What are some methods that can be used to seal an OSB subfloor from moisture? And patch small gouges (quarter sized)?

Background: Small Bathroom remodel. Tile came up with finger pressure, underneath was Gypsum Concrete (Gyp-Crete or Pyrofill). It was crumbled, mildewed and in loose chunks two foot around the tub, and a foot around toilet and vanity. Removed it all to the door threshold to set new tiles. OSB is in great shape, but there is a tiny bit of mildew on the surface in a few spots and some small gouges where the Gyp-Crete took up a quarter-sized strand. Will either go with HardieBacker 500 or self-leveling concrete to raise floor up to height to tile. To remove the mildew, I've done light sanding and sprayed bleach solution and spread it with a rag. I plan to fill the gouges with epoxy (unless there is a cheaper recommendation. Epoxy is a bit on the pricey side). I'd like to moisture seal the OSB also since moisture has found it's way down there in the past.

Edit: Decided to go with Durham's Water Putty. $2 for 1 lb. of powder that never dries out. Least expensive option I could find.

Safety Note: Gypsum Concrete contains silica which can cause silicosis. If you're going to remove it use a respirator, the type with the rubber face seal and replaceable >= N95 cartridges. Spray bottle water all over the Gyp-Crete before removing it. Don't cut with a saw or hammer it all out. Take a painter's tool with a hammer butt or a chisel, and chisel all around the edges of the room, and the pieces will lift out and pry up. Hammering the Gyp-Crete makes lots of dust, and may make fracture lines a few feet long. Possibly where you don't want it broken. When it's all out, spray more water and use a wet rag to clean up the dust/debris. Sweeping or vacuuming with a shop-vac will put all the dust in the air == bad.

2 Answers 2


If you really want to stop moisture from getting to the subfloor then a membrane under your tile would be better than sealing the OSB. Better to stop it before it gets there than trying to seal the wood.

Tile and grout are not waterproof and if exposed to water regularly the water will work its way into the sub-flooring.

Good luck!

  • Do you have a recommendation for a membrane material. Thanks!
    – user345807
    Feb 23, 2016 at 18:06
  • There are some favorites of tile installers: Kerdi, Trugard, and Laticrete makes one that just has a number 9235.
    – ArchonOSX
    Feb 23, 2016 at 21:12

You can pre-fill the gouges if you prefer & I would just go with a wood putty for that. Sealing the OSB can be a few coats of polyurethane or primer & paint, ONLY IF you aren't going to be required or suggested to use a Bonding Agent for self-leveling cement or thin-set. A Bonding Agent will also provide some decent sealing & moisture protection.

Layers of Cement Backer Board screwed & glued to the OSB with thin-set (or whatever you prefer or may be suggested) & to each other with alternating seams also with thin-set (?) will be your strongest & quietest floor...outside of getting rid of the OSB garbage & replacing it with actual plywood of thicker grade, which is slightly to strongly recommended by most manufacturers.

  • I found Durham's Water Putty to be the least expensive option @ $2 per pound. Thanks for getting me on the right track.
    – user345807
    Feb 23, 2016 at 19:57
  • That should work great & I'm sure you'll do a great job with everything. You have the right concerns & don't shy away from trying to be a perfectionist, "pro's" are very shy of that.
    – Iggy
    Feb 23, 2016 at 20:01

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