|The knee wall at left is insulated to R30, and I am in conflict with this rule, in not covering it with a vapor-permeable air barrier, "vpab." If I were to comply, I could apply drywall, or least-costly wood, perhaps 3/8" plywood. I would NOT use house wrap. But why? Why? Why? The knee wall is better and more-stably insulated than the adjacent exposed wall of a bathroom, not a knee wall, and not commanded to have vpab. For that bathroom wall I placed an over-fill of 2x4 framing, with quite-well-retained unfaced R19 batt insulation. I took the picture for good reason, though I struggle now to express that reason. With some consciousness, I did NOT do the R30 thing with this wall.|
|Here is the skylight with R11 fill among 2x4 on-flat side wall frames, and found R21 kraft fill among 2x4 upright end frames. Crossing R15 batts, and the first-layer insulation, are retained by 2x3@16" oc added framing, and screwed-on wood lath. Overall R-value a bit under R25. If a vpab is needed for insulation security of all knee walls regardless of R-value, a vpab would be demanded here too. It is not demanded, and I think no one has bothered to set standards for skylight insulation. Someone should, and, please let there not be call for a vpab.|
Finding this consulted on 1/20/2016, I want to retract that last "vpab, vapor-permeable air barrier." Do let there be an air barrier, employed in better retention and protection of insulation. A hard covering is justified, over all insulation, then to be confined all-around for best insulation value, no convective air circulation.
Here is a growing photo album of hard-covered skylights: