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questions to insulator to clarify

Posted by bc on April 12th, 2000 06:16 PM
In reply to To poly, or not to poly by The Insulator on April 12th, 2000 03:16 PM [Go to top of thread]

I'm not trying to start a war here, I just want to clarify this if we can. The original poster said he went through 4 months of prior posts and still wasn't sure and if I'm wrong then let's fix me.

Research I have done indicates that poly is a much better barrier than kraft paper. That said, there are houses built every day with only kraft paper and no poly. Even upstairs where there is more air movement through the walls. I am much less concerned in basements than upstairs because of the ground around the walls stops a lot of air movement.

I agree that poly would be a better air and even water movement barrier than kraft. But isn't "vapor"(as mentioned in both our posts) and "air carrying moisture" (as you mention and differentiate in your post) the exact same thing? Back up, upon rereading your post, are you indicating there is a difference between moving "through" the wall and moving "past" the wall? If so, then maybe thats the explanation I am looking for to comprehend all this.

We've talked about this before when comparing Tyvek and roofing felt on the outside. The Tyvek will resist air(wind) and water movement but still allow vapor(moist air) to go through as I have read and understand it. I have previously reviewed the tests done by a university on tyvek that are available on the web.

I agree that poly right on top of kraft probably doesn't have enough air space to trap moisture but I would probably slice the paper any way cause insulation doesn't always get installed perfectly flat and some even staple it inside the studs which leaves an air gap on both sides of each stud(especially when they are gluing the rock)(of course if the rock is glued, then poly isn't used either).

However, wouldn't a moisture sealer or poly on the concrete wall create a sandwich which would then cause moisture to condense inbetween especially since many stud walls are within inches of ground level? We had a poster before complaining about moisture in his walls.

While waiting on clarification, my original opinion remains the same that poly would be the best vapor barrier on the inside of a stud wall with drywall over it but most houses will not notice much of a difference with just kraft paper on insulation all the while maintaining an air space between the stud wall and basement wall. Houses are done all the time with kraft paper and they save lots of money on heating/ac when done in the basement. I would be more inclined to use poly upstairs but I see alot of them being built new with kraft there also so go figure. Maybe this all boils down to contractor choice. Catch you later.

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