Right now we design roofs to expel as much heat as possible. However I was just thinking that a home's roof could be designed to capture a lot of the sun's heat (spring and fall - and maybe even in the winter if there were no snow on it.) and so reduce heat loss through ceilings. Heat up the attic and less heat escapes the house. Add a lot more insulation and when you heat up the attic, you'd also heat up the insulation which might then radiate some heat into the house. So some sort of manual or automatic venting would be required, like a modified version of the power roof vents. Possibly black metal roofing and good membranes too, to avoid ice damning.