Radiant barriers are installed in homes – usually in attics – primarily 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.
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.
Types of Radiant Barriers
Radiant barriers come in a variety of forms, including reflective foil, reflective metal roof shingles, reflective laminated roof sheathing, and even reflective chips, which can be applied over loose-fill insulation. The reflective material, usually aluminum, is applied to one or both sides of a number of substrate materials. Substrate materials include kraft paper, plastic films, cardboard, oriented strand board, and air infiltration barrier material. Some products are fiber-reinforced to increase the durability and ease of handling.
Also, radiant barriers—which don’t provide a significant amount of thermal insulation—can be combined with many types of insulation materials. These combinations are called reflective insulation systems. In these combinations, radiant barriers can also act as the insulation’s facing material.
A radiant barrier’s effectiveness depends on proper installation. Therefore, it’s best to have a certified installer do it.
If you want to install a radiant barrier yourself, obtain instructions and safety precautions from the manufacturer. Carefully study and follow these directions. You should also check your local building and fire codes.
Radiant barriers can be installed between the roof sheathing and attic floor insulation, in cavity walls, and around door openings, water heaters, and pipes. It’s easier to incorporate radiant barriers into a new home, but you can install them in an existing home too.
An installer typically drapes a rolled-foil radiant barrier foil-face down between the roof rafters to minimize dust accumulation on the reflective faces (double-faced radiant barriers are available). This is generally done just before the roof sheathing goes on if it’s not too windy, but it can also be done afterwards from inside the attic by stapling it to the bottom of the rafters.
When installing a foil-type barrier, it’s important to allow the material to “droop” between the attachment points to make at least a 1.0 inch (2.5 cm) air space between it and the bottom of the roof. This air space has mainly two functions: it creates an air channel for the soffit and ridge ventilation system to work more effectively, and it acts as a second reflector since there are two shiny sides (one facing up/ one facing down.)
Some builders also try to attach the radiant barrier directly onto the roof sheathing prior to their installation on the roof rafters. However, a more effective method is to simply buy foil-faced plywood or oriented strand board sheathing instead. There are also metal roof shingles that have a reflective underside. If you need roof shingles, these are a practical option, but the cost of this type of radiant barrier is considerably higher than other types.
A radiant barrier installed on top of attic floor insulation is more susceptible to dust accumulation. This less desirable installation method may also trap moisture in fiber insulation and is not cost effective in cold climates. A radiant barrier installed on the cold side of the insulation acts as a vapor barrier in the wrong location. When warm moisture carrying house air leaks into the attic in the winter, it may condense on the underside of the barrier. Even a perforated radiant barrier can trap moisture in cold climates since the water can freeze in the small holes and seal them. Because of these hazards, it’s strongly recommended that you NOT apply radiant barriers directly on top of the attic floor insulation.