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.