# R-value derating for compressed insulation

• posted on September 6, 2004, 8:08 pm
Any formulas available? I have 41/2" wall cavities in an old house. I don't want to furr out and deal with all the windows and door trim extensions. Will I get more R value using 3 1/2 (r-11) with a 1" space or should I compress in 6".
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• posted on September 6, 2004, 9:17 pm
The 3 1/2" Owens Corning insulation I have is R-13, not R-11. The R-19 is 6 1/2" thick (I'm using 6" studs), so you'd have to compress it even more.
MB
On 09/06/04 04:08 pm calhoun put fingers to keyboard and launched the following message into cyberspace:

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• posted on September 7, 2004, 5:59 pm
Correction: make that 6 1/4" for the R-19, not 6 1/2".
MB
I wrote:

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• posted on September 7, 2004, 8:48 pm

Use the next size higher than the available wall space. Actually, strike that. Stuff in 2" of foam, and then squeeze in 3 1/2" insulation..
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• posted on September 7, 2004, 10:45 pm

In fact, you may gain insulation value from a slight compression of batt insulation. A couple of years ago, I asked Journal of Light Construction.
Owens Corning lists these values & thicknesses:
R30 9 1/2" R21 5 1/2" R19 6 1/4" R15 3 1/2" R13 3 1/2" R11 3 1/2"
TB