January issue of environmental building news www.buildinggreen.com had a great article on the reflective surfaces and cool roofs...

the article  explained the difference and attributes of reflectivity and emissivity. reflectivity measure  how  well material bounces back radiation, and emissivity is how good a surface is at radiating heat back  out into space. Energy star has ratings for roofing materials based on both of these qualities. you will find that light colored surfaces tend  to perform better than metallic surfaces because of the low  emissivity of  the metallic surfaces-which means they store heat better. while this  is good when looking for solar collectors or materials for tromb  walls etc, it  is not good  for building assemblies  when trying to reduce the cooling load and the heat island effect of buildings and materials. The Cool Roof Council (CRRC) publishes reflectivity and emissivity values for roofing products.

another  point that is not  talked about  when talking to reps about radiant barriers, but as these surfaces get dirty the qualities decline, so CRRC includes both new and three year values, although by using these products we are still getting ahead. the really cool thing is that manufacturers have now  developed pigments that  look like standard colors but are highly reflective to infrared light so we no longer have to buy light or metallic  roofing. I knew we could do it. technology is a great thing-sometimes. funny we  can put  men on the moon with technology, but we can't build a fish ladder that works? Hmmmm?