Fixing Basement Insulation
We recently purchased a newer home in zone 5.5. The unfinished basement is insulated with fiberglass batts framed between 2×4 spaced 24″ apart. It has poly vapor barrier against the concrete, then it looks like R-19 unfaced batts, then another layer of poly vapor. The studs are spaced about 1″ away from the poured concrete walls. I was thinking of removing the vapor barriers and batts and insulating between the studs with 3″ XPS foam. The question is how best to do that? I was thinking of using a can of spray foam between the stud and wall (after removing the poly vapor barrier) and then cutting to fit and gluing the 3″ foam between the studs. What are your thoughts on this strategy? I don’t want to move the stud wall out further because the basement slab has radiant heat and I don’t want to take the chance. Is there a better way to do this in your opinion?
Thank you for your comments in advance.
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Just to edit the above post. I was going to glue the 3" foam board directly to the foundation wall. My original post wasn't clear on that.
I'm not sure how feasible this is, but I would think sliding 1" or 3/4" foam behind the studs would be better than spray foaming each stud.
XPS is the least green foam option. It's made with a gas that has a very high global warming potential, 1400X the effect of CO2. EPS and polyiso avoid that problem.
Thanks Charlie. I was investigating Thermax last night. That might be a better solution since I wouldn't require gypsum or other fire barrier immediately afterwards. Perhaps 1" of Thermax behind the stud wall with another 2" of Thermax between the studs? I could work on it in sections (one wall at a time) as a project.
I welcome any other thoughts on this!
If you make it into a finished space, most code officials will require gypsum over it regardless of thermax or other insulation, and will likely require it even if the space is not finished. But you can have pretty much anything exposed during the construction process. Any polyiso will be safer to have temporarily exposed than EPS or XPS, so you don't need to spend extra for thermax.
An advantage of layering it is that you can offset the seams to reduce the chances of air leaks. And polyiso with a foil facer is really easy to seal well with tape, so it's easy to make the whole thing air tight.
Given the fussiness and the better-than-nothing-but-not-as-good-as-it-could-be-ness of getting something behind the studs and the fussiness and mess of cutting and fitting foam (or is thermal available to fit your bays?) I'd remove the studs (notice how easy that makes it sound) do a clean taped and air sealed job with the poly-iso resting on the existing plate, and put up a 2x3 stud wall in front of it with its plate "toe nailed" to the existing plate. Winner of the longest sentence in Q&A? This would also give you an easy way to run wiring, if you want. Only you can evaluate the fussy vs efficiency quotient. Since I can't help myself, I'd stick a water friendly foam (not polyiso) behind the existing plate, if there's room.