A Low-Emissivity Coating That Really Works

To say that there is a lot of hype about insulating paints and radiant barrier coatings is an understatement. The Internet is rife with claims of paints that dramatically reduce heat transfer—usually based on some technological magic spun off from NASA. While these products may have some relevance in the extreme conditions of outer space, manufacturers of paints containing “ceramic beads” or “sodium borosilicate microspheres” are making claims that defy the laws of physics—and independent test results—when they claim they can save significant energy in buildings. Well-engineered coatings with metallic particles can reduce radiant heat transfer, however. LO/MIT-II from SOLEC–Solar Energy Corporation has a long track record of success and is, at least for now, the only such product made with a water-based, low-volatile-organic-compound (VOC) formulation.

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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.

Of the other three products, one is LO/MIT-I, also from SOLEC, which has similar radiant-control performance to LO/MIT-II but relies on the hazardous solvent xylene, leading to overall VOC levels of 660 grams per liter. This solvent-based formulation is slightly cheaper than LO/MIT-II and is used as an exterior coating on metal structures, according to SOLEC president and CEO Robert Aresty. The other two products with high VOC levels but with emittance below the ASTM threshold are Radiance e-0.25 from BASF and HeatBloc-75 from STS Coatings. These two products have similar characteristics—HeatBloc-75 was “reverse engineered” from Radiance, according to company owner Cayce Kovacs, though she adds, “We never manufactured it from the same formula as them.” STS is working on a water-based version of HeatBloc-75 and expects to make it available during 2008. A fifth product, Sherwin-Williams’ E-Barrier, has thermal emittance slightly above the ASTM threshold and moderate VOC levels. RIMA’s test results for all these products were consistent with the manufacturers’ claims. In Texas, where there is an established network of radiation control coating companies, the installed cost for these products ranges from $0.25– $0.40/ft2 ($2.70–$4.30/m2) in new construction, and $0.40 to $0.90/ft2 ($4.30–$9.70/m2) in retrofits.

Radiant Control Coatings

*Per RIMA 2006 testing

**Does not meet the ASTM definition of an IRCCS; company literature claims a value of 0.29 to 0.32, depending on the substrate
Unlike these metallic coatings, all the rest of the products tested—those with ceramic-bead technology—created high emittance surfaces. Yarbrough notes that painting a high-temperature metal pipe with these paints would most likely increase heat loss by increasing the pipe’s surface emittance (even while making the pipe feel cooler to the touch). The manufacturers of most of these products don’t make any specific emittance claims.

Some of these manufacturers do publish insulation values, however—in one case as low as U-0.017 W/m°K, which translates to an R-value in inch-pound units of 8.5 per inch. Even if that claim is to be believed, at the recommended 6 mils of thickness it provides a negligible R-value of 0.05. Researchers at Oak Ridge National Laboratory did a side-by-side test of two exterior roof coatings, one loaded with those beads and one a standard latex paint. “We saw no difference between them,” said Andre Desjarlais of Oak Ridge. “All the benefit was due to color and reflectance.” Periodically, companies get in trouble for making spurious claims, as in a 2002 case in which Kryton Coatings signed a consent decree regarding unsubstantiated claims that its Multi-Gard coating system provides R-20 in insulation value. “You beat one down, and others pop up,” complained Desjarlais.

While no coating can provide a meaningful R-value, LO/MIT-II and its peers can reduce heat transfer by increasing surface reflectance and reducing emittance. These coatings have a metallic sheen that limits their applicability to places where appearance isn’t a concern. Other products offer similar benefits, to a lesser degree, in normal-looking paint by reflecting only invisible, infrared radiation. These can be used on exterior walls to reduce heat gain when adding insulation isn’t feasible. Oak Ridge tests of Coolwall from Tex-Cote found the potential for a 10% reduction in wall heat gain, according to Desjarlais.

– Nadav Malin