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Sprayed Foam Attic #3


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How do you go about determining the "R" rating on a sprayed foam attic ceiling? The minimum rating here for an attic is R-30. Does 4-5" equal an R-30 rating? I could not find an insulation certificate to use.


Jeff Euriech

Peoria Arizona

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According to Mason Knowles in a September 2010 JLC article, the R-value of SPF depends on the density of the foam and the blowing agent used. Low density foam in the 0.4 to 0.6 lb/cubic foot range typically has an R-value of 3.5 to 3.8 per inch. Densities of 1.5 to 2.0 lb/cubic foot yields R-values of 5.8 to 6.8.

So folk, git dat density and dat thickness and figure dem out.


PS: second paragraph ain't by Knowles, none.

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Closed cell foam is hard, almost a shiny film on top. Very hard to tear off a chunk. It's Great Stuff without the can.

Open cell is kinda......open.....fluffy....you can dig at it with your fingernail and it'll come off in a chunk.

Open cell is about R3 or 4. Closed cell is about R6-ish to 7-ish.

Some of the pics look like closed, some look like open. The underside of the roof shot looks like closed cell.

But, don't go by my guessing @ a pic.

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