Stop the Summer's Heat Radiant Barrier Tom Lane c. 1992 Tom Lane As if by magic, the magic of Physics, radiant barriers stop the summer's heat from invading your home. Radiant barriers are very cost-effective in hot climates because they sharply reduce cooling loads. In solar-powered homes, radiant barriers offer a low cost, low energy impact, solution to summertime overheating. Radiant barriers are incredibly more effective than attic fans and other passive "beat-the-heat" schemes. How Heat Moves There are three ways that heat travels: conduction, convection, and radiation. Heat transfers by conduction if the materials involved are physically touching. Convection is the transfer of heat via the movement of air or other gases. Radiation is the way heat travels through space, just as the sun's heat reaches us through 93 million miles of vacuous space. What is a radiant barrier? Radiant barriers prevent heat transfer because they will not absorb or transfer infra red heat energy. A radiant barrier reflects heat from its surface. There are two types of radiant barriers used in home construction, aluminum foil and LO/MIT-1 (a spray-on radiant barrier). The surface of any radiant barrier has what is called low "emissivity". This means that the surface is very sluggish at absorbing and transmitting radiant heat. A single low emissivity coating can reduce thermal radiation by as much as 95%. In new homes, construction grade aluminum foil is stapled to the underside of the roof deck. Or a spray-on radiant barrier is applied to the underside of the roof. This low emissivity coating prevents the radiant heat of the sun from penetrating the roof and heating the home. It is like putting your home under a shade tree! INSERT DIAGRAM Are radiant barriers the same as insulation? No, they are not. Insulation is a material the resists (slows down) convective and conductive heat transfer. Radiant barriers resist only the flow of heat by radiation, and do nothing to prevent heat transfer via convection or conduction. A well made home uses both insulation and radiant barriers. Insulation keeps the heat inside during the winter, and the radiant barrier keeps it out during the summer. How does the radiant barrier work in a home? The radiant barrier reflects heat from the roof instead of allowing this heat to penetrate the attic, and eventually the home's living spaces. During the summer, this means a more comfortable home and lower utility bills. How will a radiant barrier affect my cooling bills? In hot climates, radiant barriers can cut air conditioning bills by 8% to 20%. Radiant barriers can eliminate the need of air conditioning in the late spring and early fall months. In addition to saving power, the radiant barrier reduces the operating time of cooling equipment. This means longer air conditioner life, and lower maintenance bills. Do radiant barriers work in the winter? Yes, although the radiant barrier is much better at keeping out summertime heat. Heating bills drop only about 3% to 7% when a radiant barrier is installed. Most winter heat loss occurs through conduction and convection. Since radiant barriers are effective only against radiant heat, they do little to keep in heat during the winter. Do radiant barriers work in a ventilated attic? Yes, and they increase the effectiveness of the attic fan. Most ventilated attics do not move enough air through the attic to dissipate the roof's heat. With a radiant barrier, less heat enters the attic and the attic ventilation system is more effective. Will a radiant barrier in my attic bake the shingles? No. According to the Florida Solar Energy Center (report number FSEC- CR-GS-82), using a radiant barrier only increase shingle temperature by 2¡ to 4¡F. This rise in temperature is insignificant because the shingles are already between 130¡ and 170¡ F. Have government agencies promoted the use of radiant barriers? Foil and spray-on radiant barriers have been throughly tested by the Sate Energy Center at Cape Canaveral, Florida. The Federal Government has given GSA approval numbers for foil radiant barriers, and for LO/MIT-1 a spray-on radiant barrier (GSA# TFTC 88-CK-N115-01). What is the difference between aluminum foil and spray-on radiant barriers? The method of application is the main difference between foil and spray-on barriers. The spray-on barrier is about 20% less effective as a heat stopper than is the foil barrier. The spray-on radiant barrier usually costs less because it is much easier to install. How do you install a foil radiant barrier? Construction grade aluminum foil comes in 24 inch to 50 inch wide rolls that are 500 feet long. The foil usually has a mylar or kraft paper backing on one side containing a web of fiberglass strands to prevent tearing. The material cost is around 10 to 15 cents per square foot. In new construction, the foil is stapled to the plywood or composite board, then trimmed with a knife. The roof decking is then nailed to the joists with the shiny side of the foil facing the attic cavity. The foil can also be stapled to the inside the roof after the roof is already in place. Stop the radiant barrier a few feet below the attic's ridge vents. If you are doing the job yourself, then get some help from friends. Have a radiant barrier party. Buy several staple guns, a couple of boxes of 1/4 inch staples, a couple of 500 foot rolls of foil, and several 2 inch by 12 inch by 10 foot boards. These boards will allow you to walk in the attic without squashing the insulation or damaging the ceiling of the room below you. Don't have a radiant barrier party during the summer, you will fry and die in the attic along with the folks who used to be your friends. Do it on a cool overcast day. Apply the foil to the undersides of the roof decking. Do not apply the foil over the insulation on the attic's floor. Covering this insulation with the barrier will cause moisture to build up in the insulation, and still allow heat buildup in the attic. How do you install a spray-on radiant barrier? In new construction, the radiant barrier (like LO/MIT-1) is sprayed on the underside of the roof before it is sealed from exterior ventilation. It must be applied before the attic is sealed because LO/MIT-1 uses xylene as a solvent. Xylene, a common paint solvent, is too toxic to be sprayed by humans in enclosed areas. Typically a single man with an airless sprayer (#613 Graco tip) can cover 1,200 square feet in an hour and use about five gallons of LO/MIT-1. LO/MIT-1 can also be brushed or rolled on like a paint. Since thicker coatings have no benefit, it is best to spray LO/MIT-1 onto the surface. Material cost of LO/MIT-1 will be about 11 to 15 cents per square foot, with installation costing about 5 cents per square foot. If the house is already constructed and sealed, then LO/MIT-1 must be applied to the exterior of the roof. LO/MIT-1 can be sprayed on top of shingle, metal, or tar/gravel roofs. LO/MIT-1 can also be sprayed on east walls, west walls, and garage doors. LO/MIT-1 is only available in an architectural grey color. Where can I get more info on radiant barriers? Call the publication department of the Florida Solar Energy Center at 407- 783-0300 and ask for the following three free publications: Radiant Energy Transfer and Radiant Barriers in Buildings (DN-6), Designing and Installing Radiant Systems (DN-7), and Dealing with Heat and Humidity in Homes (EN-10). Where can I get radiant barrier materials? INSERT DB LIST Access Author: Tom Lane, Energy Conservation Services, 4110-15 SW 34th St., Suite 15, Gainsville, FL 32608 ¥ 904-373-3220 ¥ FAX 904-338-0056