How much to worry about moisture from air leaks?
I live in zone 4A (virginia). We live in a house built in 1972, so overall it’s not super efficient. I’m redoing a finished basement and adding insulation (it doesn’t have any currently), and I already have a plan for most of it. I’m going to use foamboard insulation on the walls.
I’m trying to figure out what to do at the top, however. The foamboards will be 8′ high, but there are 2×10 joists so there is ~9″ between the top of the foamboard and subfloor. There’s also a lot of piping, wiring, and more so it’s not simple to throwing some foam boards up there (can’t use caulk to seal up the seams).
Two of the walls are exterior (not underground, the ground slopes down so these walls face open air). One of them is the garage wall so the air seal is even worse; there’s no siding or barrier at all. I can feel a draft.
I could stuff fiberglass insulation all up in there, in addition to using a can of spray foam to seal any open holes. However, there will be no vapor barrier for those 10″ across the entire wall. It’s very humid here in the summer, should I be concerned about humid air coming in? Even if I sealed all the holes, humid air would find a way in (wood or concrete is not a vapor barrier). Won’t that humid air cool down as it comes in and condense? Would fiberglass insulation without any barrier eventually break down due to moisture problems?
Summed up my question is: is fiberglass insulation always doomed if humid air gets in, or is a little bit OK? I’m considering getting a Froth Pak 200 and spraying the whole top area with 2″ of foam to form a vapor barrier.
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