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Old 02-28-2012, 12:57 PM   #1
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Insulate unfinished basement walls?


We just recently finished a large addition/remodel to our home....we went from roughly 900 sq ft to 1,800 sq ft. We had a full basement under the old section and put a basement under the new addition. The two basement sections are connected by an 8' opening. The old basement section only had roughly 7'6" ceiling. In the new basement section we went with 8' ceiling so that we could finish this new section in the future. We don't plan on finishing the old section due to the low ceilings. I do however what to "tighten" things up by insulating the outer basement walls in this old section. I have seen in some homes where the outer walls were insulated below grade with some kind of insulation that had a "reflective" backing and it was just stapled/nailed to the wall. We spend a lot of time in our basement and want to make it as comfortable as possible. My questions: Is this a waste of money/time? If we do this is this the best type of insulating to do? I am open to suggestions/comments. thanks in advance for your responses.

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Old 02-28-2012, 01:02 PM   #2
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Insulate unfinished basement walls?


Since the ground temp. drops the deeper you go, the deeper insulation should be thicker. There is an online graph of this temp. vs. distance but of course I lost the link.

For payback periods of more than 10 years you probably shouldn't do it.

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Old 02-28-2012, 01:28 PM   #3
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Insulate unfinished basement walls?


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Originally Posted by Yoyizit View Post
Since the ground temp. drops the deeper you go, the deeper insulation should be thicker. There is an online graph of this temp. vs. distance but of course I lost the link.

For payback periods of more than 10 years you probably shouldn't do it.


.... Your best course of action (with bang-for the -buck in mind) would be to airseal the rim joist/sill boxes, then add rigid insulation board on the TOP 4' of the wall. You could go all the way down, but with cost in mind, the top 4' is the most effective. If you are not going to finish the space you'll need to keep fire rating in mind and use a product like thermax that does not need sheetrock over it.
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Old 02-28-2012, 01:36 PM   #4
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Insulate unfinished basement walls?


Air seal the basement first. Can foam all the wiring/plumbing holes to prevent feeding the "stack effect": http://www.wag-aic.org/1999/WAG_99_baker.pdf

Insulate/air seal the rim joists, a major source of air- both in and outgoing:http://www.buildingscience.com/docum...l_seal_rev.pdf

Insulate with foil-faced foamboard if your local AHJ will allow uncovered: http://www.quadlock.com/technical_li...Insulation.pdf

Pick an insulation system to your liking/budget, pp. 69 on Btu's: http://www.buildingscience.com/docum...study-analysis

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