Insulating Basement Walls


Here's another challenge I'm facing with my basement walls I'm hoping to get help with.
One of the stud walls in my basement (2x4 construction) has been placed offset from the cement wall so the face is about 4.5"-5" from the wall (to accomodate a vent pipe for the kitchen). I assume I should use 3.5" insulation intended for 2x4 construction rather than thicker insulation intended for 2x6 construction. My thinking is using the thicker insulation will cause it to be compressed and lose it's insulating qualities.
This means the wall is about 1-1.5" bigger than the 3.5" thick insulation. Should the insulation be placed up against the cement wall leaving an air gap between the insulation and vapour barrier, or should the insulation be placed flush with the 2x4's leaving a gap between the cement wall and the insulation?
Much appreciation for the insights and suggestions.
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lots of reasearch already done for you at: http://www.buildingscience.com/resources/foundations/default.htm then see mfg website to localize this to you: http://www.owenscorning.com /
snipped-for-privacy@glomar-group.com wrote:

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buffalobill wrote:

Which near as I can tell, gives no advised solutions for insulating basement walls on the interior side, not very helpful for existing construction.
Worth reading, though, as in humid areas particularly, vapor barriers on basement walls can do very bad things. Not thinking about having not had a previous basement that was finished did the vapor barrier on the wall in TN and on the fully underground wall it did end up w/ very bad moisture problems and failures in a very short time...
I don't really know what would be best solution overall, but agree w/ someone else that the air space behind is probably a better idea than mashing the 6" into the cavity.
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Read through it and have similar situation. My answer that seems to fit the bill is to use spray foam (they recommended rigid). Tigerfoam.com sells it for DIY.
dpb wrote:

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buffalobill wrote:

Thanks for all the feedback. The info on buildingscience.com basically said if the insulation will wick moisture then place it away from the wall. If the insulation will resist moisture then place it against the wall.
I am using Roxul Flexibatt insulation (http://www.roxul.com/sw47757.asp ) which is moisture resistant (made of basalt and slag) . The installation tips even say to place it flush against the wall.
This is great stuff to work with - easy to cut, easy to handle and install, little to no itching ... and R14 for 2x4 walls.
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snipped-for-privacy@glomar-group.com wrote:

You want to use bats sized for your 2x4 wall. Ordinarily I dont think it hurts much to compress a thicker bat slightly. In this case it can be undesirable however since it will cause the bats to touch the basement walls where they could pick up moisture. Better to have a gap between the bats and the wall so that any water on the wall can have a chance to dry out before it migrates to the wall. There are other materials which might do a better job like blown in foam.
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I had the same situation when I did my basement years ago. I left a gap between the insulation and wall. It provides better air circulation.
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regular insulation like fiberglass becomes ineffective if it gets moist:(
closed cell foam is much better either pre made sheets, or spray foam, which has better R value since it fills all the nooks and crannies.
also wonderful sound insulator
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