I live in Minnesota where the winters are dry and cold (many nights well below zero) and summers can be hot and humid. I've got a 4-level split in which I'm finishing the level that's half-underground. The ceiling above is floor trusses and there are 2' cantilevers jutting out from about half of the upper level. My question is, how do I install a vapor barrier in those areas? I've got 23"x5-1/2" insulation lying in that cavity created by the cantilever now, which seems fine, but I can't figure out how to create or install a vapor barrier. I could use faced insulation, but to tack down the edges I'd have to really compress the insulation which would lessen its insulating properties. I thought of using some 1-1/2" thick (to match the 2x4 truss lumber) rigid foam insulation (with vapor barrier) placed between the trusses and caulking the seams, but that's a ton of work and wouldn't provide enough R-value on its own.
Any suggestions? Is the vapor barrier crucial? There hasn't been one there for seven years since the house was built, but now I'll be closing off the ceiling and thereby reducing air circulation in that area, I don't want mold and mildew growing.