Basement Wall Insulation
Hello! I know this topic has been covered countless times, but looking for some insight on my particular scenario.
We purchased a home in New Hampshire (around the zone 6/7 line) with a partially finished basement; the walls were in really rough shape (old faux paneling and damaged drywall), so we’re removing the old and plan to hang new drywall. They had previously used fiberglass to insulate, but I’m wanting to do it the right way while we have the walls open. My plan was to adhere polyiso (or XPS or EPS, my mind seems to change with every post I read on here) and then fill the 2×4 stud bays with mineral wool. Pardon my ignorance, but the questions I have are:
1. Which rigid foam is ‘best’ for this application?
2. It’s going to be challenging to snake boards behind the existing walls without removing them, and the gap isn’t huge (about 1.5″ if I’m lucky). Would it be dumb to simply use mineral wool alone instead? Or perhaps in the few areas where the gap is too narrow? (Or perhaps foam board alone between the stud bays / glued to the foundation where the gap is too narrow?)
Appreciate anyone’s insight on this!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
1. You won't find a consensus, as each type of foam has pros and cons. I always spec foil-faced polyiso as it has the highest R/in, it has relatively low embodied carbon for a foam product, and I don't want any inward drying. But others make good cases for allowing some inward drying, as you would get with XPS or EPS.
With a foam layer, the choice of cavity insulation doesn't matter, in my opinion. Again, others will disagree, but once you have a relatively airtight cavity and condensation control from the foam, fiberglass batts would work just fine. You can use mineral wool for slightly higher R/in but also significantly higher levels of embodied carbon. Cellulose or wood fiber would be my first choices if available in your market, but they often come at a premium cost and in that assembly they don't have a performance advantage.
Skipping the foam layer might work but it's risky. I always include foam in basements and crawlspaces. 1.5" is the bare minimum; if you can't meet that, consider removing the wall framing.
Thanks for the response! In some areas where I might not have space for foam + stud bay insulation, I may have room for 2 layers of foam since it takes up less space - would layering in this manner reduce or eliminate some of the performance properties of the foam?
Also, I've read varying opinions re: the bottom of foam boards making contact with the slab (e.g. to cut or not cut) - not sure if you have a thought on that?
Thanks for your time!