Does insulation under the flooring + a sealed floor = vapor sandwich?
I have a pier & beam house in Austin with cold floors. I’m considering insulating the underside of the flooring, by either stapling polyiso foamboard across the floor joists and then sealing the joints with tape, or else having closed-cell foam sprayed into the joist bays. If I then seal the hardwood flooring, will that create the dreaded “vapor sandwich”, because I’d be locking in moisture from both sides? If so, can I proceed anyway if I run a dehumidifier in the living space and test the moisture content of the wood before sealing?
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