Tried a foam foundation pour for a slab, the inspector was not amused

Back in 2019 I had a client near Columbus who wanted a bonus room over the garage, but the floor joists were already in. I read about using rigid foam as a permanent form for a lightweight concrete pour, thought it would save us time and money. We laid the foam, poured the mix, and it looked great for two days. Then the county inspector showed up and told us the foam was acting as a vapor barrier in all the wrong places, trapping moisture right against the wood framing. We had to tear out the whole pour and redo it with regular plywood forms, cost an extra $3,400 and three weeks. I learned that a shortcut with foundation work always comes back to bite you, but I still wonder if someone else has pulled this off without getting flagged.

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angelac631mo ago

Kept reading about rigid foam as a permanent form and I get why it seems like a win, but it usually only works for slabs on grade where the foam sits under the whole thing and never touches framing. Your case was different because the joists were already in, so the foam had nowhere to go but right up against the wood, and that is what trapped the moisture. Even on grade pours, most code guys want a real vapor barrier under the foam and the foam rated for ground contact, not just any sheet you grab at the yard. So the inspector was not just being picky, he was flagging a real problem with how the foam was being used.

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