New answers tagged vapor-barrier
It is absolutely normal. What you do not want to see is standing water under it. I have seen moisture under my plastic all the times I visited my crawlspace, mold has never been a problem. The idea is though to have no organic material like wood, grass, anything else that would decay, trapped under the plastic.
It's not so much a vapor barrier that you need as an air barrier. The moisture is carried on air, so if you block off the basement walls from air movement, you've all but eliminated moisture transport. Vapor diffusion is dramatically overestimated, and besides, most of the foams are very vapor closed anyway. No need for an additional dedicated vapor barrier ...
From a building science perspective, the polystyrene itself is a suitable vapor barrier, if you seal the seams and edges appropriately.
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