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Old 08-23-2013, 01:04 AM  
MoreCowbell
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Default Vapor barrier question

As I mentioned in an earlier post, I am about to pour the concrete floor for my basement project. I'll be doing this in sections that match how I would have cut the control joints if I did the whole floor in one shot.

I'm wondering how to do the vapor barrier. I've been reading that even small pinholes should be avoided if possible. If I lay one big sheet of plastic, there will be some parts that will get pretty chewed up by the time I get to pouring concrete on them. On the other hand if I lay the plastic down in sections like the concrete, I'll have a lot of seams to try to seal with tape and I'm doubtful that tape would stand the test of time.

Any thoughts on this? Thanks again.



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Old 08-23-2013, 04:30 AM  
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Well, just be as careful as you can with pieces of 6 mill plastic under each pour. You can overlap each piece about 10 inches and rely on the tape. The plastic is there to stop the majority of the water, it gets cut all the time on jobs.
I have had pretty good luck with "tyvek tape" on jobs with plastic outside for years. Give it a try.Good luck.



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Old 08-23-2013, 06:35 AM  
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Pinholes don't make a difference in the vapor permeance rating except where the hole is.

Good read: http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems/?searchterm=slab%20vapor%20barrier

Okay, so we don’t need the sand layer to handle the “curl” thing. But what about using the sand layer to protect the polyethylene? Hah. The polyethylene does not need protecting. You can poke holes in it, you can puncture it, you can tear it, you can leave gaps in it, and pretty much have your way with it as long as it is in direct contact with the concrete. Huh? But, but, vapor barriers have to be continuous and free from any holes. Actually, no. Air barriers need to be continuous and free from holes, but vapor barriers do not need to be. Lots of vapor moves by air movement, not a heck of a lot of vapor moves by vapor diffusion. The concrete slab is the air barrier, and the ripped and torn and punctured polyethylene sheet is the vapor barrier. It’s that Fick’s Law thing. Diffusion is a direct function of surface area—if I get 95 percent of the surface covered I am pretty much 95 percent effective—and the parts that are left I have filled with concrete which is also pretty good as a vapor barrier. I could wear golf shoes and march around the plastic vapor barrier and not do much damage.

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Old 09-08-2013, 12:12 PM  
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Thanks guys. Good info. That's one less thing for me to worry about.

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