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.
