Check out this link.
http://www.sherwin-williams.com/pro/..._FactSheet.pdf

Radiant barriers of the foil film type typically claim about 97% effective at blocking radiant energy, while paints are down in the 60-70% range.

On a new home I would spring for the radiant barrier decking, not too sure about the paint.

http://edis.ifas.ufl.edu/pdffiles/EH/EH24500.pdf Here is a article that Jerry P. helped pay for.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002