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This is an experiment that David did to see what the cooling benefit of a bright white roof is.
From David: |
My roof needs replacing anyway, so I thought I would do an experiment to see the effects of painting it WHITE. I have already placed digital thermometers in the attic for previous experiments so I could easily monitor progress. I started off with a product made for the purpose... a highly reflective "elastomeric" roof coating. However, a 5 gallon bucket of this product costs $60.00. I figured I needed TEN of those 5 gallon buckets to cover my 1300 square foot roof. That put the cost up around $600 dollars. TOO MUCH MONEY for a little experiment. So I opted for the EL-CHEAPO method of using HYDRATED LIME mixed with 50% water to do the job. Two bags of Hydrated Lime cost $14.00 total!! It is available in the concrete department of any hardware store for $7.00 per 50 pound bag. This stuff goes on kinda GRAY looking when it is wet. But when it dries it is BRILLIANT WHITE.
Lime kind of burns your skin, so don't get it on your skin or eyes. I did all the mixing right on the roof so I wouldn't have to lug all that water up the ladder. Just kept the water hose running while worked. Painting was easier by just dumping a gallon or so at a time right on the roof and just rolling it evenly. It took about 6 hours of work to accomplish the task. Here are the results of this experiment:
House temperatures with regular roof and AC OFF......... 87-92F.
WHITE ROOF (and ac OFF)........ 79-82F.
ATTIC temperatures with regular roof.................... 120 - 140F
WHITE roof ................... 90 - 97F .
Click the pictures for full size
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Attic temperatures were reduced by about 30 to 40 degrees.... and house
temperatures were reduced about 8 or 10 degrees. Some folks might think 8 or 10
degrees isn't much... but, it is right on the edge where we don't even need to
run the air conditioner! I personally would not use AC at all.... but the wife
still likes it ON occasionally. I would like to replace my roof this summer, but
don't want to go back to the SHINGLES that absorb so much heat. It is
frustrating because I can't find an alternative that is reasonable in cost and
will keep my attic COOL. If a simple home-owner like me can accomplish reducing
my attic temperature by 40 degrees by using fourteen dollars worth of paint...
why cant these roofing companies come up with something that can reflect the
heat without breaking my bank account?
It has rained a couple times since painting... and the lime seems to be holding
on OK and not rinsing off. Don't paint if its going to rain in the next 24 hours
though. ONE neighbor asked what i was doing. I explained I was "coating my roof
with special reflective coating" to reduce air conditioning costs. If he had
objected, I would have mentioned that president Obama recommended doing this in
one of his speeches! Nobody else said anything.... might better check with your
neighborhood association/inspector before starting. The best time to paint is
during sunset. In the morning too many people are leaving for work and will see
you and ask questions. Right now, this looks "unusual"... but in a few years i
believe white roofs will be very common to reduce greenhouse gases and electric
bills. One village in Greece has ALL their roofs white.
David will answer email questions at: framistan AT sbcglobal DOT net (replace AT with @ and DOT with a period)
Gary July 5, 2010
