This Sweating Rooftop Sounds Like a Terrible Idea

A team of researchers at ETH Zurich has developed a rooftop mat made of a five-millimeter-thick polymer that can absorb water when it rains. The material changes properties along with the temperature—the mat becomes hydrophobic as it warms up, and the water is expelled, extracting heat from the building in a process the researchers compare to human sweat. The idea is to cool the interior with less reliance on air conditioning. It's hard to imagine how this is possibly going to work.

The basic physics and thermodynamics of the plan are sound—water can transfer heat as it evaporates. But roofs, for the past several hundred years, have been designed to shed water. Absorbing it and just holding it there against the building exterior is, in conventional building practice, the last thing you want a roof to do.

Look, the team in Switzerland is surely a smart group of folks. And energy-saving revolutions in building practices are always exciting. Here are a few questions the team's announcement, and its coverage on PopSci, have not yet addressed:

• Frost. What happens when a moisture-saturated mat goes below a freezing temperature?
• Mold and mildew. Is there something preventing fungal growth beneath the mat, where it meets the next membrane?
• Indoor humidity. One of the primary functions of air conditioning is collect moisture and desiccate the air within the home. This system could lower indoor temperature, but can it move moisture out?
• Waterproofing. This is probably the most fundamental question. When you have a soaking mat just sitting on top of the house for an indefinite period of time, how do you prevent that water from eventually finding its way past the outer membrane and into the roof framing? This is why conventional shingles shed rain as fast as possible—before long, standing water just works it way inside.

This development is intriguing, but it raises the question of why we'd turn our back on an established set of standard building practices. With copious insulation, and drainage designed to move water away from the structure, a building can be pretty efficient—even if its rooftop can't sweat.

A special sweating polymer mat (right) cools a model house more effectively than a mat made of a conventional polymer (left, infrared image). (Photo: Rotzetter ACC et al. / Advanced Materials)