How can I super insulate, air seal, and vent this space?
This is the top floor of my new to me house in zone 5. Under which is a fully insulated living space of 12 years. So right under that decking is standard 10 in fiberglass..
I’ve been talking to an insulation company who suggests dense packing cellulose the 2×6 walls and 2×10 accuvented rafters. Plus loose fill in the attic. I’m wondering if I should have them use membrain as the netting and air barrier so that I can then frame a second 2×4 wall/rafter DIY with roxul batts to get rid of all that pesky thermal bridging.. My plan falls apart behind the kneewalls.. Would have use xps foam board under the rafters in there I guess.. or just insulate the knee walls themselves somehow. How? I’m shooting for r40 walls/rafters. R60 roof.
Overall thoughts? Concerns? Non-foamy alternatives Bueler? Dana? Martin?
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Stephen,
Your link doesn't bring me to the right place. I don't see any photos (but maybe it's a browser problem).
The usual interior membrane used when blowing cellulose in walls is InsulWeb, not MemBrain. (You want an air-permeable fabric; MemBrain is an air barrier.) In theory, you could use InsulWeb for the insulation installation, and later install MemBrain over the InsulWeb if you wanted to -- but that usually isn't necessary. For more information on this topic, see How to Install Cellulose Insulation.
If your rafters aren't deep enough for the amount of insulation you want to install, you can beef up the thickness of the insulation cavity various ways (for example, by sistering on framing members, or by extending 2x4s below the 2x12s with plywood gussets, or by installing 2x4s under the 2x12s at 90 degrees to the 2x12s). Another option is to install an interior layer of continuous foil-faced polyisocyanurate on the interior side of the 2x12 rafters, with strapping between the rigid foam and the drywall.
https://picasaweb.google.com/m/viewer#album/107236736120604165034/6108242613380376961,authkey:Gv1sRgCOG04_bu9pLf0gE
Thanks Martin. It's working for me in Chrome.