How much energy would we save in cooling if all roofs were painted white?

Given that I actually saw a thread where someone calculated the change in the Earth’s rotation as a result of cutting all the trees, it hardly seems out of line to think that someone can actually take a shot at this:

Suppose that every roof in the States, or any other nation that uses a lot of air conditioning, were painted white instead of the usual dark color. How much energy would be saved as a result of the lowered heating from the sun? How much, if at all, would the ambient temperature change (especially in cities)?

Feel free to assume that exterior walls are painted white as well, if that helps make things easier.

Thanks,

js_ninja-whitewasher_africanus

…(interested in replies)…
My SO has often stated the desire to do this exact thing. He does go one step further. His idea requires the roofing material to flip to a dark color with the onset of Winter. He also plans to use as much solar power as possible, and feels that this idea would harness the solar power to greater gain.

I don’t know about changing the roof color, but a rather shocking statistic came to me a few years ago. The statistic was that 30% of your home’s air conditioning goes to removing the heat dumped into your house by your REFRIGERATOR. I don’t have a cite, but knowing how refrigerators are constructed, I’m not too suprised by this…
In my opinion, it seems to me we could save an ENORMOUS amount of electricity by building a duct above/behind the refrigerator that dumps to the outside. This duct could be closed during the winter to allow the heat to dump back into the house…
If I can swing it, and it doesn’t violate any building codes, I’m planning on trying to incorporate this into my house when I build it - assuming I can find a way to make the refrigerator dump to the back/top instead of out the bottom/front…

HA! I fooled them! My house doesn’t have an air conditioner! :smiley: We just open a window if it gets hot, and let the ocean breeze do the rest.

In any event, wouldn’t the really smart move be to put the oven and the refrigerator back-to-back? One dumps heat, one uses it…

Hah! In Torrance you might be able to do that. Try it where I live.

By the way WWII buddy of mine (The ugly one one the left with the Garrison Cap.) runs the Contemporary Center on Sepulveda Blvd. If you want fine furniture and interior decor advice, see them.

Oh my! I think I just broke a rule. It was all in fun guys, I don’t know him and he’s actually a bum on skid row.

Better still to make the roof out of active mirror tiles that focus the sun’s rays on a communal hot water tank or boiler for a steam generator to charge battery-powered street lamps, or something.

I mentioned this to someone not entirely unfamiliar w/ building & stuff and he said that keeping the white in the winter would be better because you don’t want the roof to be warmer than it has to be—otherwise you can melt the bottom-most snow and end up with an ice-covered roof and get water backing up under the shingles (and the leaks that’d ensue).

Taking a wild shot at a real answer I come up with zip, zero, nada, zilch. Unless you can, as suggested byLyllyan , switch to black in the winter. On the whole, country wide you’d loose about as much heating capibilities in the winter as you’d save in air conditioning in the summer.

Thinking a bit more I’ve now come the well reasoned conclusion that you’d have a net loss. My guess is that the average house, in the contential US anyway, runs the furnace more than the ac.

My $.02, YMMV.

My guess is having a white roof would save a lot on the A/C bill, if your roof was not insulated, or if you didn’t have attic ventilators or fans. But with both, I doubt a white roof would make much difference at all.

Did anyone bother to check Google?

Study Confirms Energy Savings of White Roofing.

Saving Gracefully: California’s Shortages Rekindle Its Efforts To Conserve Electricity.

White Reflective Roofing: How to Reduce Energy Consumption and Cool Urban Hot Spots.

Roofs Reflect Better Savings.

White Roofs: Cutting Costs at the Top.

A bit less enthusiastic:
A study shows factors other than color affect roof system energy efficiency.

Overall, there appears to be little doubt that white roofs can significant lower energy costs in hotter, sunnier climates. Farther north, the need for heat in the winter means that additional steps must be taken, like insulation. Since I have insulation in my attic I put a light-colored roof on my own house. (Pure white gets dirty so quickly that there can be little extra to be gained and it is more expensive.)

I can’t imagine how you would come up with an answer to the OP, though.

That’s a real problem where I live. The ice gets up to 2 feet thick on parts of my roof. The ice creates a damn near the edges. When snow on top melts it creates a lake.

You have to put a membrane under the shingles. Bitithane. And or heat tape. Your best solution is the bitithane though.

And a word to the wise. If you deciede to put down bitithane, don’t ask your friends to help you. Invite your enemys.

um… the great majority of the heat that your refrigerator dumped came from the environment anyway(the rest comes from dissapation of electricity as mechanical energy and whatever heat those microorganisms inside your fridge made). thus, your air conditioner would be venting the exact same amount as it would without the fridge. the vent idea seems to be valid though…

Well, you can do the math here. An efficient refrigerator may use something like 660 Kwh/yr , 55 KwH/month, or 0.076 KwH/hr. The conversion from KwH to BTU is 3413, so unless I’m making some erroneous assumptions, a refrigerator is pumping 260 BTU/hour into the room. A typical air conditioner for a kitchen-sized area might be rated at 10,000 or 12,000 BTU/hr. So I’m thinking that your 30% figure is off by a factor of 10. It’s more like 3%. It might even be less than that, because as the air conditioner runs, it cools the kitchen, reducing the amount of work that the refrigerator has to do.

Intuitively, I think the above calculation is in the ballpark, because when I stand next to a refrigerator, I might feel a slightly warm draft. When I stand next to an air conditioner, I feel a dramatic change in temperature.

Note that there’s no way you’re going to save an “ENORMOUS” amount of energy, because the amount of heating were talking doesn’t represent an enormous amount of extra cooling. Assuming you air condition for 3 months/year, and all of the heat generated by the refrigerator is considered undesirable, that’s only $15.00/year of extra air conditioning.

BTW, your stove pumps out pretty much the same amount of heat as the refrigerator (lots more heat over a shorter period of time) and it does it during prime air conditioning time, so you’d probably save more money by simply grilling outside during the summer.

Except for opening the door as you take stuff in and out. Unless of course you eat outdoors. But then what’s the point of having a airconditioned house?

We could simplify the whole thing (in a thought experiment) by replacing the fridge with an electrical heater with a similar energy consumption rating to the fridge - I suspect the amount of heat added to an environment by a fridge is not insignificant and it isn’t solely based on the amount of heat generated by the bugs in your food.
The insulation is less than perfect and you keep opening the door, so the temperature inside tends to rise, causing the compressor to run for longer, releasing more waste heat (i.e. heat from the machinery, not pumped heat from inside the fridge) into the room. Also, in a warmer room, a fridge will have to work harder to maintain the same low internal temperature - harder work = more waste heat.

I admit to not having read the links particularly carefully, but it seems all these studies are done in places like Florida where proper insulation is the exception rather than the rule. Is there any data on whether roof colour makes a difference when your attic insulation has a value of R40? (I believe current building code in Saskatoon mandates R40 in the roof, not positive.) I have a hard time believing roof colour would make any difference whatsoever in a recently-built home here.

And, for those who don’t know, good insulation is just as effective at preventing a house from overheating due to hot temperatures as it is at preventing a house from freezing due to cold temperatures. Pink is your friend.

I’m inferring from this Staff Report, but dark colors radiate heat faster than they absorb it if there’s a wind. Given that in the summer you’re trying to keep the suns heat OUT and are better off with a reflective surface, and in the winter trying to keep what heat there is IN and are better off with something that doesn’t radiate heat as well, I would suspect that white is a better choice year-round.

Anyone with a better grasp of the science involved care to comment?

Gorsnak, did you read my last paragraph? Or the last link, which referred to Chicago?

I carefully qualified my statement to say that in the north a combination of insulation and a light-colored roof is the best year-round solution. My wife works for a utility and looked at the research from the industry before we changed roofs. All of it said that light-colored roofs reduce energy costs.

Since the majority of houses in the north already have insulation - newly-built ones include it automatically and most older ones have been retrofitted over the years to cut energy costs - then the only real issue is roof color. And so for every part of North America light-colored roofs are more energy efficient than dark-colored ones.

I did read your last paragraph, and I didn’t see an answer to my question. I hadn’t read the last link closely, but now I have, and my question remains. Would roof colour make any difference whatsoever with a roof insulation value of R40? The study doesn’t say. It talks about comparing R19 to R20, etc, but this is a rather different thing. Heat transfer (in either direction, obviously) approaches zero when you get up that high.