Vapor Barrier Under Plywood.

 
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Old 12-21-2017, 12:37 PM   #1
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Vapor Barrier Under Plywood.


I'm laying 3/4 plywood over t&g 2x6 decking. Not many gaps, just a few old holes from pipes,etc. It's in a kitchen and baths.

Is it preferred to lay something like asphalt impregnated Kraft paper under it as a barrier from underneath, or is it even needed in this case?

At this point, I am assuming it will be covers with sheet vinyl or LVT of some sort.

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Old 12-21-2017, 03:10 PM   #2
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Re: Vapor Barrier Under Plywood.


Nothing? Nobody? I agree it's a bit of a simple question, but I thought I'd at least get a go to DIY outta you mugs.

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Old 12-21-2017, 03:15 PM   #3
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Re: Vapor Barrier Under Plywood.


What's it over? Crawlspace? Conditioned space?
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Old 12-21-2017, 03:41 PM   #4
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Re: Vapor Barrier Under Plywood.


Tar paper if fine, but mostly it is just used to help eliminate squeaks but alas we also get to DaVinci's question (any major water issues underneath also). As for a "Barrier" i.e. plastic the answer is a definite NO
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Old 12-21-2017, 03:43 PM   #5
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Re: Vapor Barrier Under Plywood.


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Old 12-21-2017, 04:12 PM   #6
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Re: Vapor Barrier Under Plywood.


The kraft paper would work as would 15lb felt.

An option would be to go over it with a belt sander and glue and screw it.


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Old 12-21-2017, 04:48 PM   #7
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Re: Vapor Barrier Under Plywood.


Crawlspace with insulation with vapor barrier side up. I'll probably just do it as I've found a few more gaps as I prep it. I have it, so I suppose my questions more a, is there a reason not to.

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Old 12-21-2017, 06:58 PM   #8
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Re: Vapor Barrier Under Plywood.


Thanks for posting to Contractor Talk. The folks over at DIY network would be happy to help you.

I don't see any reason not to (no plastic), but why not foam the gaps and move on?
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Old 12-21-2017, 07:15 PM   #9
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Re: Vapor Barrier Under Plywood.


It seems that the old houses always had felt between layers of flooring. As mentioned before, I have been told that it was to eliminate squeaks from the two layers moving against each other.
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Old 12-21-2017, 07:17 PM   #10
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Re: Vapor Barrier Under Plywood.


Wait, a "crawl space with insulation with vapor barrier side up".

Does that mean Kraft faced FG batt insulation in the bays and the faced side is touching the underside of the 2x6 sub-floor?

Then the finished floor will be sheet vinyl or some such?

Hmm...I think this is a bad design then. You would have a semipermeable Kraft face under the 2x6s then another semipermeable on top of the 2x6s then 3/4" ply then another semipermeable vinyl flooring.

This is not good.

At the very least if you go ahead with this then I would suggest installing 1 1/2" of rigid foam under the floor joists.

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Old 12-21-2017, 08:44 PM   #11
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Re: Vapor Barrier Under Plywood.


I agree with Scipio.
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Old 12-21-2017, 09:50 PM   #12
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Re: Vapor Barrier Under Plywood.


Luckily I ran into a few more mouse houses and had to deal with that today, so no subfloor went down.

I was thinking the same thing, which is why I wanted to ask. Lots of vapor barrier and nowhere for any vapor to go. Unger was thinking the same thing as well.

I'll leave it out. That was kind of the way we were heading. This makes me feel better about it and not me just being lazy.

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Old 12-21-2017, 11:24 PM   #13
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Re: Vapor Barrier Under Plywood.


Quote:
Originally Posted by ScipioAfricanus View Post
At the very least if you go ahead with this then I would suggest installing 1 1/2" of rigid foam under the floor joists.

Andy.
My take on it would be to use 2" foil sided foam, tape all the seams and foam around the edges. Keeps the rodents out, and stops air flow. They would have nice warm floors

Then again...I am not a fan of vinyl flooring of any type.
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Old 12-22-2017, 08:16 AM   #14
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Re: Vapor Barrier Under Plywood.


Quote:
Originally Posted by ScipioAfricanus View Post

At the very least if you go ahead with this then I would suggest installing 1 1/2" of rigid foam under the floor joists.

Andy.
I'd worry this could trap moisture in the FG. It may condense on the foam.
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Old 12-22-2017, 09:06 AM   #15
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Re: Vapor Barrier Under Plywood.


Kitchen and bath with sheet vinyl, somebody is going to dump a bucket of water on the floor and swab it like it has scuppers.

Ground contact or potentially very wet areas under sheet vinyl I'd go with Roxul, no vapor barrier. Fiberglass wicks the stuff everywhere, and loses its insulation ability when wet.
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Old 12-22-2017, 10:18 AM   #16
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Re: Vapor Barrier Under Plywood.


This is a remodel. I'm not messing with the current insulation, just the floor.

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