Cool Roofs

In the discussion of white roofs to combat Global Warming and save energy
I feel at least 4 important aspects are being missed.

The first is that truly reflective roofs would be the case of them being
made with roof tiles that are coated with metal, i. e. mirrors (oxidized
aluminum). Flying over cities with roofs made of this material would be
difficult, but the reflective index would be near 90%.

Another aspect is air pollution. If the "whiteness" of the roof tiles were
due to titanium oxide being used; city air would benefit doubly. Under the
influence of this oxide and ultraviolet light (the sun), this white dye
acts as a catalyst that destroys smog molecules with a high efficiency.

More research should be done on a new family of materials that can
radically alter their properties when a voltage, magnetic field, etc. is
applied. Some can be switched from a conductor to an insulator, i. e.
reflective to absorptive. Having grown up in Milwaukee, where the hot,
sweltering summers are as bad as in the US Southwest, this "one way for
summer and flip a switch for winter" could have an appeal even for the
colder parts of the US.

Last case concerns something one often sees here in Europe on the camping
grounds. When the sun beats down upon the caravan, people place a tent
over it. The caravan, being in the shade, stays cooler. By installing a
double roof, with an in between gap, the house can stay cooler. If the
external roof were also to be constructed of solar cells or solar thermal
aggregates, then a home owner could save energy doubly well.