I think that having roof shingles that change from lighter to darker and vise versa, could have huge positive implications in a building's heating and cooling systems. For homes in changing environments, darker colors would utilize the sun's thermal heating and reduce heat loss through the roof in the Winter. In the Summer, lighter colors would help cool the roof and minimize energy to cool the building. In urban settings, I believe this may also help alleviate heat generated by the roofs of numerous, compacted buildings. I think it could end up being a standard in "Green Architecture".
I could envision perhaps a dial that the homeowner could use to adjust the lightness or darkness of the roof. With asthetic issues, I would also imagine the shingle to have tones that would match the color of the home.
I'm not sure of the technology and how the shingle would be created, but I could see how each shingle could connect with one another most likely at the top. An then all shingles could be tied in together.
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If the materials become available for this idea, instead of a dial to adjust temperature, a thermostat could adjust to do that automatically. It would probably be expensive, but hinged roofing tiles that could raise or lower like spines to change heat exchange rates might also work.
I believe their are some new dyes that change color with temperature. One that goes from black to white as the temperature rises would do the trick simply and cheaply. I don't know how much they cost but the energy savings would be considerable. Whats more they could be painted on to any suface:- walls; iron; tiles, you name it. With all this talk about greenhouse gases this is exactly the kind of solution that could really work. Well done!
In warmer climates, such as the southern US, roof heat gain in the summer is a problem much greater than any benefit likely to be gained from having darker roofs in the winter. Therefore, it is possible to make most of the gain in efficiency that you are seeking with these color changing roofs by simply painting the roof a lighter color. I believe there are special roof paints and coverings designed to reflect heat. Thermal reflectivity should be considered whenever a roof is being replaced. In hotter climates, a thermal reflective barrier can be attached to the underside of the roof, but better attic insulation and ventilation are often a better deal.
Shingles are expensive enough already, but to coat an entire roof with actively changing shingles would probably be extremely expensive.