I think you would find rafter cap or furring method using atticwrap could be blown against. I've blown against different types of housewraps (woven and spun) face stapled, not lip stitched, against studs to dense pack levels, they are in my own shop in 2-3 bay segements if you'd like to take a look, it looks like a patchwork quilt of testing in there and nothing has stretched beyond the initial install pillowing. The pillowing can be rolled out if you have access.

"I think what Dana is saying (and I agree with) is if you use the Tyvek product, it is a flexible wrap spanning from joist to joist" - what do you think Durapac or Insulweb is?

Dana, I'm not getting your 2" nail gap comment. A 1 1/4" roofing nail through standard asphalt product penetrates the underside of the deck about 1/4". You nailing shingles with 16d commons or what? You shouldn't be doing that, the heads are too small.

greentree: I was thinking (wondering) if you installed the Tyvek with only 2" clearance from the roof deck with rafters 24" o.c. it might not be sufficient to clear the nail points after pillowing up from dense packing, not knowing just how much initial stretch to expect. Also, you'd still want an average of at least an inch or so of clearance after you're done to have sufficient cross sectional area that the vent channel can really do it's job.

Rolling it flush with the studs/rafters is one thing, but we're talking about making a ventilation channel at the top edge of the rafter bays to allow the roof deck to dry downward. How would you go about flat rolling the channel between the rafters??

I thought most blowing meshes were polypropylene, and most housewraps were polyolefin, but I don't know how much stretch to expect. The purpose made products are designed to limit the pillowing, but housewraps are designed to other parameters.

Thanks for the real-world feedback on how it dense-packs, and how it holds up over time.

Jon
Any idea how much that attic wrap costs?

No idea, but I guess the radiant barrier aspect of it is worth a little extra over plain Tyvek.

Dana, the housewraps I used did pillow more, but not an obscene amount over fabric net. Used with their rafter caps I think you could have a successful installation based on the depth of the cap and the tensioning action it would provide. The problem I had with the housewraps is the inadequate air relief when blowing the cavity, it did seem to have more spots that were inconsistent and needed addtional material added after the initial 1 hole fill, I did not finish the typar bay and left it incomplete to observe settling in that bay over time, the other wraps used were woven. Maybe rolling netting versus atticwrap over trusses and using a rafter cap would be the ideal way to do it if you had to go that route to insulate.

If you had to install the tyvek (or netting) prior to roof sheathing, you could take a chance with weather and insulate prior to sheathing the roof to verify the installation and roll the cavities if necessary, no?

I like Izerarc's idea of using the TJI flange as a base for solid backing, 'cept I might try an XPS panel versus plywood.