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.

lots of reasearch already done for you at:
http://www.buildingscience.com/resou...ns/default.htm
then see mfg website to localize this to you:
http://www.owenscorning.com/

wrote:
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.

wrote:
Here's another challenge I'm facing with my basement walls I'm hoping
to get help with.

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?

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.

buffalobill wrote:
lots of reasearch already done for you at:
http://www.buildingscience.com/resou...ns/default.htm
....

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.

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:
buffalobill wrote:
lots of reasearch already done for you at:
http://www.buildingscience.com/resou...ns/default.htm
...

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

buffalobill wrote:
lots of reasearch already done for you at:
http://www.buildingscience.com/resou...ns/default.htm
then see mfg website to localize this to you:
http://www.owenscorning.com/


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.