This home built year-2000 has garage roof trusses rising to second-story roof level. It gives contrast to thoughtless knee wall construction where insulation was an afterthought. The wall shell is closed with tightly-fit 1/2" OSB. (The second-floor ceiling and attic floor, still with insulation pour on drywall, remains thoughtless and dangerous in such grand new housing. I head toward hard-shell covering of attic floors too. Insulation may stay at rest against gravity. Heat and misstepping persons, do not.)
The OSB shell was planned for in the design of trusses. I presume 2x6@24" joist bays are over-filled with R21 batts opposite the OSB.
This fits checklist, FRAMING Critical Details, www.northwestenergystar.com: "All knee walls are backed with a rigid material or other supporting material (e.g. wall to attic, skylight shaft, wall to porch roof, staircase to attic) (Thermal Bypass Checklist, Items 2.3. 2.4, 2.5).
This example conflicts with Energy Trust of Oregon Weatherization Specifications for existing homes, where no exterior wall is required, and in its place is called: WA 1.5—Open Wall Open walls that separate conditioned from unconditioned space, such as in garages that face a conditioned space, shall be sealed for air leakage, insulated to a minimum of R-15 (or the cavity must be filled), and covered with a vapor permeable air barrier to limit human contact - - -
It is NOT about human contact, but only the overfill and retention of insulation. An outside barrier that can not pressurize contained insulation, has no value. I am distressed that contractors in Oregon are compelled to place house wrap in this application. There instead should be movement toward complete hard containment, as in new houses.
Better than this simple hard containment, recognize too there may be insulation opportunities with crossing layers, achievable from the "back" side. A framer of a new house might not fathom the needs of an insulation installer. OSB pre-applied as in photos above, will have difficult application to better insulation of skylights.