According to the FSEC (Florida State Energy Center) a radiant barrier in the attic can reduce home cooling needs by +/- 9%
FSEC-CR-1231-01. A reflective white roof can reduce cooling needs by 18-35%
FSEC-CR-1220-00. Those improvements include results for several homes that already had powered attic ventilation.
Thanks to BuilditSolar.com
Passive Cooling Techniques for both links.
I've been very pleased with our cool roof (not white, but a good Solar Reflectance Index). We're midway through our second summer with the cool roof and it has made a big difference in our comfort level.
I'm still looking to improve our home's passive cooling, but the roof definitely helped during a recent heatwave.
We're Having a Heat Wave, a Tropical Heat Wave | EcoNewMexico.com