The 2006 IRC (section 314) has been almost entirely re-written on foam plastic. I have been having issues where builders are not only told this is acceptable, but are recommended to do it for the 2006 Energy Code. I will not say who, but they teach lots of energy classes. In some cases this is where the builder has a 2×4 studed attic knee-wall and instead of making it a 2×6 wall he uses R13 and 2 layers of R3 Foam Plastic which bring s it to the Required R18.

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