Basement insulation
DenmarkDave
| Posted in General Questions on
Hi GBA. I am in Denmark ME and have a question about insulating my basement. The house was built with 1″ of XPS insulation against the inside foundation wall in the basement. A 2 x 4 stud wall was built pinning the foam board to the foundation. I would like to avoid ripping out the studs to add more insulation. Can mineral wool be added to the stud cavity without causing other problems like condensation?
thanks
Dave
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Replies
Dave, going by prescriptive ratios, and assuming the seams in the XPS are taped, you could install up to R-10 batts with minimal condensation risk. Standard 3 1/2" batts could result in moisture accumulation. It would be safer to have closed cell foam (HFO-blown for greatly reduced environmental impact) sprayed between studs, or you could install rigid foam (preferably recycled) and seal them in a cut-and-cobble approach.
Installing HFO blown foam between the studs is an expensive waste, but it would work from a moisture control point of view. Keep it to an inch (not more) and it would then be fine to install R8 soundproofing batts (eg Johns Manville Sound-Shield or it's Owens Corning competition) , which are nominally R8 when compressed to 2.5".
It's often possible to just bang out the studs and rotate them on the stud plate without re-cutting, which leaves just enough room for 2" of foam board between the flat side of the stud and the original foam. With R12 foil faced polyiso and an air gap to the gypsum board the total stackup would perform at about code-min. or slightly better. If the 1.5" deep cavities are filled with compressed R8s or split R13/R15 batts it'll do a bit better than code.
If sliding in 2" foam board behind the turned studs is too awkward (often is), backing off to 1.5" foam (R9) and installing R8 compressed to 2" would also get you to code-min performance with margin.
The turned stud approach does sometimes lead to wall-flatness issues, so pay attention. If a stud is a bit bowed it's usually better to put the convex side facing the room, not the foundation. If it's bowed badly, turning the concave side toward the room and installing blocking & shims behind it to straighten it up can work.
Thanks for the suggestions. Turning the studs is a great idea - that will work well for the utility area without making it smaller.
thanks again
Dave