Perimeter Evaporative Roof Cooling

Just read this article in Backhome Mag, no 113, and it talks about how, even with insulation, about 60% of summer heat enters through the roof. This system uses water to cool off the house, and for every gallon that evaporates, over 8000 Btu of heat are "sucked away." The system (PERC) lowers AC electrical costs (even by half) and extends roof tile life span. You can even create a water catchment system so that the runoff water waters your garden etc.

Evaporative cooling is much of what makes a green roof cooler.

Suzy, can you describe the system a bit more?  Suitable for retrofit?  What climates does it work in? 

Sure Yukkuri, The author took conventional lawn sprayers to the edges of his roof, on the gutters. He put them to a timer. They can also be hooked up to a thermostat, which causes them to spray when the roof surface is hot enough to warrant evaporative cooling. I would assume this would work in any climate where your roof got hot enough. In the article, there is a chart showing money-savings in different parts of the states. The savings are greatest throughout the South, yet still save a few hundred per bill in New England. The chart also shows reduced tons of CO2

This seems like a huge waste of water and the power required to pump. If you're dealing with a conventional asphalt roof your best bet is to insulate heavily and ensure that you have adequate ventilation. The most common stupid error I've seen with asphalt is that people use black shingles. This not only causes excess heating but also reduces the life of the shingles considerably. If you're building your roof from scratch a green roof incorporating soil and plants would be better than just spraying water around. If the grass is allowed to die back naturally during the hottest driest part of the year most of the suns rays strike the dead grass and are converted to heat well above the soil layer so little of that little heat is transferred down through conduction. There's no need to water a dormant roof. You mentioned new England where summer temperatures are not generally extreme. For any of this to be necessary you would have to be living in a house where many design and usage errors have been made. Lack of insulation, thermal mass, shading and venting would be the prime cause of overheating and these are the items which should be addressed. A much more economical means of using water for cooling would be to use a swamp cooler within the house and roof pond within the attic as is common in the desert Southwest.

hows about using a soaker hose on both sides of the ridge? just dripping all day into your rain barrels. even when you factor in evaporation, i have a feeling if you are on city water, you could come out ahead if you didnt have to run your ac so much. i do have a black asphalt roof as it were.

An asphalt/composite shingle roof can also be painted with white reflective roof coating which is supposed to cool the home interior by 10 degrees F.  We'll be painting our roof this Fall or Winter.

http://www.builditsolar.com/Projects/Cooling/WhiteRoofExperiment/WhiteRoof.htm

Are you going to paint it yourself?

My roof is old enough to be replaced but there is nothing wrong with it.  I didn't know you could paint asphalt shingles. 

I'm kind of thinking that maybe it would be worth trying?  But I would have to hire it done so that might not be worthwhile.

Curious the tempature difference the color makes on the same house?  how hard could painting a roof really be? You are willing to replace it anyway,,, right? Or attract an awful lot of birds to pewp on it'

My husband has rods in his spine so cannot climb up there to do it.  I have tried to do roof work but my balance just doesn't cut it.  So we have to pay others. 

gotcha, i forget these things. my bad, also some roofs are quite steep. it would seem to me the paint would help it hold together and last longer too?

My husband and I will be painting our roof ourselves.  Eek....

And yes apparently the coating can help a roof last longer.

This stuff:  http://www.homedepot.com/Building-Materials-Roofing-Gutters/Henry/h_d1/N-5yc1vZar6rZ2wn/R-100051140/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

I've tried evaporative cooling recently during a heatwave on my black flat asphalt roof- It made zero difference to inside temperature, mind you the water on the roof pooled, so it worked more like thermal mass and held the heat in. I'm with other on this one, it's better to keep the roof cool in the first place-shade, color, insulation.

It does work.

Yes it takes power and water, but less than AC. Yes shade is better, but getting mature trees takes a LONG time or a LOT of money.

You do not want the water to pool or run off, if it does you are putting down too much. Just enough to "sweat"

Use a radiant barrier and vent the attic well. Water can be very expensive in some markets. Also, spraying water on the roof can cause mineral staining and salt buildup, if it matters. For more effective use of water, use an array of misters. To shade roof, put up a lightweight trellis and plant fast growing vines, or grapes if you aren't in a hurry. Deciduous vines are best if you want the sun to warm the roof in Winter.