Attic knee wall material selection – Tyvek vs plywood
CZ 3A/4A
2001 gut reno
Our attic knee walls need an air barrier on the attic side of the insulation, and I’m looking for guidance in choosing the right material.
We have plenty of Tyvek commercial wrap left over from the exterior, and if we use this, we would ensure that the top & bottom plates are sealed (most likely with Siga Wigluv tape and/or a solid bead of sealant behind the Tyvek). We would then tape all of the seams and over any cap nails used to secure it to the framing. Are there any other areas of concern we aren’t thinking of that would make using Tyvek a bad choice?
Alternatively, if we go with something like plywood, is there a minimal thickness we would need to use for it to be effective?
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Replies
I just went through this with my own attic and knee walls. I'm in Phoenix so I had to work to maximize insulation as much as possible on those vertical surfaces which can't, of course, use blown-in loose fill.
I ended up going with Hempwool batts in the stud cavities, and then rigid Polyiso foam boards with a radiant barrier across the framing. The foam was taped neatly with radiant barrier tape to close all gaps and cracks. Essentially the same assembly I have on my exterior walls, but thicker foam and no exterior cladding. (Fortunately, there is no rain or UV light in the attic.)
In my case, the insulation firm also air-sealed all the top plates and penetrations, so I'm not too worried about air leaks through the knee walls themselves. If you detail any HVAC penetrations, your Tyvek solution should be fine. But I would definitely recommend outboard insulation in addition, to try and make a more robust assembly overall and keep that attic heat where it belongs.
Thanks for sharing your experience! We have a couple places where the space gets a little tight, as we have to fur out the walls some to meet code too. So I'm not sure if using rigid foam insulation will work well for us.
I usually use polyiso for this too, since it gets you a good air barrier and also some continuous R value on the attic side which helps a lot with thermal performance. I think the housewrap would work too, but would be a lot more prone to damage and more difficult to seal well. What I usually do is use polyiso of 1" or thicker (so that it's strong enough to stay flat against the wall), then tape the seams and seal it to the perimeter framing with a bead of polyurethane sealant. I attach it with a few screws and insulation washers for long term support in the middle area of the panel, and also to hold it in place while the polyurethane cures.
Bill