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s2m 12-07-2012 09:11 PM

Utterly confused - air barriers when finishing attic?
So I'm totally confused. I'm finishing my attic, and working on the insulation right now - it's got to be up to code for building inspections.

I've furred out the rafters to accommodate R30, and I know that I have to put in baffles for those sections against the roof to allow air circulation. But the knee walls - evidently they need an air barrier? Tyvek or some other kind of home wrap on the outside, then insulation (R18?), then drywall?

That's fine. So are the baffles.

But do I need to put some kind of air barrier on the roof section, the part that will be covered with R38 at the top?

I'm just a little confused. The air barrier is supposed to stop wind wash, which I understand is moving air destroying the insulating properties of the batts?

So is it just the knee walls that need this air barrier? And why not the ceiling section too? Surely that is equally exposed to the outside air?

I'd like to do this right, first time. As it stands, all I have managed to find, in an afternoon of solid research, is that the knee walls need to be sealed with some kind of air barrier.

Is it just the knee walls though? The sloping parts of the ceiling against the rafters need no air barrier, nor does the flat ceiling? And if not, why not?

Confused...and thinking that I should have got a contractor to do this for me...:(

Windows on Wash 12-08-2012 08:24 AM

Air barrier on the roof section should be an airtight drywall approach.

There are a ton of threads on kneewall insulation methodologies.

Gary in WA 12-08-2012 09:32 PM

Where are you located?


s2m 12-10-2012 09:49 PM

Located in the fine Commonwealth of Virginia.

Perhaps my question could be put far more simply: is wind wash not a concern in the ceiling (horizontal) of a finished attic?

Surely this is as open to the outside as the space behind the knee walls? Or is there relatively little air movement above the ceiling compared to behind the knee walls?

Windows on Wash 12-11-2012 08:17 AM

There is air movement.

Most kneewall applications are best served by covering them with rigid foam so as to eliminate the thermal bridging of the studs at the same time as adding R-Value to the wall assembly.

Gary in WA 12-11-2012 10:28 PM

Perhaps you can tell us exactly which one;


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