David's White Roof Experiment

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

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)

Update from David: July 2013:

Many people have emailed me about their interest in "hydrated-lime-painting" their roof over the past few years. Most everyone wants to know how it holds up over the years. I kept the coating on my old roof for two years. I would have kept it on there longer, however, my roof needed replacing and secondly my wife did not like the color. We never got any complaints from neighbors or inspectors but did get a few puzzled looks. One winter when it snowed, I said to my wife, "LOOK honey... everyone's roof is white now! Isn't it BEAUTIFUL!!!" but she was not convinced so, in the end I went back to the standard brown shingles to make her happy.

However, I installed some sheets of Styrofoam INSIDE my attic on the upper rafters. With my new ridge vent going all the way across the roof's peak, the Styrofoam has an airgap above it that allows most of the heat to go up and out. This system is INVISIBLE from outside the house. However, this Styrofoam method is NOT as effective as the white hydrated lime paint on top of the roof. My attic temperatures go up to about 115F rather that the 100F I got with the hydrated lime method. So it is a compromise between cooling the attic down... and cooling the wife down.

To answer the question about how well the painted roof will hold up? ... I didn't see much if any loss during the 2 year test. I put the stuff on THICK because it is really cheap. Most people, I believe are put-off by the NAME. It is also known as WHITE-WASH. This makes us think it will wash-off. Of course it will... but so does every other paint eventually. This stuff holds pretty darn good as long as it gets a chance to dry...which only takes a couple hours. Basically you don't want it to RAIN too soon after applying it. After half a day of drying, you are OK.

By the way, I have always been happy to answer all the emails that come my way. Thanks to Wolfgang for his efforts and data gathering on his home. Someday, this roof coating will be much more common I hope.

David M. Mundy

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Gary  July 5, 2010