Air sealing dryer vent (insulating basement) without setting house on fire

[Mr.IncrediblyBored]

I think XPS is acceptable, just make sure to use tuck tape between all the seams. Though that does draw another issue, on it's own it does not have enough R value, so you'd want to put batts over it. That would then put the vapour barrier behind the batts. The R value of the foam might be enough to counter the moisture though. One thing some people do is do the floors and walls, that way you can seal the floors too, then you lay a plywood subfloor over the XPS. Everything is all taped together creating a nice sealed envelope.

I've also read elsewhere that rigid foam is not considered a vapour barrier, but first I hear of that myself so not sure how true it is, as even the shows like Holmes etc do sometimes use it.

From what I could gather, xps is a sufficient vapor barrier when it's at least 1.5" thick -- 2" recommended. I taped the seams and spray-foamed the edges.