Insulating Basement Walls

Should the insulation be placed up against the cement wall

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.

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.

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.

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

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

This is great stuff to work with - easy to cut, easy to handle and install, little to no itching ... and R14 for 2x4 walls.