Polyiso with asphalt backing

A homeowner planning to insulate basement walls and a slab asked about a cheap rigid foam board found on Facebook Marketplace: polyiso insulation with an asphalt facing. The question was whether 2 inches of this material would block moisture vapor from passing through, since the basement has stayed dry for ten years.

An experienced reply explained that the asphalt-faced type is likely similar to a kraft-paper or fiberglass facing, which lets some vapor through rather than blocking it. Polyiso foam itself, at 2 inches thick, only slows vapor slightly rather than stopping it — it's the facing on the foam, not the foam itself, that usually determines whether a board acts as a true vapor barrier, and foil-faced polyiso is the version that blocks vapor. For someone who wants a real vapor barrier on a basement wall, plastic sheeting (6-mil poly) installed against the masonry, with the foam board anchored on top through the plastic, was suggested as a straightforward option. Taping the seams between foam boards helps stop air from sneaking behind the insulation and condensing against the cold wall.

The reply also addressed a common worry: sealing a basement wall completely with a vapor barrier is fine, and the wall does not need to "breathe." The one thing to avoid is trapping air-permeable insulation, like fiberglass batts, between the wall and the vapor barrier, since that combination can lead to moisture problems.