White roof

[postvmvs]

I know the summer is a long way off, but I am always thinking about ways to reduce energy use and one of the biggest culprits in the summer is air conditioning. Where I live, it is common to have a dark asphalt shingle roof, which gets ridiculously hot in the summer. Basic physics dictates that a white surface absorbs less heat than a dark one. So a white roof would save on cooling in the summer. I think the biggest reason white roofs are not common is the aesthetic preference for a dark roof.

While one could install white shingles, that would involve replacing an existing roof. Not a cost that one would want to bear if the roof is otherwise in good shape. There are white elastomeric roof coatings that are sold commercially, but they are relatively expensive, and the manufacturers only warranty the coating a few years and sometimes not at all for sloped asphalt shingle applications. From my searching I found a posting from someone that used traditional white-wash to coat their roof:

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

I haven't done this myself, but I am thinking about coating the south-side of my roof (the side not visible from the road) with a white coating. The white-wash method is intriguing due to its very low cost. Hydrated lime is very alkaline, so I am little concerned about its compatibility with asphalt shingles and the general lack of information, as this is not a common practice.

[paddedhat]

IF you were talking about a house in FL. or AZ. I might be inclined to agree that it's not a completely bad idea. In NJ, I seriously doubt that it's worth the trouble. Not only are you going to end up with a temporary coating on your roof, with some pretty nasty run-off when it rains, but you are really only coating half of it anyway. It will look like shit, and not make your neighbors happy in the least. As a fellow mid-atlantic resident, I really don't think that it gets hot enough, for a long enough duration to justify coating a visible shingle roof.

[Goldielocks]

An alternate solution is to investigate ways to shade your roof.

That is one reason why solar panels are great, but a big tree or lattice shading on the hottest parts may work too, and you can paint it to blend in and still be effective.   Be careful about attaching things that the wind will tear off.

Also, when you go for new shingles, you can look for grey versions with a bit of reflective sparkle, those help, too, and blend in well with neighborhood.

White washing your skylights in summer is an option, too, like greenhouses do.

[kendallf]

I think I'd pass on white washing it, but there's a definite energy savings due to lighter colors.  The metal roof I put on my house was Energy Star rated in light colors (mill finish i.e. silver, white) and not in dark colors.  If you get to the point where you're replacing it, consider metal.

[TVRodriguez]

We live in Florida and have a Spanish tile roof on a 1300 sq ft one story house.  DH and I painted the back half of the roof white (it faces our tree-full backyard and is not visible to anyone without a drone).  We used some special roof paint that DH bought (he got the local hardware store to order it for him for $200/can--I think it was two cans).  He's been after me to paint the front half, but our neighbor did that and it gets really dirty really fast so I vetoed.

I can't say the exact effect it's had on our electric bill bc we did a bunch of other stuff, too (insulated attic, let temp rise to 80 before using a.c. unless someone is particularly hot, hang clothes dry and use dryer sparingly, etc.).  We have a pool, though, and the pool pump alone runs us about $65/month.  We figured this out when we had to be out of town for several days and turned off everything else but left the pump on so we wouldn't return to a green pool (it cycles a couple of times a day).  Our lowest bills hover around $100/month, but our average is about $150.  This is down from over $200 on average before we took some of these measures.

[BudgetSlasher]

I have a few things I would consider in that situation

First, where you live. For example a place where it is HOT and sunny almost all of the year (say Florida where shorts and flip-flops are Christmas attire) you might gain something. In a cooler climate with a lower heating demand you might not gain as much. In a climate with a significant heating load, you may loose some heat in the fall shoulder season and into winter (at least until snow makes your room white for the winter . . . if you have snow that sticks), even with the lower sun and fewer hours of sun the, which would have to be made up by the heating system. Where I live I would would love free BTUs in the fall to spring and would gladly take the house warming a few extra degrees during the summer days.

Second, I would think that how you home is built would impact how much of a difference it makes. For example a home with an insulated roof deck and no ventilation would likely see a greater reduction when compared to a house with an attic that is unconditioned, well vented, and has the same amount of insulation on the floor.

Third, the condition of your roof. As you point out no one would likely volunteer to pay for a new roof, but you also shouldn't volunteer to put in money and time to paint a roof white that will need to be replaced soon (or before a reasonable payback period). But, it certainly should be a consideration in the next roof; just from my experience the attics under light colored metal roofs are cooler than the ones under dark asphalt roofs.

Fourth, cost of alternatives. I would compared the cost of doing a roof painting with upgrading the attic insulation (say to R-60) which would also keep the house cooler (and warmer too). True a white roof will still help at that point, but at a diminished amount.

It is an interesting idea that I have toyed with with in the past when I lived in warmer climates, but only as a thought experiment as I was renting at the time. If you move forward with it please let us know the results, I would be very interested to see how it works, but in terms of reduced cooling load and possibly increased heating load.

[postvmvs]

The house is a ranch style with an unconditioned attic space. When I bought it, one of the first things I did was increase the attic insulation from a nominal R-19 to a nominal R-49 (I also insulated the rim joist in the basement and garage doors in the attached garage). I agree attic insulation is a great easy way to get energy savings, for both heating and cooling seasons. From my experience the cost of cooling via air conditioning in the summer, has always outweighed the cost of heating via natural gas in the winter, even though I wouldn't benefit nearly as much as say someone in Florida.

The roof on the south-side of the house isn't visible to neighbors, as the back of the property abuts forest/park land, so I would be the only one to see it, not that I live in fear of what the Joneses might think. I would estimate that the existing roof has about 5 years of good life left, although could be made to last 10 if needed. I would certainly choose a more reflective roof material when the time comes.

What really makes the white-wash method described in the link appealing is the low-cost of materials. A 50 lb bag of lime is 5-10$ and few bags would cover the whole roof, versus 500-1000$ for a commercially available roof paint/coating. Really, the main thing dissuading me is the lack of information on this method from multiple sources, i.e. if everyone were doing it, I would jump off that bridge too.

[Drifterrider]

I spoke with a roofer on the same subject.  He said the surface temperature of the white would be cooler but it wouldn't be much cooler in the attic.  He said for best all around results (longer life of the shingle, lower energy cost) make sure you have vented soffits and a ridge cap.

[lthenderson]

Best case you are going to change the temperature of your unconditioned attic a few degrees and leave yourself with another coating on your roof that you will now have to periodically maintain/touchup to look nice. I don't think it would be remotely worth it.

[worms]

This technique was used on slate roofs in some places in southern England in the 1800s, at a time when farm workers often lived in the attic space of agricultural buildings.  At that time, lime was readily available on the farm and used for a wide range of purposes.  Since making white-wash and painting the buildings was part of the annual cycle of work, adding a layer to the roof was cheap and easy to do.  White-washing the roof helped reflect the sunlight off the roof in summer and repeated layers over the years possibly helped keep slates in place and filled cracks, making the whole roof more wind and water tight.  The practice wasn't widespread and died out. 

If you are going to do this, then you would need to make sure that you do it at a time of year when the limewash will carbonate slowly rather than simply dry up and blow/wash off, so you want a cool, damp period (like an English spring!) and avoid times when frost is expected.

Padded Hat mentions "nasty run-off" but in practice the water coming off the roof will at worst be slightly higher pH than normal and since it will be highly diluted by the rain, any effect should be minimal.  (I limewash my cottage walls and my most weather-exposed side of the house has a border below it in which I grow heathers and a rhododendron - if the amount of lime washing off into this border was excessive, it would very quickly show up as a chlorosis in the rhododendron).

[postvmvs]

Thanks for the info worms, are you in the UK?

I think at this point I will set up a test rig in the spring of a few shingles (I have a couple of bundles lying around) on a piece of plywood to practice whitewashing it. From doing some research, various forms of limewash and limeputty are traditional finishes in countries such as Germany, Denmark, Ireland and others. They have some advantages: low material cost, breathability, and some resistance to mold. The disadvantages are the higher labor cost of multiple initial coats and the periodic reapplication required.