| Radiant-Barrier.Org |
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A radiant barrier is a thin layer of aluminum film 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 attic, wall and floor radiant barriers include both dollar savings and increased comfort. The largest benefit of using a radiant barrier is reduced air-conditioning costs in warm climates.
Radiant barriers aren`t new. They have been in use since the 1960s and in common household use since the late 1970s. All new buildings should incorporate a radiant barrier as part of the total insulation system. Radiant barriers are an absolute must in warmer climates. They have become increasingly popular because tests by the Florida Solar Energy Center (FSEC) and Oak Ridge National Laboratories throughout the 1980s and 1990s show that they work, and also because manufacturers have improved the quality of radiant barrier materials making them easy and affordable to install.
While other types of insulation are made to resist or impede the flow of warm air, a 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 radiant barrier can save as much as 15 percent on air conditioning energy costs.
Radiant barrier installed under the roof trusses
Without a radiant barrier, when the roof gets very hot (as in summer), it radiates solar-generated heat down into the attic insulation. 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 barrier 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 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, 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 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 radiant barriers in cooler climates by keeping radiant heat inside the house. Adding a 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.
Garage without a radiant barrier
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| Aluminum Foil is the Best Radiant Barrier- The Physics of Foil |
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. 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 aluminum foil facing an airspace like an attic, wall cavity, or crawlspace creates a radiant heat barrier. Aluminum foil reflects back 97% of radiant energy that strikes it.
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| Types of Radiant Barriers |
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The most common type of radiant barrier material is one or two layers of aluminum foil with a backing material like polyolefin. The strength of the backing material is important since un-reinforced foil will tear easily.
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. Foil faced roof sheathing is also available to provide a combined decking and radiant barrier.
Aluminum foil radiant barrier comes in rolls
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| Benefits of using an Aluminum Foil Radiant Barrier |
- 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-19 to R-13).
- 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
- Moisture-proof - will not allow moisture to pass through in either direction
- 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
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| Installing a Radiant Barrier |
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General notes
A radiant barrier must face an airspace like the attic or crawlspace in order to work. You must maintain a 3/4-inch or greater airspace.
If you are using the 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.
You do not have to install the aluminum foil with the shiny side up. Most products have one shiny side and one dull side. This finish look is created in rolling mills during the process of making the aluminum sheeting. 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 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.
Places to install a radiant barrier
- Roof - There are two options for 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.
- Attic - For new construction, the attic radiant barrier can go either under the rafters, under the trusses or under the roof decking. For retrofit applications, the 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.
- 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. Aluminum foil radiant barriers are also vapor barriers 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)
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| Conclusion |
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Radiant barriers will not replace other forms of insulation. Because heat is also transferred by conduction, there will always be a need for materials like fiberglass, cellulose and foam. But because radiant energy is the dominant form of heat transfer in a building, the addition of a radiant barrier will be a very valuable building improvement for both comfort and energy savings, especially if you live in a hot climate.
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| References |
LaRue, Jim, and Ken Collier. "Radiant barrier insulation; a money saver for hot-climate homes. " The Family Handyman. 47.n5 (May 1997): 102(2)
Miller, Stephani L. "Silver lining: radiant barriers are an effective but little-known tool to reduce heat gain and loss. (Trends). " Building Products. 14.3 (May-June 2003): 41(1)
Melody, Ingrid (with revisions by Fairey, P. and Beal, D.) "Radiant Barriers: A Question and Answer Primer." Florida Solar Energy Center. 2005
McKnight, Diane(Revised), "Radiant Barrier Attic Fact Sheet"
Oak Ridge National Laboratory. June, 2001
Parker, D.S., J. R. Sherwin and M. T. Anello, "FPC Residential Monitoring Project: New Technology Development - Radiant Barrier Pilot Project", Contract Report FSEC-CR-1231-01, Florida Solar Energy Center, Cocoa, Florida. January 2001.
"Understanding and using Reflective Insulation, Radiant Barriers and Radiation Control Coatings",
Reflective Insulation Manufacturer`s Association. May 2002
Barr, Jodie, "Insulate with Reflective Insulation", www.building-insulation.com, 2007
Barr, Jodie, "Consumer Information on Reflective Insulation", www.reflective-insulation.org, 2007
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