Rigid foam Install in Basement
Yup, you caught me. There are some things about code that are great and wonderful, and others that aren't and todays code for basement insulation is horrible. They make it very hard to do properly AND prevent mold/mildew. The most important thing about basements is moisture control, not insulating. What good is having R13 or R38 basement wall that's got moisture build-up and moldy? That wall will certainly exceed code... you'll likely be ripping it out later for moisture issues. Code mostly treats basements like any other floor, when it shouldn't. I'd rather have R10 basement walls that are dry and let my foundation breath/dry preventing moisture issues than R38 that's wet and causes me to throw everything out in my basement and I mean everything. By code, he can install R15 batts in the stud wall and call it a day... he only needs R11 that is much better than code. It sure won't insulate or perform like R10 foam, sure sounds like he will have to rip it later too from water & mildew issues, and when the fiberglass gets saturated with water it won't be R15 anymore, but that wall exceeds code and what I have found professionals often do because it's cheap and easy and meets code. Code requires a barrier for any insulation whose perm is under 1. Put up a stud wall, insulate in between the studs (with fiberglass, mineral wool, or cellulose), put a vapor barrier up, and then drywall over that. Frequently done, meets code, but the #1 design to create a studwall that will look like a science experiment gone bad. You might as well sealed your studwall & insulation in a well sealed plastic bag and throw water into it on occasion there ain't nothing good that can come of it but it meets code and code is a minimum. Where in code does it explain fiberglass should not be installed in a basement or when it should? That the bottoms of basement walls can't dry to the exterior so need to dry to the interior and preventing that has known to cause issues (so bottoms of basement walls should not be covered with polyiso but the tops are fine)? Or that wall paper is a barrier so should be banned from being installed in a basement... any pro will install wallpaper in my basement if I ask and I'll likely be ripping it all out later because of moisture issues that will ensue. Likewise basement floors also should dry to the interior and if you put vinyl/laminate over a plywood subfloor you've just created a barrier and your plywood will be soaking but every pro I know will happily install vinyl over my plywood subfloor in my basement without mentioning that.
He can install the 2" of polyisocyanurate making R12 for the whole thing and that will meet code (the dead air space adds R1 = R13) ... and risk moisture problems not letting the bottoms of the foundation dry. But, this option meets code.
Or 3" of XPS foam, but that is too thick he's now in the situation he's putting more emphasis on insulating vs. the risk of moisture problems (3"-4" is actually okay in parts of Alaska).
If you're that worried use 2" XPS foam, you are putting a slight emphasis on insulating vs. moisture control and the dead air space adds R1 to the deal (you have to seal it for fire drafting reasons anyway) and every building inspector I know calculates dead air space as R1 = R11 which is what you said. If you need R13 like Bob said, I think your best option is to insulate the wall with 1.5" XPS with all seams & holes taped (just a pinhole can let in up to a cup of water/season) and fill in the stud walls with unfaced mineral wool. Unfaced because you have to let the foundation breath, and the foam will move the condensation point inside it. Not something I prefer over just foam. I hate fiberglass/mineral wool in any foundation stud wall but the most important thing is XPS against the foundation (or polyiso on top and XPS on the lower) and then insulate the stud wall how you see fit just no vapor barriers, any moisture vapor that comes through the foundation you gotta let it into the living space. Make sure you use a dehumidifier.
Last edited by Piedmont; 12-15-2009 at 04:15 PM.