I do live in Maine, but I also live in the middle of the state’s largest urban heat island (such as it is, in Maine.) So believe it or not, there is a brief stretch here– roughly from July 4th to Labor Day– when the weather has the potential to get uncomfortably warm. Out of those two months there are maybe eight hot nights when I lie awake in the second floor bedroom, sweltering, thinking about sleeping amidst the creepy-crawlies in the basement/dungeon. Now and then I get a fleeting urge to buy an air conditioner, but it never rises to the level of action.
This year, as spring wore on, I started to think whether there were any passive strategies I might use to get the house a little cooler (other than fans, which I already employ extensively.) My first fantasies involved heat-sensitive, color-changing exterior house paint– say, a paint that is black in the winter, but white in the summer. This is a nice idea, but so far as I can tell, the technology isn’t quite ripe yet. ( This company) makes some cool paint that does almost what’s called for– but at $429 a quart the paint job might cost more than the house is worth.)
Temporarily changing the color of the roof, though, struck me as relatively simple.
We have a flat roof with a black asphalty-rubber surface. I don’t know how much solar heat it absorbs in the winter (it’s usually covered with snow), but in the summer it gets “wicked” hot– empirically, hot enough to scorch bare feet, on a sunny hot day. I resolved to try covering the roof in some sort of white fabric. A la Christo.
I went through various material options, almost settling on scrap Dacron sailcloth. But I decided to try plain old white cotton first, buying 400 sq. ft from eBay for $36. It’s cheap, and if it was a failure, at least it’s biodegradable. Plus, it can absorb a little water, which I thought would provide opportunity to try a secondary strategy of evaporative cooling (by spritzing it on extra-hot days.) It did occur to me that cotton would be susceptible to growing mold when damp, but I thought it would get enough sun between rainstorms that it would more or less get sanitized.
That was before I knew it was going to rain like the bejesus all of June and into the first week of July. I carried the heap of folded fabric up on the roof the first week of June, but it started sprinkling on me, so I decided to wait till the rain stopped to unfold it. I had to wait over a month. By that time, it was getting kind of slimy. Ew. But I spread half of it out anyway, to test.
Here’s how it looks:And good news: it works. When we finally got a sunny day, I took temperature measurements at the surface. The black part was 99.0F (37C). The white part was 82.6F (28C). And this, I should mention, was at 5pm, as the sun was getting pretty low– at noon, probably a bigger differential. Anyway, I’m convinced that covering the whole roof will be enough to lower my upstairs house temperature at least five degrees on the hottest of days, which is all that’s really needed. Can’t wait!
(P.S. While I was waiting for a break in the weather to roll out my white roof, the U.S. Energy Secretary came out in favor of white roofs too. His suggestion is that eliminating heat absorption from black roofs could stop global warming. Up north here, though, a black roof is a benefit in the cold months.)