Felt paper vs 6mil plastic


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Old 01-28-19, 07:54 PM
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Felt paper vs 6mil plastic

Is felt paper or 6mil plastic better as a vapor barrier for osb over a crawl space?
 
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Old 01-29-19, 05:34 AM
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6mil plastic is a better vapor barrier. Before sheet plastic was invented tar paper was used so you may see it in old houses but plastic is the way to go.
 
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Old 01-29-19, 07:19 AM
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Felt paper is not a vapor barrier, water soaks right through. But why the osb, are you wanting that over the top of the vapor barrier? In general a vapor barrier over a dirt floor in a crawlspace should get as little traffic as possible. Even storage would be at risk unless walls are protected also and the space is conditioned.

Also, you can get a much larger piece of plastic for a continuous cover with minimal taping.

Bud
 
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Old 01-29-19, 07:52 AM
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I was thinking the roofing felt was being used on the bottom side of the floor joists [that is how much of my house is] and not laid on the ground ??
 
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Old 01-29-19, 08:01 AM
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I saw his other thread and think you are correct. Still would like to hear what is on the ground, concrete or dirt. If dirt that should also have a vapor barrier over it.

Thanks
Bud
 
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Old 01-29-19, 10:30 AM
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It's not dued but like a grey later of very fine rocks or dirt - not sure what it's called.
i plan to cover that with 6mil.
But then the floor above also needs plastic. The osb is the subfloor and would go over the plastic then flooring in top.
​​​​​​So, in order:
dirt, plastic, crawl space air gapg, joists, plastic, osbh, hardwood floor.
 
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Old 01-29-19, 10:36 AM
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I don't know about the plastic under the OSB but it's common procedure to lay roofing felt between the subfloor and the hardwood.
 
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Old 01-29-19, 10:46 AM
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Don't be too obsessed with vapor barriers. Thay have now learned that diffusion through building materials is not the most important issue in regards to moisture vapor, it's air leakage. Seal off the air leaks and let the assemblies dry in both directions (at least one). Here's an article to help explain.
 
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Old 01-29-19, 11:01 AM
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The vapor barrier on the floor of the crawlspace is the one you need.

Think of all the holes you would put in the one under the OSB by fastening down the different layers.
 
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Old 02-08-19, 05:46 AM
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I have seen that you can also put a foil vapour barrier on the underside of the joists? Would this be useful to keep heat in the room above?
So, 6mil on top of the dirt, then foil barrier on underside of joists a couple feet above, then the joist bay empty, then osb, then tar paper, then hardwood floor?
 
 

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