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Radiant Barriers

How Radiant Barrier Systems Work

A radiant barrier reduces heat transfer. Heat travels from a warm area to a cool area by conduction, convection, and radiation. Heat flows by conduction from a hotter material to a colder material when the two materials are in direct physical contact. Heat transfer by natural convection occurs when a liquid or gas is heated, becomes less dense, and rises. Thermal radiation, or radiant heat, travels in a straight line away from a hot surface and heats any object in its path.

When sunshine heats a roof, most of the heat conducts through the exterior roofing materials to the inside surface of the roof sheathing. Heat then transfers by radiation across the attic space to the next material, either the top of the attic insulation or the attic floor. A radiant barrier, properly installed in one of many locations between the roof surface and the attic floor, will significantly reduce radiant heat flow. Thermal insulation on the attic floor resists the flow of heat through the ceiling into the living space below. The rate at which insulation resists this flow determines the insulation's R-value. The amount of thermal insulation affects the potential radiant barrier energy savings. For example, installing a radiant barrier in an attic that already has high levels of insulation (R-30 or above) would result in much lower energy savings than an attic insulated at a low level (R-11 or less).

All radiant barriers use reflective foil that blocks radiant heat transfer. In an attic, a radiant barrier that faces an air space can block up to 95% of the heat radiating down from a hot roof. Only a single, thin, reflective surface is necessary to produce this reduction in radiant heat transfer. Additional layers of foil do little more to reduce the remaining radiant heat flow.

Conventional types of insulation consist of fibers or cells that trap air or contain a gas to retard heat conduction. These types of insulation reduce conductive and radiant heat transfer at a rate determined by their R-value. Radiant barriers reduce only radiant heat transfer. There is no current method for assigning an R-value to radiant barriers. The reduction in heat flow achieved by the installation of a radiant barrier depends on a number of factors, such as ventilation rates, ambient air temperatures, geographical location, amount of roof solar gains, and the amount of conventional insulation present.

Several factors effect the cost-effectiveness of installing a radiant barrier. You should examine the performance and cost savings of at least three potential insulation options: adding additional conventional insulation, installing a radiant barrier, and adding both conventional insulation and a radiant barrier.

Selection Criteria

First, you should determine if the radiant barrier will be effective in your climate. Radiant barriers tend to offer a much lower potential for energy savings in colder climates. The reasons for this will be discussed in a later section.

Although emittance, or emissivity, is the most important property of a radiant barrier, most barriers on the market today have the same emittance values (between 0.03 to 0.05). Therefore, you should consider other characteristics (strength, flammability, availability, and cost) before you choose a particular brand of radiant barrier.

Tensile strength, or resistance to tearing, is an important property of radiant barriers, especially for the "do-it-yourself" installation. The most effective way to test brand strength is to obtain samples and try to tear them by hand. Although some brands are nearly indestructible, others tear easily. A barrier that tears easily may rip at fastening points. The types that are least susceptible to tearing usually have a woven mesh between two sheets of foil. This mesh is often comprised of fiberglass thread. Be aware that not all mesh provides adequate strength. Loosely woven mesh tears more easily than tightly woven mesh. An alternative to mesh is the "bubble-pack" radiant barrier. This has a polyethylene air pack (typically found as packaging material) sandwiched between two layers of foil. It is almost as strong as a mesh radiant barrier.

You should also check the flammability rating of any radiant barrier you are considering. Although aluminum foil is not flammable, other materials that make up a radiant barrier may be. Plastics, untreated Kraft paper, and some adhesives burn. When you install a radiant barrier in the attic, choose one that has a Uniform Building Code (UBC) Class I, or National Fire Protection Association (NFPA) Class A, flammability rating.

Installation

You may retrofit radiant barriers or install them in new construction. There is a variety of installation options for radiant barriers. Traditionally, in new construction, you drape the radiant barrier, foil-face down, across the tops of the roof framing before applying the roof sheathing. The barrier droops between the supports, leaving a 1.5 to 2.0 inch air space between it and the roof sheathing. Some builders prefer to attach the reflective insulation directly onto the roof sheathing prior to installing the sheathing. Foil-faced plywood panels are now available; they may be used adjacent to an airspace to accomplish about the same result within the same installation time. The roof shingle industry is even looking into laminating radiant barrier material onto shingles. You may also install a radiant barrier on the underside of the roof supports. This is a popular retrofit technique. For single-sided radiant barriers, remember to orient the reflective face downward, towards the attic, to minimize the effect of dust accumulation. Studies have shown that dust accumulation on radiant barriers can significantly reduce their effectiveness. You can install a radiant barrier on top of attic floor insulation in both retrofits and new construction, however, this application is more susceptible to dust accumulation. Placing the barrier on top of attic floor insulation may also trap moisture in some locales.

In cold climates, a radiant barrier on the cold, or outer, side of the insulation increases the risk of moisture accumulation on the warm side of the barrier. Leakage of warm, moist air from the house interior can condense and accumulate as frost on the radiant barrier. When this frost melts, water may drain back into and possibly through the insulation. This may damage ceiling material below. Even perforated radiant barriers can trap moisture in cold climates. Because of this, many energy specialists, various energy organizations, and the Reflective Insulation Manufacturers Association recommend not applying the radiant barrier directly on top of the attic floor insulation in cold climates.

In all radiant barrier systems, the radiant barrier must be directly adjacent to an air space. When you use a single-sided radiant barrier, the reflective, or shiny, side of the radiant barrier should face downward. This reduces the dust accumulation on the shiny surface that causes a drop in efficiency. Many manufacturers produce double-faced radiant barriers, which have foil on both sides. Although this does not make the barrier significantly more effective, it does eliminate the facing question. Usually, double-faced barriers cost little more than single-faced barriers.

How Effective are Radiant Barriers?

As noted above, radiant barrier systems successfully reduce heat gain. During the summer, an attic radiant barrier, combined with existing R-19 attic insulation, may reduce heat gain through the ceiling from 16%-42%. For single-story houses, typically about 15%-25% of the summer cooling load is due to ceiling heat gain. The heat gain reduction from a radiant barrier installation will usually result in a total cooling load savings of 2%-10%-possibly as high as 15% in attics insulated to R-11 or less. Higher savings occur when retrofitting less efficient buildings. Buildings with little to no attic insulation and a high volume of attic ventilation typically provide the most dramatic energy savings from a radiant barrier. The hotter and sunnier the climate is, the more beneficial the radiant barrier installation becomes.

In new or remodel situations, installing a radiant barrier may reduce the cooling load sufficiently to allow installation of a smaller capacity air-conditioning system. In 1991, the U. S. Department of Energy (DOE) published the Radiant Barrier Attic Fact Sheet, which shows how to calculate the economics of radiant barriers and added ceiling insulation. It includes an Energy Savings Worksheet with an example.

You can also use recent utility bills to estimate annual radiant barrier savings. For each summer month you typically operate your air conditioner, subtract an amount equal to your minimum spring or fall bill (when no heating or cooling is needed). Add the savings for each of the summer months to obtain an annual cooling cost savings. Compare this annual cost savings to the expected cost of the radiant barrier installation to determine how many years it will take to pay for the installation.

The decision to install a radiant barrier system is not always based on a simple payback calculation. In cases where spaces adjacent to the attic overheat during the day, installing a radiant barrier system may be the most economical option. By lowering the room temperature a few degrees, you may avoid the expense of reconfiguring or installing an enlarged cooling system.

Because radiant barriers redirect radiant heat back through the roofing materials, shingle temperatures may increase between 1° to 10°F (17.2° to 12.2°C). This increase does not appear to exceed the roof shingle design criteria. The overall effect on roof life, if any, is not known.

If you live in a cold climate, installing a radiant barrier is not generally recommended. Radiant barriers are most effective in blocking summer radiant heat gain and saving air-conditioning costs. In cold climates, air- conditioning is usually a much lower priority than heating. Cold-climate homes also tend to have high attic insulation levels. Although the radiant barrier may be somewhat effective in retaining heat within a home, it may also block any winter radiant solar heat gain in the attic. At least one test in Minnesota with radiant barriers placed over R-19 attic floor insulation, found no significant reduction in heat loss. In Alberta, Canada, the net effect of the radiant barrier was only a 5% average reduction in total annual heat loss through the roof. This amounts to about a 1% savings in total annual energy consumption for heating.

For additional information contact:

Davis Energy Group (DEC)
123 C Street
Davis, CA 95616
Phone: (916) 753-1100; Fax: (916) 753-4125
Internet: (E-mail) info@fsec.ucs.edu; (World Wide Web) http://www.fsec.ucf.edu

Reflective Insulation Manufacturers Association (RIMA)
c/o Innovative Energy Corp.
Attn.: Robert Wadsworth, Membership Chairman
1213 West 145th Avenue
Crown Point, IN 46307
Phone: (800) 776-3645; Fax: (219) 662-7990
Internet: (E-mail) rima@mail.icongrp.com; (World Wide Web) http://www.rima.net/

EREC is operated by NCI Information Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of Energy. The statements contained herein are based on information known to EREC at the time of printing. No recommendations or endorsement of any product or service is implied if mentioned by EREC.

Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com

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