Sealing batts in rafters in unvented attic insulated from exterior and interior – GreenBuildingAdvisor

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