Thank you for a very informative website. I am a residential energy auditor and we are having to answer customer questions – unfortunately after the installation in question has already been done – about how radiant barriers work and how radiant barriers should be installed. My general comment to customers has been that in our area – Seattle, Washington – our temperatures do not get hot enough, often or long enough for a radiant barrier to be very effective. Is my assessment correct or do I need to re-think this a bit?
Essentially you are correct. There would be some times, on very hot days or even milder days with clear skies (is that an oxymoron in Seattle?), where the sun beats down on an uncovered roof that a radiant barrier could provide added comfort as well as reduced energy use. This is especially true if the AC ducts run through the attic as often is the case. But as a general rule, it might be difficult to get an ROI unless the homeowner bought the radiant barrier at a contractor price and installed it themselves.