First the original poster's question was, "I was looking for a product called radiant? I think. It's like a white paint for the underside of the roof.It is sprayed on from the attic with a paint gun and it reflects heat cutting utility bills.I have checked at home depot and lowe's and they didn't know about it". He wants a product for the underside of the deck and not the top. Second, the use of radiant barriers on the attic side of roofs of all types, including wood decking, have been around for quite some time in the Sun-Belt. They are very effective, have been researched out the kazoo, and do not accelerate deteriation of wood decking materials. They do raise the temp of the roof, but the amount is miniscule compared to the cost savings in cooling the structure. Many building designers actually design homes and buildings using radiant barriers. You Yanks build houses with basements... we build houses with radiant barriers. You guys talk funny... we don't. Go figure.