What is wrong here?

Distress that prompted the previous post might not be understood, without direct posting of relevant photos and captions.

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.

This is found insulation of a typical skylight shaft, in a row-house complex with 2x4@24" oc attic trusses. Insulation is all pretense, stapled crazily over framing and not preventing complete bathing of shaft drywall, in convected attic-temperature air. Construction was in 2002, and the installation would not pass inspection, then or now. At least sixty skylights are visible in satellite photos of the row-house village. Attics are poorly insulated in other ways, missing the R19 cover of loose-fill in the lee of any object, and at any point of maintenance. A bath fan installer might think rebuilding of fragile insulation is futile.

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:
https://picasaweb.google.com/108533770292578040917/AirtightHardCoveringOfSkylights