New mat cools buildings by letting their roofs sweat


October 5, 2012

A PNIPAM mat, shown on the right, keeps a model house cooler than a mat made from conventional polymer, shown on the left (Photo: Rotzetter ACC / Advanced Materials)

A PNIPAM mat, shown on the right, keeps a model house cooler than a mat made from conventional polymer, shown on the left (Photo: Rotzetter ACC / Advanced Materials)

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We're used to the thought of humans sweating to cool down, but what about buildings? Researchers at ETH Zurich have applied the biological cooling mechanism to the task of keeping a building cool, and in the process have hit upon a novel and inexpensive method of cooling houses which could prove useful for homes in both developed and emerging nations.

Led by Wendelin Stark, professor at ETH Zurich's Institute for Chemical and Bio Engineering, the researchers have developed a mat designed to be placed atop roofs. If it should rain, the mat soaks up rainwater like a sponge. When direct sunlight subsequently warms the mat to reach its internal temperature threshold, the collected rainwater “sweats” from the mat's surface and is turned into vapor, cooling the building below.

The mats are constructed from a special polymer known as Poly(N-isopropylacrylamide) – or PNIPAM – which features a water-permeable membrane that allows it to soak up and retain water. However, once the PNIPAM’s internal temperature reaches 32 degrees Celsius (89.6ºF), it begins to shrink and adopt hydrophobic properties, forcing the water back out through the same membrane whence it came.

The research was carried out to scale on small model railway houses (Photo: Rotzetter ACC / Advanced Materials)

So far, the researchers have tested the new mat on a small scale, covering the roofs of model railway buildings with 5 mm (0.19 inch) thick mats, while using a lamp to provide "sunshine." The team found that a model house fitted with the new mat remained significantly cooler than a model house sporting conventional polymer.

According to calculations by the ETH Zurich researchers, even a thin mat could potentially save up to 60 percent of the energy typically expended by air conditioning during a day of strong July sunshine. The team further posits that the sweating mat could be ideal for use in developing and emerging countries located in warm regions of the world, as it should prove very inexpensive to produce.

However, Stark's team also cautions that work still remains to be done to ensure the technology works as effectively on a larger scale – one question which still needs to be answered is whether or not the mat is frost-proof, for example.

The research is not patented, so we may well see a company take up the challenge and bring a version of the sweating mat to market.

Source: ETH Zurich via PopSci

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Adam Williams Adam scours the globe from his home in North Wales in order to bring the best of innovative architecture and sustainable design to the pages of Gizmag. Most of his spare time is spent dabbling in music, tinkering with old Macintosh computers and trying to keep his even older VW bus on the road. All articles by Adam Williams

Interesting for sure. But as you probably know, a gallon of water weighs almost nine pounds. So the roof structure would need to be upgraded to carry the additional load(s).

Ralph Spurlock

Yes, I thought too that weight would be a significant issue.

This is a new twist on the 'Coolgardie Safe' which was common before electric refrigerators, and the old hesian water-filled bag that we used to hang on the front of farm vehicles to keep the water cool.

Martin Hone

An Imperial gallon of water weighs almost exactly 10 lbs. Not a coincidence. It consists of 160 oz. And one liquid oz. of water weighs almost exactly 1 oz. Also not coincidence.

Brian Hall

for this to work it needs to be where there is..

a. a lot of rain (or a supply of wastable water) to get the thing wet, and also

b. low humidity so the water can evaporate

where would that be, exactly? a river in a desert?



I'm working on designing a viable building system for northern Nicaragua. The most used roofing system here is corrugated metal. A sprinkler or mister mounted at the ridge would probably work as well as this product, without the added weight and cost of heavy framing. Also, how would one attach the membrane to the roof frame? Would it evaporate both inside and out? Cost? even window screen is ten times more expensive here than in the U.S. I had to give up on latex concrete with window screen reinforcement. There is no recyclable paint, so, cost per square meter was over $3.00 just for screen and paint. Next attempt will be ferro cement.

Richard Cobbs

American gallons are 8 lbs but that isn't much as roofs are designed for far more. Walking on it requires 300lbs.sq' for example not to mention snow loads.

This isn't that new and could be done by using various soil water retainers or other means like felt used for 3000 yrs at least!!!


@ Ralph Or if you've gone metric (like almost everywhere else in the world) 1 litre of water weighs exactly 1.00 Kg 8^)


Given the weight and possible mold issues here, wouldn't it make more sense to capture the water then use drip system to the let the building sweat. If it must be in the form of a coating, make it much thinner and add water as it evaporates.

Robert Harvey-Kinsey

I think you would be better off insulating the roof and where appropriate use a swamp cooler. In places with lots of rain you can use the runoff from the roof to power a water wheel to power what ever device you choose.


The massive electricity demand in the summer in warm places like Texas comes from air conditioning units cooling bad insulated homes. Similar problems are in Australia.

Then somebody came up with the idea of powering air conditioning units with solar panels ... of a bad insulated house.

Just insulate your freaking home.

Fretting Freddy the Ferret pressing the Fret
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