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"effective" R-value for spray foam insulation?

2457 Views 1 Reply 2 Participants Last post by  PaliBob

I've been in contact with some contractors for spray foam insulation for the underside of a new construction roof. Although they say the foam has an R-value of 7 per inch, they say the 'effective' R-value is double that because there is no air movement within the material. Thus, I don't need 7+ inches to get R50, I'll only need 3.5-4 inches. Is this true?
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