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Old 09-08-2009, 05:13 PM   #1
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Basement Insulation R Value


I am in the middle of a basement finishing project. My basement is dry and has only had a minute amount of leakage over the years, which was eliminated by sloping the earth away from the house. My city-approved plan calls for stud perimeter walls 1 inch out from the concrete basement walls. I applied 2 coats of Drylok, per City's reqirement. The City is also requiring R-21 insulation between the studs. My problem is that R-21 fiberglass insulation is 5-1/2" thick and the distance from the front of studs to the cement wall is 4-1/2". So, when I install the insulation, I will have to compress it 1 inch for it to fit. I believe this reduces the R-value. Anybody have any input or suggestions, or am I going to be okay?

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Old 09-08-2009, 05:58 PM   #2
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Basement Insulation R Value


You don't say where you are located, but you might want to check out the ORNL insulation web site (Oak Ridge National Laboratory). They have a calculator where you plug in the type of house you are in, and they recommend an insulation value. For a basement in my zone (Northeast US), the recommendation is R11 for an interior basement application. Of course, your recommended value might be different, and your local code controls anyway.

I see a problem for you. Given that you only have 4 1/2 inches of space, you cannot get fiberglass to make an R21 value, and I don't think foam works either. Not to mention that you are not covering the studs, so you will get some thermal bridging through the studs, reducing the amount of R value further. You should probably ask the city what they recommend, it might be that they are effectively requiring 2x6 stud wall construction, which seems like overkill for a basement. As you noted, compressing a batt of fiberglass that normally requires 5 1/2 inches to 4 1/2 inches will reduce the R value proportionally.

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Old 09-08-2009, 06:21 PM   #3
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Basement Insulation R Value


Compressing the insulation does reduce the R value
In addition the 1" gap they required is NOT to be filled with insulation
That is to be free air to allow the cement to breathe water vapor & dissipate

With fiberglass the best you can do us R15 in 3.5"
R21 requires a 2x6 wall

R21 in your wall will only give R15 - no gain over R15 fiberglass batts

Why do they want R21?
Is the basement above ground?
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Old 09-08-2009, 07:19 PM   #4
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Basement Insulation R Value


Only the upper 2 feet is above ground, the rest is below grade.
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Old 09-08-2009, 07:21 PM   #5
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Basement Insulation R Value


I agree that this seems like overkill. We live in the northwest and generally have pretty mild winters compared to the northeast. Also, the basement is often warmer than other parts of the house in the winter due to heat radiated off the ducts. I am tempted to take the stance that since the bldg codes people approved the plan for a 1" gap, it was THEIR mistake from the get go.
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Old 09-08-2009, 08:31 PM   #6
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Basement Insulation R Value


Add 1/2" ply furring to the studs and install 4 sheets of this: http://www.lowes.com/lowes/lkn?actio...065&lpage=none No vapor barrier, I hope.
Be safe, G
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Old 09-08-2009, 10:24 PM   #7
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Basement Insulation R Value


Quote:
Originally Posted by tjeieio View Post
I am tempted to take the stance that since the bldg codes people approved the plan for a 1" gap, it was THEIR mistake from the get go.
Yup, and when they make you tear the insulation out what will you do?
That is what our Inspector would do
A gap is exactly that - a gap - not to be filled
It would not be THEIR mistake, it would be YOUR mistake to properly understand & build/insulate properly

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