A Low-Emissivity Coating That Really Works

Radiant-heat control products can reduce heat transfer in attics and in certain mechanical system applications. To be effective, however, they must produce a low-emittance surface, like the low-emissivity coatings on high-performance windows, and the best way to get that low emittance is with a low-emissivity foil product. Foils come in rolls adhered to plastic films for strength or attached to the underside of roof deckings. The best of them have emissivities as low as 0.03 when they are new—as they get dirty and scuffed their performance declines.

There are no paint-on or spray-on coatings that perform as well as the foil products, but for applications where it just isn’t feasible to roll out a shiny film, a coating may be the best option. ASTM Standard C 1321 defines an “interior radiation control coating system” (IRCCS) based on a measured thermal emittance no higher than 0.25. In 2006 the Reflective Insulation Manufacturers Association (RIMA), which represents suppliers of both foil and coating products, hired insulation specialist David Yarbrough, Ph.D., P.E., of R&D Services to determine the emittance of a range of coatings on the market. Based on independent testing of 17 products from ten manufacturers, Yarbrough found four that met the ASTM definition of an IRCCS. Three of these four are solvent-based, making them high in VOC emissions. LO/MIT-II (the name is a contraction of “low emittance”) is the exception, with a VOC level of 170 grams per liter.