Purpose of vapor barrier in attic?


Old 10-25-08, 07:24 PM
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Purpose of vapor barrier in attic?

I have an attic in an old building built in the 50's. The insulation up there is fiberglass with a paper-like vapor barrier.

This vapor barrier only stretches between the joists, it does not continue under the joist like it would if it were plastic spread across studs in a new construction job.

Since there are so many gaps in the vapor barrier, is it doing it's job? What exactly is it doing in the attic?

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Old 10-25-08, 08:28 PM
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Join Date: Dec 2005
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Paper backed insulation is not a VB. VB in an attic is to keep the RH outside so it will not come into the living space.
Old 10-27-08, 07:41 PM
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The kraft paper holds the insulation in place during construction and perhaps keeps it from settling in walls over time. But as a vapor barrier, you are correct, it isn't doing much. I think it was intended to be meticulously stretched between the framing, overlapping the previous one in hopes of achieving some degree of air barrier. The reality is it just gets stapled up.
But that's not a big problem. The rule for vapor barriers is, "install them on the warm side of the insulation". I live in cold country so that is on the inside. People in Florida would put it on the outside. So where does it go for people who heat and air condition? In the middle isn't always practical, so they leave it out. NJ is pretty much in that middle group, so not having one is ok.
Your real concern is air movement. If the air can't move, then the moisture can't get to cool surfaces where it can condense out. Air seal everywhere you can, you avoid moisture problems and you save on heat. Outlining all of the places you should air seal is a long list, so see what you can find on the web about the topic. If you need more input post here or send me a message.
Brick Town, 12 years

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