away from any surface and heats anything solid that absorbs the incident energy. Radiant heat transfer occurs because warmer surfaces emit more radiation than cooler surfaces.
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. A radiant barrier 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:
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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.
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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.
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The angle the incident radiation strikes the surface—a right angle (perpendicular) usually works best.
All radiant barriers must have a low emissivity (0.1 or less) and high reflectivity (0.9 or more). From one brand of radiant barrier to another, the reflectivity and emissivity are usually so similar that it makes little difference as far as thermal performance. (Most products have emissivities of 0.03–0.05, which generally corresponds to a reflectivity of 95%–97%.) Also, the greater the temperature difference between the sides of the radiant barrier material, the greater the benefits a radiant barrier can offer.
Radiant barriers are more effective in hot climates than in cool climates, especially when cooling air ducts are located in the attic. Some studies show that radiant barriers can lower cooling costs between 5%–10% when used in a warm, sunny climate. The reduced heat gain may even allow for a smaller air conditioning system. But in cool climates, it's usually more cost effective to install more than the minimum recommended level of insulation rather than a radiant barrier.
What Is A Radiant Barrier ?
Heat penetrates buildings in three ways :
Conduction , Convection , and Radiation . Radiation is the transfer of heat by electromagnetic waves known as thermal radiation . Thermal radiation is a form of invisible light that travels through the air from a warm to a cool surface . Surfaces like roofs heated by the sun , radiate or emit their heat through airspaces like attics, to cooler interior living spaces . A radiant barrier is a low -e surface placed in the path of this thermal radiation to block heat transfer . The "E" in low -e is also known as " emissitivity " , a measurement of how much heat is blocked. The most cost effective away to achieve this barrier is with an aluminum foil or spray coating . Only Gold or Silver works better .
The Impact of Radiant Energy on Your Home – And Energy Costs
Radiant energy is energy that comes in waves from the sun or comes from an inside source, like a furnace. On warm days, when the sun’s rays strike the roof, the roof absorbs heat, which increases temperatures in the attic. Eventually, this heat is transferred throughout your home – resulting in increased air conditioning costs. On cool days, when radiant energy comes from an inside source, it travels upward and escapes through the attic. But in either situation, energy costs are literally going through the roof.
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