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Old 06-29-2006, 02:42 PM   #1
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Insulating 3" Wall Cavity


What would be your insulation of choice for a wall framed with 3" rough sawn studs (i.e., the precise width of the studs varies from two and seven eighths to three inches)?

R-11 fibergalss batts are designed for walls at least three and half inches deep. If the loss of insulating capacity isn't great, I suppose the most cost effective option is to compress the fiberglass batts some.

The other option would be rigid insulation either two and half or two and three quarter inches thick. I could be wrong, but I'm guessing that the R value for even two and three quarter inches of polystyrene or similar rigid insulation won't exceed R-11 by much, if at all. If that's true, is there any other good reason to use rigid insulation in this situation?

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Old 06-29-2006, 05:06 PM   #2
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Insulating 3" Wall Cavity


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Originally Posted by Riddler
What would be your insulation of choice for a wall framed with 3" rough sawn studs (i.e., the precise width of the studs varies from two and seven eighths to three inches)?

R-11 fibergalss batts are designed for walls at least three and half inches deep. If the loss of insulating capacity isn't great, I suppose the most cost effective option is to compress the fiberglass batts some.

The other option would be rigid insulation either two and half or two and three quarter inches thick. I could be wrong, but I'm guessing that the R value for even two and three quarter inches of polystyrene or similar rigid insulation won't exceed R-11 by much, if at all. If that's true, is there any other good reason to use rigid insulation in this situation?
Foam insulations rarely make sense in a cost-to-benefit ratio in most applications.

For yours, you can achieve a R-11 or higher by putting either R-13 compressed fiberglass into the cavity space or using high density R-15 fiberglass.

Although you could achieve an R-21 with 3 inches of some foam insulations, the cost will remain excessive and take many years to pay off before you see any actual energy savings.

Fiberglass remains the cheapest insulating material at the best price in most cases.

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Old 06-29-2006, 05:56 PM   #3
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Insulating 3" Wall Cavity


Thanks Manhattan, I had not come across high density R-15 batts for 2" x 4" stud walls, but I'll look into it now. You're quite right, installing this type of product in a slightly shallower space would likely yield decent performance without the higher costs associated with foam board.
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Old 06-30-2006, 01:14 PM   #4
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Insulating 3" Wall Cavity


You can special order high density fiberglass from places like Lowes and Home Depot. It costs a lot more than standard fiberglass but far less than foams.
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Old 06-30-2006, 10:24 PM   #5
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Insulating 3" Wall Cavity


It also depends on where you live. In the northern climates an R-11 or 15 isn't alot. I am assuming these are exterior walls? When you compress a fiberglass batt it loses some of it's insulating capacity. If you sqeeze to large of a batt in there you'll lose the benefit of the increased R value.
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Old 07-01-2006, 07:13 AM   #6
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Insulating 3" Wall Cavity


DaveH is correct.

Compressing fiberglass will lessen its ability to insulate and therefore lower its R-value.

If you use a high density R-15 fiberglass in a 3" cavity, one would end up with about an R-11. It's not much but may be adequate for even cold locations if additional insulation is added in floors, attics, or on exterior walls if practical.

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