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We are doing a bathroom remodel of a Florida home built in 1995. For the walls in the tub area, they used treated lumber along the concrete slab and regular 2X4 for the wall frames. We have a large tub and there are two support walls for the length of the large tub and two end 1/2 walls and then covered them with green Sheetrock and 6"X6" bath tile. When I opened it up, there is black mold on the existing lumber. I immediately sprayed it with bleach (but did not scrub it and tried not to disturb it) and will have that black mold lumber removed professionally.

My question is, once that black mold is gone, I want to go back with treated lumber for that entire area so I won't have the pest (insects Florida is known for) and mold issue again.

Can I put treated lumber into those sheet rocked (actually going back with cement board) walls? I will use galvanized nails & screws for framing & the cement board that will be attached to the treated lumber. But, I keep reading 'no' you can't use treated lumber because they have chemicals. But I think they are talking about the entire house, and I can understand that for an entire home, but for what I am going to use it for I did not know if I could or not.

What I will be building is:

  • Two 2"x4" stud "1/2 walls" (56" long by 40" tall)
  • Two 2"X4" stud "tub support" walls 60" long by 12" high??

Would this be safe if it is covered with cement board and new tile?

Or, is there anything I can paint on the lumber to help with these pest and mold issues for/in the future?

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    I know treated lumber is not recommended to be used where skin will touch it/sit on it, or where it can be licked/chewed on, but covered up it should be safe. I do not know if it is up to construction standards for load bearing walls.
    – crip659
    Jun 21, 2023 at 19:25
  • Treated lumber is going to give off fewer chemicals than paint. So that should not be a consideration. Jun 21, 2023 at 20:29
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    I don't think treated lumber is going to prevent mold from growing, if the conditions are right (moisture).
    – SteveSh
    Jun 23, 2023 at 11:20
  • Did you find insect damage to the walls during your demo? You only mention mold (not all mold is bad for humans, BTW, not even all black mold - don't believe everything you see in the media - if you're really concerned, have yours tested) in your list of issues, then mention that you want to go with PT lumber to prevent pest and mold issues. Unless you've got termites (and PT won't fix that), not sure what kind of pests you've got that are destroying your walls. Beavers, maybe?
    – FreeMan
    Jun 23, 2023 at 12:30

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You have mold because moisture is getting in. It's getting in because you don't have a moisture/vapor barrier in your walls that surround wet areas.

You'll have moisture getting in from the wet areas and from the slab. To deal with slab moisture, build the walls that sit on the slab with a pressure treated sill plate sitting on top of a sill-seal gasket. Nothing above that needs to be pressure treated. The walls that are exposed to water and vapor need to have a moisture and vapor barrier like liquid applied membrane, special foam-based wallboard, or at least plastic behind the wallboard. There are different systems and if done right, they will keep moisture out of your walls.

Cement board alone is not a moisture barrier: they aren't damaged by water themselves but will absorb it readily. If you use cement board, put plastic behind it OR some kind of membrane on top of it (but not both). You may want to waterproof the floor as well. It won't be damaged by water but if it somehow saturates (shower floor? overflow from tub?) that moisture can spread under the walls and dampen other areas.

Pressure treated lumber shouldn't be necessary inside walls. They should be dry enough on the inside that regular lumber will function with no issues and if it isn't then mold will grow on materials other than the lumber anyway.

I recommend that you seek advice on the John Bridge tiling forum. They typically give very good detailed advice over there and will troubleshoot your situation if you give them enough information.

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