In Berkeley, we are fortunate to have such events as Science at the Theater, where Lawrence Berkeley National Lab researchers give talks on their work at the Berkeley Repertory Theatre. The lectures are free and get a pretty sizeable audience.

On Monday, October 11, I was in the audience as researchers from Lawrence Berkeley National Laboratory (and the beloved Art Rosenfeld) gave a presentation titled “Cool Roofs, Cool Cities.” The post below consists of Part 2 of my record of the presentation – Ronnen Levinson gives an overview of cool roofs. All portions are included in chronological order.

An ellipsis (…) indicates that I was not able to capture the words or thoughts skipped. The presentation is transcribed as accurately as possible – punctuation choices are mine. I also added any images.

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In North America, we have cool roofs, usually white roofs – and we have them on commercial buildings, because the roof can’t be seen from the street … In some other parts of the world, you might see white roofs on pitched surfaces, like in Bermuda … In North America, you might also see cool colored roofs … With a commercial roof, without much insulation, and with extended operating hours, might save about 15% on AC bill by adding a white roof …

What if you put white roofs on about 80% of commercial roofs in the United States? … We assume they get soiled … What we found is that each year you would save about $735 million dollars, and save the equivalent carbon of taking 1.2 million cars off the road … The lifetime energy savings has a present value of about $11 billion … and there is no extra cost to choosing a white roof – so this is free money. Everyone likes free money …

But we don’t like to see white roofs from the street – except in Florida, which we’ll look at more later. So what can we do for roofs we can see from the street? … Near infrared makes up about half of the energy in sunlight, but you can’t see it … so maybe we make the surface reflect normally in the visible spectrum, but we try to make the surface reflect as much of the invisible infrared part of sunlight as possible … …

Let me tell you about some of the cool colored roofs you might find … you can get cool concrete tiles … and you can get cool clay tile. You can get cool metal (often used on fast food places) … We’ll do a little show and tell … …

[ A graduate student named Pablo shows off asphalt shingles, metal shingles, clay tiles, concrete tiles, and some white roofing membrane.]

One of the interesting cases is fiberglass asphalt shingle – it goes on many houses in North America because it’s not expensive … It’s black, and another problem is that it tends to crack …There’s crushed rock on the surface. Any rough surface is less reflecting than any smooth surface … Also, these little bits of crushed rock cause the surface to have a thin coating … These are the 800-lb gorilla of the residential market … Typical shingles on your home now might reflect 10% of sunlight … … We now have a different process of applying colors to the granules. A lot will reflect up to 35% of sunlight, and if you’re willing to go with a bright white shingle … can reflect 62% of sunlight. These are still in development in the lab, but we’ve been working with manufacturers, and in the next few years, we hope to bring these to market.

Asphalt shingle failure – Image credit: Wikimedia Commons

… Another aspect we’re working on is the issue of maintenance … the roof begins as a bright white roof, and after years, gets grayish … Initial reflectance might have been 80% but might fall to 55% … The first thing to understand is that plastic roofs can leach plasticizers, which makes the surface sticky … and things grow on this … There are agents that you can add, photocatalysts, which can help break down oil and soot – they are already used to help keep windows clean (popular in Japan) …  If you add these, you  can also change the way water flows over a surface.

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Part 1 is posted here. Part 3 is posted here. Part 4 is posted here.

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