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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?

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  • Have you installed a vapor barrier over the top of the ground in the crawlspace?
    – RetiredATC
    Commented Aug 2, 2022 at 18:55
  • Yes, there was one there but it wasn't covering some edges and bunched up other places. I replaced it with some 15mil and maybe this year I will get some fans in there.
    – Rick Tilley
    Commented Sep 21, 2022 at 23:33

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The best thing you can do is provide adequate ventilation for your crawl space. I do not know your area but I had a similar problem many years ago in Ohio. We cut several ventilation holes in the block for the crawl space and the problem went away as the contractor said it would.

I would suggest insulating the coolant lines, it takes a lot of energy to condense vapor to water especially as warm as it is there. That will pay for itself in a relative short time. You could also place a small fan to force air through when it gets humid out. A simple humidistat would control it for you. Extra benefit the AC will work better.

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  • I'm in Florida. There are vents but circulation can be improved. I put a new vapor barrier in and removed some of the vent obstructions. I still see water pooled a bit under the coolant lines. Condensation on the lines was pretty bad this summer. Still things are fairly dry considering.
    – Rick Tilley
    Commented Sep 21, 2022 at 23:35

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