Vapor Barrier Under Subfloor for 1880's Home?


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Old 06-28-13, 12:13 PM
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Vapor Barrier Under Subfloor for 1880's Home?

Hi Everyone,

I'm renovating an 1880's home (down to studs). In part of the house, I pulled up all of the tongue and groove subfloor in order to redo the sills and sister the joists. I will be putting plywood down over the joists and then reinstalling the old tongue and groove on top of the plywood.

My question is, can I install a plastic-type vapor barrier UNDER the plywood, so between the plywood and the basement-facing floor joists? Reason being, the basement is rather damp with the old brick walls, floors, etc. I do notice a few areas where mold has formed on the underside of the old tongue and groove subfloor (and yes, I realize that's a whole different topic/resolution). I think plywood would be even more prone to mold and I want to keep that from happening. So I'd like to install something that mold can't even grow on (well, within reason). I don't think even tar paper would meet that requirement. I'm thinking something like those liners that they use in craw spaces. Does anyone foresee an issue with this idea?

And a side note, I guess I wouldn't install a vapor barrier on top of the subfloor if I do one below the subfloor, correct?

Thanks!
 
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Old 06-28-13, 12:27 PM
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The more you encapsulate that moisture problem down there the worse it will get. If this floor is not insulated, and you or the next owner adds air conditioning, then you will have a cold vapor barrier right next to humid basement air. You can guess what that creates.

A basement or crawl space needs to be defined as conditioned or non-conditioned and then the air barrier, vapor barrier, and insulation are installed in the same plane.

A crawl space liner is probably a better place to put your efforts.

Bud
 
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Old 06-28-13, 01:06 PM
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You're right Bud. I'm probably making the problem sound worse than it is though because my sump pump was down for a while (hence, more moisture than normal). It's just an old basement but is for the most part dry.

But to your point, is there a good recommended brand/type of permeable barrier? That way I'm not "trapping" it, but it would meet my goal of a bit more mold resistance than just straight-up plywood. Maybe even just paint the underside of the plywood?
 
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Old 06-28-13, 02:14 PM
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It varies, but something like half the air you breathe in your house comes from that basement and you already know what is down there. Start with a humidity and temperature meter so you will know what you are dealing with. RH above 50% will grow mold. Even if you try to protect the plywood, the floor joists and rim joists will provide sufficient food for the mold.

The objective is to protect your home and provide healthy air to breathe. Secondary is the energy costs for that uninsulated unsealed old basement.

If mold starts to grow on the bottom of the new plywood, you have a bigger problem than the damage it will do to that wood. You are doing a complete remodel, so now is the time to decide which approach you want to take. Insulate the floors with a VB above as originally stated or insulate the basement walls and vapor seal the floor. (and rob a bank to pay for it all )

Bud
 
 

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