In 2007, Oak Ridge built a roof with phase-change material between foil layers; attic temperatures fell about 22°F, with utility savings estimated at 8%

As the summer season approaches and heat starts rising all over the US, air conditioners have to work overtime, causing electric bills to go up. In several cases, a fair share of the load on your AC is silently caused by one of the most overlooked parts of the house: the attic. Positioned right under the roof, it absorbs the heat coming directly from the sun and is rarely insulated enough to resist it from reaching the interiors. It may be just another piece of mundane infrastructure, and yet, it plays quite an important role in how much cooling a home requires.

Engineers at the Oak Ridge National Laboratory (ORNL) in Tennessee, which is part of the Department of Energy, were looking to solve just this problem almost 20 years ago. According to a 2007 ORNL article, a prototype roof and attic roof system utilizing a phase change material between two layers of reflective aluminum foils reduced the temperature in the attic by 22 degrees Fahrenheit, potentially lowering the cost of utilities by 8%. This is not a marketing statement from a consumer product company, but the result of federally funded materials research conducted in a national laboratory. Yet almost two decades have passed, and this discovery remains obscure to the outside world.

What was actually happening inside that roof


The concept isn't inherently difficult to grasp, if you remove all the jargon. The principle of a phase change material is the same as that of an ice cube in a cooler: it absorbs heat and continues to absorb heat without causing the temperature of the surroundings to rise, and it releases heat back when it cools down at night. The material developed at ORNL by a group led by Bill Miller and Jan Kosny was an inorganic compound that avoided the corrosion, instability, and risk of fire problems that plagued previous phase-change materials in the 1970s and 1980s. During lab testing at ORNL's Buildings Technology Center, on a day with an outside temperature of 92°F, a conventional attic reached 127°F, while the ORNL system stayed at 105°F, according to ORNL's own 2007 announcement of the project.

Construction science usually takes its cues from the construction industry, which means that it works pretty slowly. Phase change roofing was not a success on the same level as solar panels or spray foam insulation. That's not because it inherently requires tearing off and rebuilding an existing roof, later ORNL field studies show PCM-foil assemblies have also been installed as retrofit layers over existing roof decking or asphalt shingles, and even as blown-in attic-floor insulation, without demolishing what's already there. The harder sell is that a PCM system is still a custom, multi-layer assembly built for a specific roof, not a single product an installer can simply staple on the way they would a plain radiant barrier, and in a construction industry that doesn't take the attic seriously to begin with, that added complexity has been enough to keep it out of mainstream roofing catalogs.

The research did not stop in 2007

The story did not end with the first prototype. Kosny and his co-authors continued to develop this idea in a subsequent 2014 peer-reviewed study titled “Thermal load mitigation and passive cooling in residential attics containing PCM-enhanced insulations,” published in the journal Solar Energy. This paper shows that PCM-embedded insulation assemblies in the attic of a southern U.S. climate building reduced peak-hour cooling loads by 25 to 35 percent and could delay the peak cooling demand by several hours, a significant benefit when peak cooling demands are highest during the afternoon. That 25–35% figure describes a narrower measure than the 2007 estimate, it's the drop in cooling load during the hottest hour of the day, not a projection of the whole month's utility bill, so it isn't directly comparable to the 8% overall savings estimate from the original prototype. The science, that is, passed the test of further investigation, but it was never put in mass-market roofing catalogs.

Why should this matter

A 2017 U.S. Energy Information Administration analysis shows that almost 9 out of 10 households in the U.S. have air conditioners and that cooling consumes about 18 percent of the typical household's annual electricity consumption, which continues to rise as summer temperatures rise. Attic performance has a major impact on that figure.

The ORNL roof wasn't touted as a miracle solution, but it's a reminder that some of the most promising solutions that are adjacent to climate may be dull, unflashy, as in materials science, and without a marketing budget. Nearly two decades on, the technology still hasn't made it into standard residential construction, a gap worth keeping in mind for anyone evaluating a home's attic or roofing options.