Generally R-values are additive; I would expect the 1 1/2" stuff to have an R value of about 7.6. I'd prefer the 1 1/2" myself if I had that much space to fill, to eliminate cutting and pasting. An important consideration in installing this stuff is to be sure there are no gaps around the edges (next to the studs, etc.) which would allow air to leak around the insulation.





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