I've witnessed first hand that a strong vapor pull from a crawlspace up through the subfloor can happen when there is a big temperature difference. In this case a single room was cooled to 68 degrees all summer in Florida. The rest of the house was cooled to 75. Vapor drive over two years caused the MDF underlayment to rot and the plywood floor underneath started growing mold.
I have read that an interior vapor barrier is not recommended in hot humid climates. Vinyl flooring is basically an interior vapor barrier, there is no getting around that. The ideal is that the floor can dry to the outside and to the inside.
Obviously MDF is a bad idea. But what techniques can be used to floor vinyl plank that does not trap moisture between the subfloor and vinyl plank? I do not want to insulate underneath because it would hide any mold growth.
I am already addressing sources of moisture and improving the vapor barrier. Currently I am considering using two layers of plywood subfloor:
- The original 1/2" plywood and replacing 3/4" MDF with 3/4" plywood.
- Then stapling a layer of 30lb asphalt roofing felt, and
- Then staggering a layer of 15lb asphalt roofing felt on top of that.
- Then either waterproof laminate plank or vinyl plank.
My idea for drying to the interior is, vapor can escape out at the edges of the room where the baseboards are. I would try to seal all air flow but may leave a couple vent holes.
What improvements can I make to better handle any vapor that ends up under the plank flooring? Or are there waterproof flooring alternatives I should be considering?