Say “insulation,” and the first thing that comes to mind is thick, bulky, wool-like materials designed to be stuffed into exterior walls and under roofs to retard the transfer of heat or cold into and out of a building.
But there is another category that almost acts like insulation: radiant barriers. Wafer-thin materials with no inherent insulating properties, radiant barriers block heat transfer through roofs and, some companies say, walls. They include reflective metallic foils that are used alone or bonded to other materials, as well as liquid coatings that are painted or sprayed onto interior surfaces.
Not every shiny metallic material or coating reflects heat, however. Only products that are engineered or formulated to repel infrared radiation and meet specific industry standards qualify as radiant barriers under current regulations. Radiant barriers must have an emittance rating of less than 0.1 to meet ASTM C1313 and C1371 specifications. Liquid interior radiation control coatings must have an emittance rating of less than 0.25 to meet the ASTM C1321 standard.
Radiant barriers have been used in home construction for years, mainly in the hotter Sun Belt states where heat-blocking materials are needed and can be most effective. Early adopters used jerry-rigged foils and even metal sheeting to deflect at least some of the heat. But not all of those materials performed as intended; some products — such as tin and galvanized steel sheeting tacked to the underside of roof rafters — transferred ambient heat into the building.
Eventually, the practice of using reflective metallic foils as heat barriers caught on, and manufacturers began delivering products. Some performed as advertised, but because the science of heat reflectivity was not well understood until recent research, unsupported and sometimes wildly inflated claims of effectiveness often obscured the value of reputable products.
Industry research is divided as to the value of radiant barriers. Independent studies by the Florida Solar Energy Center and Oak Ridge National Laboratory show that radiant barriers in high heat-zone areas can reduce air conditioning costs by as much as $150 per year, but savings decline steeply in regions with lower ambient temperatures.
Other research has demonstrated that radiant barriers can improve heating and air conditioning in all climate zones, and a recent ASHRAE study conducted by ORNL showed that adding a radiant barrier generally improves the heat-blocking performance of standard insulation systems.
Michael Morris is a carpenter, author and journalist covering the home-building and remodeling industry.
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RBS radiant barrier sheathing is an oriented strand board with a perforated aluminum foil bonded to the interior face to prevent moisture buildup in the roof deck. (woodbywy.com)
ThermaWrap uses the optimum balance of Tyvek weather barrier and adds a low-emissivity metallized surface. (dupont.com)
LiquidFoil attic barrier is a vapor-permeable reflective paint for under-roof surfaces. It blocks up to 84% of heat radiating into or out of a structure. (henry.com)
Solarbord OSB radiant barrier sheathing has a 3% emittance rating and 97% reflectivity, which can reduce a home’s air-conditioning requirements by up to ½-ton. (norbord.com)