# Insulation question

• posted on September 29, 2008, 7:40 pm

I am insulating the ceiling in my garage. The ceiling is 2x8 16 on center with dead space above. I am using R-30 batts. Now, I know these will stick up about 2.5" and that's fine for now. But if someday I put a rough floor up there for storage and compress it, what's the downside if any? Does compression hurt the R value? It seems to me I would still be better off then using R-19.
Thanks, Jim
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• posted on September 29, 2008, 8:32 pm

For every inch you compress subtract the inch R value rating. If 10" is R35 than 9 will be R31.5
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• posted on September 30, 2008, 3:03 am
Yes, compression hurts the r value. AND if it's trusses, you can't put anything up there.
s

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• posted on September 30, 2008, 1:44 pm
On Sep 29, 11:03 pm, "Steve Barker DLT"

I'm missing the math, but this is a garage with 2x8 rafters spanning 19'. I will be putting R30 (9.5") up there and if I do put some plywood on top for a little storage it will compress it to 7.5". So it sounds like even with the compression I'm better off then the alternative of R19.
Am I off base?
-Jim
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• posted on September 30, 2008, 2:00 pm
No, you are correct. 7.5 inches is greater than 6.
steve
You still can't load the bottom chord of a truss however. Are they trusses, or conventional rafters and ceiling joists with support poles?
I'm missing the math, but this is a garage with 2x8 rafters spanning 19'. I will be putting R30 (9.5") up there and if I do put some plywood on top for a little storage it will compress it to 7.5". So it sounds like even with the compression I'm better off then the alternative of R19.
Am I off base?
-Jim
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• posted on September 30, 2008, 3:17 pm
On Sep 30, 10:00 am, "Steve Barker DLT"

They are conventional rafters and ceiling joists, 16" OC 19' span. I know I'm being thick, but given my example of compressing R30 by 2.5", what do I end up with?
-Jim
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• posted on September 30, 2008, 3:27 pm
23.625
arrived at that figure by dividing 30 by 9.5 and getting about 3.15 then divided 19 by 6 and got about 3.15 then multiplied 7.5 times 3.15
s

They are conventional rafters and ceiling joists, 16" OC 19' span. I know I'm being thick, but given my example of compressing R30 by 2.5", what do I end up with?
-Jim