Sealing batts in rafters in unvented attic insulated from exterior and interior
In my unvented, conditioned attic, what can I use to cover the batts in the rafter bays behind my knee wall? Climate zone 4a.
I had the roof insulated from the exterior using polyiso according to cathedral ceiling best practices from GBA. I also added additional insulation using unfaced batts on the interior side between the rafters.
I will have drywall along the cathedral ceiling from the peak down to the knee wall and the up the knee wall as shown in my attached sketch.
I’ll have some grills/perforation in the knee wall to have conditioned air get behind the knee walls.
What can I use to cover and enclose that batt insulation so the fibers are contained.
Drywall is an option, but I don’t plan to use that space much, so ideally I could use something lower cost without creating a foam sandwich—e.g., membraine smart vapor retarder, tyvek, something else?
Thanks,
Luke
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Replies
Luke,
Air-tight, vapour-open. I'd probably use Tyvek.
Thank you!
Commercial tyvek is probably better than "regular" here, just so that it holds up a bit better over time. I'd probably use some 1x2s over it along the underside of the rafters too, so that I wouldn't have to worry about staples tearing through.
Bill
Thank you!
You could use a thin layer of unfaced EPS if you wanted more R-value, but it’s not ideal unless you’re absolutely sure the assembly can dry to the interior. If you go that route, check availability and pricing— home depot vs lowes https://help-center.pissedconsumer.com/lowes-vs-home-depot/ might offer different options or brands depending on your location.
anrew,
Can explain that? I'm not sure I understand.
I interpreted it as eps thin enough to still be vapor permeable enough to get you not only a seal, but also some extra r value (eg no more than an inch). I used 2 in EPS like this in my rim joists so my mudsill with no capillary break could dry to the interior. That said, given my roof assembly and it's critical dependence on drying to the interior, I wouldn't be comfortable with this
Thin EPS is pretty fragile stuff if unfaced, and the usual facer is polyethylene (or similar), which is a vapor barrier. Fiber faced polyiso would be a much better choice here since it's much more rigid than EPS in thinner sizes, while still being somewhat vapor permeable.
I would not be comfortable with either material in this situation though, since you really want something that is very vapor open here, and neither rigid foam material really qualifies as such.
Bill
What about finishing the drywall down to floor level, finishing the flooring out to the drywall, and then put up little sliding doors, shelves, or something along those lines and use it for storage?
I think drywall would be the low-cost solution here.
"I think drywall would be the low-cost solution here."
With the joints taped and mudded enough to be air tight and painted enough to be a moisture barrier?
No need to be pretty but affective as air and moisture barrier.
Walta
Walta,
With these hybrid roof assemblies, you want a good air-barrier, but they also benefit from being vapour-open. Tyvek Commercial would get you 28 perms. Painted drywall about 5 perms.