Um, isn't radiating the sun's heat away counterproductive? I thought passive solar was supposed to be a good thing. Summer is uncomfortable for a month or two, sure, but winter gets cold, and I don't want to sacrifice winter heat for a little more coolness in summer.

I'm a little surprised it's taken us this long to realize that black roofs are terrible for heat. ;-(

Chris
http://amateureconblog.blogspot.com/

Stupid question: in an ideal world then (and of course in climates where winter is cold), wouldn't it be best to have a roof that was white in summer and black in winter?

For the average home (excluding appartments in high rise buildings in this arguement), solar gain through the roof is lower in winter because the sun's angle from the horizon is reduced over summer and because the sun spends less time in a clear sky, with incident rays close to perpendicular to the roof's angle. Snow caused reflection also figures in. THerefore, with the exception of the southern most perimeter of the US, where snow buildup hardly exists, solar gain control in summer is useful for average annual energy efficiency. Conversely, it is most beneficial during winter to have low emissivity coatings on all windows (not just south facing ones) to keep heat inside the building, and adding to subjective comfort. This last statement applies strongly to the northern tier of states, and increasingly less to (because of Climate Change) to southern states. Most people I speak to have difficulty with these ideas as they are not uniform geographically, are counter-intuitive, deal with invisible forces, change over the seasons, and may be changing over time. Buidling codes are behind the times, adding further consternation. Even building contractors often get it wrong, commonly arguing that money can be saved in northern states by putting low-E glass only on south facing windows, which is completely wrong. Doing that makes the cost of the ones you do put in as low E a waste.

CTP, if you're not going to be using that solar energy for anything you would be better off going with an earth-ship design than a colour-changing paint job.

In the winter case, having a black roof wouldn't get you much because all you would be doing is heating insulation; you still need insulation to keep the place warm on cloudy days and at night. Truth be told, if you're in the market for a house in a cold climate and you see a "bare black-shingled roof" (also look for large icicles) turn the other way and run! It's a sign that heat is escaping due to a serious lack of insulation.

You're much better off going for a proper solar-thermal energy capture system - the type that uses evacuated glass elements - or photovoltaics (which will perform much more efficiently at lower winter temperatures) to make productive use of the winter sun.

For warmer climates (including summer time) I would think a photovoltaic solution makes the most sense. The brighter the sun, the more energy you have to power what little A/C you might need.

...I much prefer the green-roof concept. If I'm not going to use it then I might as well let some plants take advantage of all that "wasted" sun.

John,

Good comments about the counterintuitiveness of many energy saving technologies. To add to the confusion a bit, the low-e coating on the north windows should be on on the inner pane (assuming double-paned windows). Basically, you're reflecting heat back into the house, whereas on south windows you're relecting the heat outside, so the coating is on the outer pane.

One cool roof product not mentioned in the links page is a special type of radiant insulation paint - SuperTherm paint.