A foil radiant barrier is composed of a thin layer of reflective low emissivity film. It is placed in a building airspace to block radiant heat transfer between a surface that will radiate heat (such as a hot roof) and a surface that will absorb heat (such as conventional attic floor insulation). The benefits of using a foil radiant barrier in the attic, floor, wall or garage include both dollar savings and increased comfort. The largest benefit of using a foil radiant barrier is reduced air-conditioning costs in warm climates.

While other types of insulation are made to resist or impede the flow of warm air, foil radiant barrier reflects back infra-red energy from the sun so it does not penetrate the building. It can also reflect back radiant heat inside the house so it does not escape. The concept is simple: each unit of radiant heat energy that is reflected away from your home in summer and each unit reflected back inside during winter means less operation of your air conditioning and heating systems, less wear and tear on your equipment, and less money you pay in utility costs. Studies show that an attic foil radiant barrier can save as much as 15 percent on air conditioning energy costs.

Without a foil radiant barrier, when the roof gets hot  in the summer, it radiates solar-generated heat down into the attic. Conventional types of attic insulation such as fiberglass, foam, and cellulose primarily reduce heat transfer by trapping warm air. These materials have high radiant heat transfer rates and are very good radiators of this absorbed and retained heat. As surfaces radiate infrared rays in all directions, the heat trapped in conventional insulation during the day will radiate down through the ceiling into the air-conditioned living space at night even if the night cools down. This means you will have to run the air-conditioning system even at night wasting energy. Using a radiant foil insulation will prevent this problem. An aluminum foil radiant barrier in the attic will block up to 97 percent of this heat from reaching the insulation and the air-conditioned living space below. 

A foil radiant barrier will cut air-conditioning costs by blocking a major portion of the downward heat gain into the building in summer. In the milder weather of spring and fall, foil radiant barriers can save additional cooling dollars. While outdoor air temperatures are comfortable much of the time, solar energy still heats up your roof, insulation, and ceiling to temperatures that can make you uncomfortably warm. An attic foil radiant barrier will stops almost all of this downward heat transfer so that you can stay comfortable without air conditioning during mild weather. Although not as significant, heating savings also accrue from the use of foil radiant barriers in cooler climates by keeping radiant heat inside the house. Adding a foil radiant barrier to un-insulated, unconditioned spaces such as porches, garages, barns, and sheds can expand the use of these spaces by making them more comfortable as well.

Heat flows from a hot or warm medium to a cold medium in three ways:

• By radiation (infra-red heat rays) from a warm surface to a cooler surface through an air space
• By conduction through solid or fluid materials resulting from direct contact
• By convection, which involves the physical movement of air - warm air rises

Heat moves through wall cavities, between roofs and attic floors or between floors and basements by a combination of radiation, conduction, and convection with radiation being the dominant method of heat transfer. Research shows that control of radiant heat transfer is the core of heating/cooling climate control.

Radiation is the transfer of heat (infra-red radiant energy) from a hot surface to a cold surface through air or a vacuum. Radiation is the dominant method of heat transfer in a building accounting for 65-85 percent of all heat transfer through walls, ceilings, attic and floors. Aluminum foil is an effective barrier against radiant heat transfer because it reflects back almost all of the infrared radiation striking its surface and emits very little of the heat conducted through it.

The thermal resistance of insulation is measured in R-value, or resistance to the movement of heated air by convection. The higher the R-value the more effective the insulation will be in resisting convective heat transfer. Prodex Total AD5 has a 15.67 R-value. R-value does not measure an material's ability to resist radiant heat. Mass insulation like fiberglass or foam board primarily slows convective heat transfer. In contrast, reflective low emissivity foil facing an airspace like an attic, wall cavity, or crawlspace creates a radiant heat barrier. A product with an emmisivity of .03 (Prodex Total Insulation) reflects back 97% of radiant energy that strikes it.

The most common type of foil radiant barrier material is one or two layers of low emissivity reflective surfaces with an inner substance. Radiant barriers (with no inner substance) are not sold on this website. For more on this subject visit: Comparing radiant barriers - The page includes photos of products, descriptions and recommendations.

Multi-layered foil products use a central layer of foam, plastic bubbles or fiberglass for additional insulating value. This reflective insulation is very cost effective as it gives you protection from both radiant and conductive heat in one.

• Energy Savings - Research conducted by the Florida Solar Energy Center (FSEC) has shown that the installation of a radiant barrier can have significant economic benefits. Attics account for approximately 22 percent of the heat gain in a typical Florida house. Installing an attic radiant barrier will reduce heat gain through the roof by about 40 percent. This equates to a savings of 8 to 12 percent on the annual electricity costs for air conditioning in a typical Florida home. The summer peak energy use savings was as much as 27%. This study also showed that the average peak reduction from the radiant barrier system was three times greater than that produced by added insulation (a change from R-13 to R-19).
• No significant mass to absorb and retain heat
• Very low emittance values "E-values" (typically 0.03 compared to 0.90 for most insulation) which significantly reduces heat transfer by radiation
• Eliminates condensation within the ceiling and walls when properly installed as a vapor barrier
• Nontoxic and non-carcinogenic - does not irritate the skin, eyes, or throat and contains no substances which will out-gas
• Radon retarder - will limit radon penetration through the floor

Places to install a "foil radiant barrier"

• Foil radiant barrier attic installation - For new construction, the foil attic radiant barrier can go either under the rafters, under the trusses or under the roof decking. For retrofit applications, the foil radiant barrier should go under the rafters. You can roll the barrier out over the attic floor or existing insulation if you live in a climate where dust will not settle on the barrier and you don't use the attic for storage. Part of the effectiveness of an attic radiant barrier is dependent on a well-ventilated attic.

• Radiant floor insulation install - Between your floor joists to control heat transfer.

• Foil Radiant barrier roof installation - There are two options for foil radiant barrier roof installation. Drape the radiant barrier over the tops of the rafters before the roof deck is installed or attach the barrier to the underside of the roof decking material prior to installation.

General notes

A foil radiant barrier must face an airspace like the attic or crawlspace in order to work. You must maintain a 1/2-inch or greater airspace.

If you are using the foil radiant barrier as a vapor barrier also, all seams and joints must be tightly taped. If not, in an area such as under the rafters, you don't need to get the joints airtight. This will not improve the performance as the barrier is meant to stop the penetration of radiation, not convective warm air flow.

Foil Radiant Barriers work by reflecting infra-red energy so the aluminum foil would work just the same if both sides were dull.

Dust collection on the reflective surface of the foil barrier will reduce the reflectance and effectiveness over time. Dust is very conductive and will allow the foil to absorb rather than reflect thermal radiation. If your insulation has foil only one side, put the foil side toward the airspace. In the attic it would be facing down from the rafters.

How does a foil radiant barrier work? During the summer months the heat from the sun penetrates the roof. If a radiant barrier is present, it will reflect that heat back out of the house, thereby helping to keep the house cool. In the winter, the barriers can reduce the amount of heat lost through the ceiling.

Foil Radiant barriers can be installed in both homes and businesses. In a home they are usually installed in an attic, either on the floor above the insulation, or on the roof rafters (better). The important thing about installation is that the radiant barrier's reflective surface faces an open air space, which is required for it to work properly. Also, if it is used on the attic floor, it must allow water vapor to pass through, which is important during the winter months to keep condensation from forming.

It has been shown that a foil radiant barrier effectively reduces cooling bills during the summer months. Several studies have been done in states with prolonged warm weather seasons, such as Florida and Tennessee. They have found that a foil radiant barrier in certain conditions can reduce cooling costs by 2 to 10 percent. The percent reduction for winter heating costs would be less, since radiant barriers are more effective in keeping heat out.

Another plus for foil radiant barriers is the cost of installation. Costs for any type of insulation can vary widely, depending on the type of material and where it is being installed. However, a foil radiant barrier is generally less expensive than traditional insulation. On average, radiant barrier can cost 25% less than traditional insulation. 

Retrofit attic installation of a radiant barrier

• Ceiling - Use a radiant barrier for cathedral ceilings where there is no attic space above the ceiling.
• Walls - The radiant barrier can be applied as whole house wrap. The benefits of using a radiant barrier for house wrap are multiplied by its properties as a vapor and moisture barrier.
• Floor - Use a radiant barrier in the floor above either an unheated basement or crawl space. Some foil radiant barriers are also vapor barriers (Prodex Total) that prevent ground moisture from migrating into the living space above. Laboratory experiments and computer modeling suggest that floor radiant barrier systems may exhibit R-values as high as R-7.5 to R-8.0 for reducing heat loss to basements and crawl spaces. (Source - Reflective Insulation Manufacturer's Association)

Because radiant energy is the dominant form of heat transfer in a building, the addition of a foil radiant barrier will be a very valuable building improvement for both comfort and energy savings, especially if you live in a hot climate.