Saw a crew in Nashville using foam underlayment on a slab last Thursday

The GC was all about it for sound control but I'm still sketched out about moisture trapping under there, anyone dealt with that on a concrete pour?

2 comments

2 Comments

eric_carr26d agoMost Upvoted

Wait they put foam UNDER the concrete pour? Like right on top of the slab before the fresh concrete went down? That is absolutely BONKERS to me. Foam is basically a sponge for moisture vapor and trapping that against a curing slab is asking for mold and delamination nightmares down the road. I've seen too many floors bubble up and peel because of trapped moisture under the finish. That GC better have a really good vapor barrier plan because that sounds like a lawsuit waiting to happen once that floor starts failing in a year or two.

the_mia25d ago

Hang on though, isn't closed-cell foam basically its own vapor barrier if you tape the seams right? I've seen guys use that rigid polyiso board under slabs for years, especially in cold climates where you need the insulation value. The key is making sure you have that vapor retarder on top of the foam, not underneath it. If they laid it right and kept the foam dry before the pour, the concrete curing off-gasses but the foam isn't gonna wick that moisture up like a sponge. It's more about how they handled the joints and if the concrete mix had enough low water content to not bleed into anything. Plenty of commercial jobs use this setup for radiant heated floors too, it's not automatically a disaster.