R-13 or R-21, that is the question??


Old 03-27-10, 06:13 AM
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Join Date: Feb 2008
Location: PA, USA
Posts: 28
R-13 or R-21, that is the question??

Hey all.

Reached the final hurdle of our insulation 'improvement', and for the final phase, I want to replace all the old wall insulation in a livable attic space which has either collapsed or been destroyed by critters (mice have way too much time on their hands!!!).

This is an older house, 1930's, so the studs are 2.75 inches deep which would only allow me to use R13 (a little compressed at that!). Is it worth me going to all the trouble of adding 2 x 4 'extensions' to each stud to allow me to gain enough depth for R21, or is the difference in the R values so negligible that all the effort is not worth it??

Would really appreciate any comments/help.

Thanks as always, these forums are a life saver!

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Old 03-27-10, 06:24 AM
Join Date: Oct 2008
Location: New England
Posts: 9,996
Hi Dellguy
If you will be heating that space, then the savings would justify increasing the insulation. Fiberglass insulation will function under minor compression, the loss occurs due to the reduced depth. A 2.75 inch wall would give you about 75% of the R-13 or R-10 and then you have to calculate the studs and EECH, you are down to R-7 or 8. Two inches of rigid foam with drywall over that is an option that uses less space and covers those studs. But padding them out works also.

Old 03-27-10, 06:58 AM
Tolyn Ironhand's Avatar
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Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 12,189
I know you can get a better R-value with a spray in foam in a smaller space. Have you looked into that? Prices around me are about $2 a square foot.

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