Spray foam inside with foam board outside?
I have a wood structured home with fiberglass insulation in each bay, OSB sheathing, a 1” layer of foam board (on all areas that have siding on them, but not under brick facade), a house wrap (also only in sided areas), and a mix of cement board siding and brick facade. Home was originally built in the 1970s and was resided this year.
My question is, as I refinish interior rooms, what is and isn’t safe to do in terms of insulation? Can I tear it down to the studs, pull out the fiberglass, spray foam ~1” thick to seal and reinstall the fiberglass? Is it safe to sandwich the OSB between foam board and a vapor barrier on one side and spray foam on the other side? Is it safe to spray foam behind brick facade? Do I just need to get open-cell foam to avoid moisture issues or can I use closed-cell? Would open-cell still air seal?
The base fiberglass is only R11 and my exterior walls are only 4” studs. I’d really like to achieve a passive house standard – at least in terms of total energy use per square foot. So any advice on how to insulate best (recalling that we already dumped money into an exterior remodel) from here would be greatly appreciated. I am concerned if I do it wrong I could cause mold issues and would prefer to avoid this.
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It would be safer not to have the interior spray foam layer in terms of drying ability, especially if your exterior "foam board" is a vapor barrier (such as foil faced polyiso or poly faced EPS). The spray foam layer will limit drying ability of the OSB sheathing.
I'm not a fan of "flash and batt" in general, but especially in walls like this. Spray foam is expensive, not particularly "green", results in an irregular surface making it difficult to get a batt to fit properly, and adds an extra step in the construction process. On top of all of that, it doesn't really gain you all that much in many cases. What I would recommend here is to air seal the wall the "old fashioned way", using caulk and canned foam. I would then put in mineral wool batts, which will give you R15 in a 2x4 wall, which is what you appear to have here (or do you actually have full 4" depth studs?). You probably don't really need an interior side vapor retarder with 1" of exterior continuous insulation, but it depends on your climate zone and you didn't mention where you are. I would put in an interior side smart vapor retarder anyway for some extra insurance, but would use MemBrain since it's probably the least expensive option. You could also used kraft faced fiberglass batts, just be sure to get high density R15 batts. I would detail the interior drywall air tight too. This will give you a pretty robust wall assembly of somewhere around R19-R20 depending on the type of foam board you have on the exterior.
You're not going to get up to passivhaus levels here without doing a lot more to that wall. Shooting for "pretty good house" is probably a more realistic target here, and you will notice a difference.
Bill