Open Question

The Rated R-Value of Foam is related to density...

In the foam insulation industry, r-value is measured and varied according to the density of the cured foam and is expressed on a per inch basis.

Density is measured in pounds per square foot.

Open Cell foam has a density starting at 0.5 and can be as high as 1.5 pounds per square foot.

Closed cell foam for insulation purposes, can be as high as 7.6 pounds per square foot and even higher in some roofing application foams or specialty foams.

So, if you are having an open cell foam installed in your walls at a nominal R-3.6 per inch in a 3.5" wall cavity, you would achieve an R-Value of 12.6 in that cavity. Of course, the r-value test is slanted towards fibrous insulation in that it doesn't take into account air sealing, which foam does extremely well and is why code officials are not generally concerned with a 12.6 r-value if code calls for R-13 because they know that insulative value across the wall is much better served by a superior air barrier than a few "Rs", which are the measurement of resistance to heat transfer through the insulation material absent air infiltration.

Likewise, if you installed a closed cell foam at R-7 per inch for 3.5 inches, you would have R-24.5
in a 2.5 wall and superior air infiltration reduction.

Or you could put in just 2" or the R-7 foam and achieve R-14 within the sealed cavity.

There is more information out there, and probably on this site, about what the R-Value test really is, but that's the way I understand it.

Hope it helps.

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