What is the best way to seal under the subfloor in a crawlspace?

Question

My wife and I were about to install hardwood flooring, when we discovered that the carpet underlayment was trapping moisture in our subfloor.  The crawlspace underneath the room has had a moisture problem for a while that I had thought I'd taken care of (more or less) but would still see moisture gathering down there after a particularly heavy rain.  To solve this, we installed a french drain that appears to have done the trick.  

Now that the moisture levels are back to normal, we want to ensure that we don't install these beautiful harwood floors, only to have a spike in moisture and cause our floor boards to cup.  Is there a way to seal the subfloor from underneath??

i_was_myself · Answer

Rigid Foam board would probably work the best. Either mount it to the underside of the joists with glue and the appropriate fasteners or cut the board to fit between the joists, but leave a quarter inch gap on both sides, then spray a bead of spray foam to fill the gap. No less than 1.5 inches, but you can use the cheap foam board for this.

fenx · Answer

You may have done this already, but make sure you put down a good thick sheet of plastic over the entire floor of the crawl space.  This will really help keep moisture down.

Anonymous · Answer

Uh,possibly a raccoon or critter,i comprehend there are some snow tires decrease than the circulate slowly area and probably a team of spiders..as quickly as I moved in right here i found 50 lb boxes decrease than the circulate slowly area with rice,beans,flour and sugar..the folk residing right here until now stored them for some form of non secular reason,i've got because thrown them away.

John M · Answer

Lay 6 mil plastic over the floor and then lay 1/4 inch Luan plywood then prepare your floor.

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What is the best way to seal under the subfloor in a crawlspace?

4 Answers