roof whitewash/blackwash for cooling/heating

Hello,
In reading on http://builditsolar.com experiments on whitewashing an asphalt shingle roof with hydrated lime the thought occured;
Why not whitewash each June then darkwash each September to fit my cooling and heating seasons here in Connecticut.
(the darkwash would be done by adding dark earth pigments to the hydrated lime)

Has anyone tried this?
My roof is 12 years, light grey shingles,, moderate slope.

Thanks for any advice,

Bill

That sounds like a lot of work. The idea has occurred to me, but I have too many projects going on to take on that kind of a yearly chore.

I am reminded of something I saw at a museum somewhere in the Southwest. It seems that it was women's work among Native American tribes in that area to go out each spring and slop on a fresh coat of mud onto the adobe hogan to keep it in proper repair. Since the walls and the roof were adobe, that meant getting up top and slopping some up there as well.

One difference with the adobe is that they were in an arid climate and every year they would be replacing the small amount of material that thunderstorms had washed off. But in your case, I take it you are talking about a coating that doesn't wash off. What's it going to look like after 10 years and it has 20 layers of white/black/white/black? What does that build-up do for the life of your asphalt shingles?

Thanks John,
I'll have to attach the links from build it solar. The first guy who wrote about it, Dave, did a hydrated lime whitewash on year 23 of his asphalt roof with the need to replace the roof soon. He collected a lot of data in regards to cooling since he was already doing other experiments with insulation/venting in his attic. Yesterday I read about Wolfgang who in Texas realized great savings on his electricity bill.

A simple hydrated lime + water mix should have a bit of erosion and chalking off. My understanding is that this makes calcium carbonate which is the same as the grains on an asphalt shingle.
- would the erosion balance out with the addition of 2 coats yearly?
- how much damage will I do walking the roof and rolling it?
Between using 3 cords of wood and back-up electric oil radiators for winter heat. Then using room air conditioning June thru August it may be worth my while to get up on the roof twice yearly.
*Actually thinking of only doing the south facing side of the roof. Hoping 4 bags lime + will have to price the dark pigments (saw some on realmilk paints)
I think it will be a worthwhile experiment.

I'm on a deadend road. Only the google/NSA satellites should notice my roof doesn't match.

I'll mention to my wife the tradition of having woman paint the roof

Bill

Not sure if about your details but if you have insulation on the attic floor, then you need to start by air sealing this plane and then adding more insulation. Peel back existing insulation around lights, interior partition walls and penetrations and sealing with caulk or sprayfoam. Air sealing your basement, rim joist or crawlspace walls will also reduce the total amount of infiltration that makes it through the roof.

Blown cellulose or fiberglass is very cheap and an easy DIY project. These efforts are a much more proven strategy than what you are proposing and they should decrease humidity issues in the basement/crawlspace. I would at least start with weatherization before moving to experimental techniques.

I'm with Brian. Also, when shingles are replaced, use white. The shingles last longer, air conditioning is reduced and insulating snow loads are retained. Less icicles are produced. Black shingles give heat when it's unwanted but not when it is needed.

Lime is a base. Oils are acids. I would expect regular lime coatings to destroy the shingles by making them brittle as oils are attacked. Then there's all of the foot traffic which does shingles no good.

I can't imagine a situation where this would make sense.

Lime coating can work to reduce heat on masonry walls and it extends the life of bricks and mortar. It can also be used to delay flowering of fruit trees by keeping trunks cooler in spring.

The roof tends not to be a good place for solar gain in winter and cooling in summer at a latitude like Connecticut. The sun is high in summer, shining a lot on the roof when you don't want it. And the sun runs low in the southern sky in winter, not really giving any gain to a dark roof. So it would be better to keep your roof well insulated and use your solar gain and shade cooling energies on your south facing wall, where they will have more effect.

Thank you for bringing this idea up.
Following your link taught me alot about the uses of lime.

Thanks all for the informative replies.
@Brian- Gary the founder of builditsolar gave much of the same advice re: insulation and air sealing first, though he would be interested in data if I did do black/white wash yearly. Their's not much room in the attic, 20" at peak. I have gable and soffit vents.

@Dale- good caution that things could go wrong adding other chemicals to a mfg. roof. Risks of premature shingle failure.

@Rebecca- solid advice to concentrate on south wall shadings. Thinking about awnings, reflective window films and shades. Also transplanted my trumpet vine and will build trellis's on south wall (hoping it takes, then I hope I can control this monster of a plant.

I'm still thinking of doing this - for the sake of science (tho nights above 46 f are running short)

My thoughts are 1/3 hydrated lime S: 2/3 water. I believe I can use up to 10% carbon black for pigment. Carbon black fades in mortar which is what I want it to do in time for the June whitewash.

I'm looking for a cheap source of carbon black pigment if anyone knows one. (or maybe I can use the blender on woodstove charcoal I have saved)?

2 bags of lime - $18, not sure of carbon black price (cheap per pound by the container, outrageous $$ by the ounce in art stores) will cover my south facing roof.

Bill

PS, I have painted a roof white in the past for cooling. I arrived in the local Casa Gringolandia, in hot and humid Managua, Nicaragua to find my compadres had pulled down the dropped ceiling in hopes of more airflow in the house. They created more airflow!- 170 f air in the attic below the red rusted roof now mixed with the ambient 95 f. creating horribly hot conditions.

On arrival, using my limited youthful experience of hot cars, I made a plan to paint the roof white. Due to economic conditions I travelled by bus to Costa Rica to find the 20 gallons. I returned in a leg cast, shared a taxi from the border with a Norwegian Pharmaceutical mfg. and a young teacher to Managua. (Managua is one of those interesting Capital city compromises, like Brasilia and Washington, DC, located between 2 opposing city factions of liberals and conservitive parties in the hope of keeping the peace. It didn't work)

The white roof worked. It went from truly unbearably hot, to regular this sucks hot.

Later we added an attic fan. Then 6 mil plastic dropped ceiling abd finally replaced the wood cielo raso.

I was (and still am) oblivious to the aesthetics of the white roof. I offered to paint local family and friends roofs white, but no one accepted. They liked their (hot) green, red, blue roofs

I limewashed my old roof and it helped keep the house cool enough that we didn't have to replace the aging A/C unit. I wouldn't bother putting a dark coating on the roof to get more solar gain in the winter- over the course of the summer, dirt will land on it and reduce the albedo effect, for one thing (just get up there and do a good rinse with a hose at the end of spring), and the lower sun angle and lower intensity of the solar rays during the winter means the solar heat gain in the winter is a lot less, anyway- I ran the calculations once before, but they took forever and I don't have the numbers with me, but the reduced cost of summer cooling saves you more money than the increased cost of heating in winter, if you just calculate BTU's lost and gained vs. heating and cooling loads.

Besides, when it snows the roof is white anyway, and the snow is a decent insulator. It's just a lot more work than it's really worth. Making a few solar aluminum can heaters (or, how I'm going to make mine, aluminum gutters, since I think it looks nicer) and building mounting for them on a wall with good solar aspect will give you a lot more useful gain than having a darker roof in the winter. Remember, heat rises, so having a hotter attic in the winter doesn't do you as much good. I live in Texas and with a medium gray roof, it gets up to 120-130 in the attic by 10:00 in the morning, but since heat rises it's not AS HORRIBLE as it could be. I'm doing what I can to fix that right now, but even with completely inadequate insulation in the attic, my ceiling is only in the high 90's.

To keep a house warm, instead of all the work I'm doing to keep my house cool, I'd put in some passive solar wall heaters that can be taken down at the end of cold weather, like these http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm

And possibly put a lot more insulation in my attic. In a lot of ways, keeping warm is easier than keeping cool! I miss living in a cooler climate.

I am from India, basically a hot country and to using lime painting my roof to keep my rooms at top floor cooler, but as you want a permanent heating and cooling solution, if will suggest you to use some thermal blanket like material as used to cover glaciers. Researching such cloth is worth and you can paint your roof dark black and cover it in Summer. In long run its more cost effective.

When I put in my second story addition, I chose the most reflective asphalt tiles I could find. I also put in a ridge vent and a radiant barrier under the roof joists which formed an air channel from the soffit vents to the ridge vent. The walls were also covered with a radiant barrier and I set the siding on 3/4" lath to form an air channel from vents at the base of the wall to the soffit space. I am able to keep the original floorspace and the addition (about 27' x 64') cool with the existing high-efficiency evaporative cooler, though it could use a more powerful fan.

My wife is from Guayaquil, Ecuador. Her parent's home has a sealed attic which acts as a thermal battery, radiating the day's solar gain until about 2 or 3 in the morning. They could really use some venting, but I think it is a social status thing to have the finished ceiling and folks don't trust that screens will hold up over time, especially unseen, letting vermin into their attics, so they keep them sealed. I did spend some time in a top floor apartment with an open attic (tin roof) that had screened soffit and ridge vents. It was bearable during the day and cooled off immediately at night (near the Equator, so its 12 hours of day and 12 hours of night, all year), but it was considered low class because it didn't have a finished ceiling. Style over comfort is the rule.

I have thought a lot, over the years, about how to build or retrofit a home there that could be comfortable without air conditioning (at least during the dry season). I had one of my brother-in-laws interested in trying out the reflective, insulating paints that are marketed here in the states, but he passed away of a sudden illness before he had a chance. That was quite a few years ago and since then, I have not heard much about them. They may not have been as effective as advertised.

I agree that it would not be cost effective to be switching coverings from season to season. Concentrate on reflecting the sun from the roof, seal and insulate, and use passive solar below the eaves and/or solar panels for heating.

Bill McGee wrote:
Why not whitewash each June then darkwash each September to fit my cooling and heating seasons here in Connecticut.

Hi, folks.  I realize this is an ancient thread,  and I normally don't like raising zombies,  but there's an important point that has not been made and I registered here just to add this for anyone who comes across this in the future.

Bill's question comes from a very common misunderstanding of how radiant heat behaves.

A white surface will not only reflect radiant heat much better than a dark surface,  it will also RETAIN radiant heat better than a dark surface.

It turns out that REFLECTIVITY has a reciprocal property known as EMISSIVITY.

A white surface has a high REFLECTIVITY and a low EMISSIVITY.  Black is the reverse - high emissivity and low reflectivity.

Ever touch a chromed bumper on a sunny day? It's  HOT.   That's because it can't emit the heat it picks up from the surrounding atmosphere.  Yes, it's reflecting the sun's energy very effectively,  but it's not radiating heat effectively.

Why is the SR-71 Blackbird painted black?

To COOL it down. It generates tremendous heat from friction with the atmosphere and the black color allows it to RADIATE much of that heat.  A light color would RETAIN too much of that heat and destroy the plane.

So painting a roof black for the winter is a great way to cool your attic.

IF you could make it white for the daytime and black at night, you'd have the ideal summer roof.  For winter,  leave it white 24x7.