Pier-and-Beam Construction: Insulating the Floor
Finalizing floor insulation details in Zone 2A, pier and beam, totally open crawlspace. Rim joists were insulated during framing. I have a lot of 3″ reclaimed felt-faced polyiso on hand and plan to cut&cobble it in the joist bays (w/thermal bridging, that’s a total assembly R-value of 14.1, code-compliant… and based on a Manual J that showed almost zero return beyond this thickness).
I need to air seal it, but no sealant or tape sticks reliably to this material. Many articles on GBA, like this one, recommend putting a continuous layer of rigid foam or plywood on the bottom of the joists. I don’t want to do that because it will prevent me from inspecting the joist bays for critters, leaks, mold, etc.
What if I ran a layer of *clear* 10-mil poly sheeting under the joists instead? Would the air gap between the foam and the sheeting be a problem? If so, I could instead run a strip of poly sheeting in each bay…
(Yes, it’s a vapor barrier, but so is the polyiso (to say nothing of the tile above the subfloor… which is a double-vapor-barrier problem I don’t know how to get around…).)
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