Radiant barriers are installed in attics to reduce summer heat gain, which helps lower cooling costs.
The barriers consist of a highly reflective material that reflects (or more specifically, re-emits) radiant heat rather than absorbing it. They don’t, however, reduce heat conduction like thermal insulation materials.
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:
- 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.
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.