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I'm renovating my bathroom and I need to insulate the floor. It's a concrete slab with 2x4 joists set on one brick piers.

The home was built 50 yrs ago.

There was no insulation at all in the floor. And the bathroom is freezing in the winter and hot in the summer.

If I slide in pieces of rigid foam board will I have problems with condensation rotting out the 2x4s?

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Your plan is what most people & contractors do. But, latest basement testing has revealed that even with foam board or spray-in foam you need 6-mil poly against the concrete & then also on top of it. Under-it stops moisture & air from wanting to come in & On-it stops moisture & air from wanting to penetrate it. Here's a really good video breakdown I recently ran across of what's been determined.

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When its cold below the floor, the warm most air from the bathroom is include to travel in that direction. If anything, cold air will enter from cracks in the floor connections and enter the room - keeping that area dry.

My first reaction was also to use rigid foam insulation. Placing a layer of tyvek or plastic sheeting over the foam and joists, under the subfloor would fully eliminate any chance of a problem.

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