Mold Issue Subfloor

6 years ago

I had decided to replace the vinyl wood flooring in the master bedroom this weekend. Paint, molding, etc.

Upon removing the old vinyl flooring, immediately noticed dark spots on the foam underlay. Removed that and instantly the smell of mold hit me. There are two very bad spots of mold on what I assumed was the subfloor as well as two lighter spots.

We paused, sealed off the room, masked up and opened the window in that room to get fresh air. Using a shop vac to catch the dust, we began tearing out the first 4x8 sheet of subfloor to find a bigger mess.

Evidently the previous owners had a leak in the main bathroom adjacent to the master. They had disclosed this. The entire bathroom had to be gutted to the floor studs and a new tub, surround etc replaced.

What seems to have happened was the moisture seeped thru to the subfloor in the master. But, instead of replacing the damp, weakened subfloor in the master... They laid 1/4 sheets of plywood on top of the subfloor and installed the new vinyl.

In reality there is 3/4 subfloor, two separate layers of 1/4 plywood, and then the foam and vinyl on top. It was a shocking find.

We have searched all areas where we are seeing mold, and can not find any active leaks. Even went under the house and checked the studs/braces from below.

I think the smartest remedy is to completely gut the floor and install all brand new subfloor and inspect the joists for any mold.... Treat with concrobium, and start fresh.

Any suggestions or other things I am not thinking of would be highly appreciated.

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Comment (1)

SJ McCarthy
6 years ago
Do what you are planning to do. You REALLY SHOULD inspect the joists. Sigh. I know. It is a complete pain. I get it. Here's why.

Vinyl STOPS moisture from moving UP into the air. And it stops dry air from hitting the wood. The foam pad (probably should NOT have been there...vinyl doesn't like regular underpad...but sometimes it allows for expensive mass-loaded vinyl) made it worse.

Imagine the wood underneath is wet. In reality it should have been allowed to dry. A mold retarder could have been sprayed over it and allowed to dry once more. Assuming the joists underneath were NOT affected (former owner should have done it...but didn't) then the new layer of ply goes down. The vinyl goes down after the moisture levels prove everything is 'dry'.

Now imagine NONE of that happened (sounds like it didn't). Now imagine SEALING IN the 'wet' in the wood by throwing down new ply. Now imagine sealing THAT up with foam (non breathable material) over the wood (wood does NOT like this...a vapour barrier should NEVER be used over wood...ever). Now you SEAL THAT set up with floating vinyl over top.

Now leave it alone for a year or two. The moisture in the wood goes UP to the new plywood (like all chemicals, water moves DOWN it's own concentration gradient...it moves from wet to dry). That moisture soaked the top layer of plywood. That plywood tried to 'sweat' off the moisture but there was FOAM in the way. That foam was also sealed from above with vinyl.

The moisture that wanted to sweat off, it got caught in the foam and dripped BACK DOWN onto the wood. More moisture (from below the wood...like basement or crawlspace) moved into the bottom plywood (cause it is drier than the air the moisture is in). That plywood moves the moisture UP where it hits the foam. Again MORE moisture drips down onto the plywood.

This goes on until you unsealed the subfloor. You really need to check those joists. If you cannot see them from the bottom then you have to view them from the top.
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