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Water and Polyiso Foam Sheathing

If I were to use polyiso sheets to insulate a basement wall and the basement eventually flooded, what would happen to the polyiso?

Also, I'm considering using polyiso sheathing on a new house for the 1st time instead of XPS. What happens to the edges of the polyiso when exposed to rain (potentially heavy rain or days of rain) while under construction?

Bruce Miller

Asked by Bruce Miller
Posted Nov 12, 2012 9:08 PM ET


3 Answers

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I imagine that if the basement ever floods, you would have to evaluate the damp building materials (including the polyiso) on a case-by-case basis. If the flooding were shallow and brief, the polyiso might dry out would work fine. If the flooding were deep and prolonged, everything goes in the dumpster.

As far as rain exposure during construction is concerned, use common sense. If the house has wide roof overhangs, everything should probably be fine.

The most vulnerable part of the polyiso is the exposed edge that has no foil facing. If you are leaving such an edge exposed to the rain on a side of the house with stingy roof overhangs, and you have to leave the jobsite overnight when rain is predicted, just staple up a scrap piece of housewrap to protect the exposed top edge. That's just common sense.

Answered by Martin Holladay
Posted Nov 13, 2012 7:56 AM ET


It's hard to leave it in the rain long enough to matter, but if you float in in a pond for a week before putting it up on the siding you might want to toss any that are noticeably heavier than the rest. :-) Some builders will use FSK tape on the top edge as a flashing and tape seams as they go, which is pretty protective of foil-faced iso. While iso is somewhat hygroscopic, it's not like a sponge- it takes some real time and soaking to get it to take on much water.

When installing it in the basement it's good practice to leave an air gap between the exposed bottom edge and the slab to avoid slow wicking of curing moisture (or ground moisture) into the iso. In an extended flooding situation basically anything below the high-tide mark should be considered dumpster fodder- you'd want to be able to inspect and decontaminate the foundation up to the high-tide mark anyway.

Answered by Dana Dorsett
Posted Nov 13, 2012 4:52 PM ET


Thanks Martin and Dana. I talked to a Hunter Panel technician today and he echoed Dana's comments. He told me it was not necessary to cover the edges during the framing stages no matter how much rain fell because the polyiso just would not absorb a material amount of water in a vertical wall application. If the Hunter technician told me something that is untrue, let me know.

Answered by Bruce Miller
Posted Nov 13, 2012 7:09 PM ET

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