Thanks Jon and Mike for your comments. I think I'll abandon the idea of having celotex under the siding. Moisture problems seem inevitable. One thing I didn't mention is that I have an exterior wall with celotex already attached (over osb and tyvek). Rather than trash it, I'd like to salvage it and thought it could reused by installing it under new drywall in the interior of the house (all new drywall required in all perimeter walls). Any opinions about putting celotex under drywall? It seems like it should be OK since it can act as a vapor barrior. ...On the topic of vapor barriors, I've read that a high quality primer paint can act as an adequate vapor barrior.. what do you think? Thanks, Genevieve