40x60 insulation question

[bleazenb]

Can I install kraft faced R-19 batts and use a 6 mil plastic vapor barrier. Is it wrong to have both the insulation paper and plastic both acting as a vapor barrier on the warm side of the barn. Thanks for any ideas on my question.

 

[moserjj]

Yes, wrong. One or the other usually. Sometimes I've pulled the paper off if I wanted to use a plastic barrier but that's a bit of a pain. I suppose you could cut a bunch of slits in the paper and then cover it. You don't really want 2 vapor barriers or you'll trap moisture

 

[ishiboo]

Plastic over kraft paper is fine - it's just redundant. I used to do it, now I buy unfaced insulation. Taped and mudded drywall itself is nearly airtight as well. Since there is no airflow or volume of moist air between the two, it will not cause any issues. (Plus, there is no temperature gradient between the two next to each other as there is one on each side of an insulated wall!)

What you need to avoid is having vapor barriers on BOTH sides of a BREATHING wall... IE, if you had wood walls, you wouldn't want kraft paper on the warm side and a plastic vapor barrier on the outside. The outside gets a breathable house wrap.

 

[bleazenb]

The walls are going to be 7/16" OSB, so I want the best draft and vapor barrier I can get. That is why I thought the kraft stapled to the 2x4's and plastic overtop of that would work the best till I was told that I should only have 1 vapor barrier. But like you said I think that applies to having one on the outside wall and a second one on the inside wall, which I know is a no-no.

 

[Highbeam]

Good question. The kraft paper is not very airtight anyways, look close and see the perforations plus the non airtite seal at the studs. For ease of installation, the kraft paper can be better.