i live in new england, and i'm insulating a couple of second floor bedrooms, and would appreciate any advice on my proposed plan.
where i'm insulating, there's a small knee wall, then a short section of sloped ceiling (actually the roof rafters), then a section of flat ceiling made from collar ties. instead of insulating the knee wall, i'm insulating the rafter bays from the soffit up past the collar ties, and this is where i have questions.
the rafters are only 2x6, so i had planned on installing kraft-faced R-21 fiberglass insulation in the bays (with the facing towards the living area), then attaching 2" R-10 extruded polystyrene (XPS) foam board (held in place by strapping for the drywall), and taping all foam boards with aluminum tape.
the XPS has a perm rating of 1.1 - 1.4, so i'm not considering this a vapor retarder and don't believe it should present a "double vapor retarder" problem even when combined with the kraft-facing.
i'm assuming the kraft-facing will server as the true vapor retarder. i've read elsewhere on this site that the retarder should follow the "one third, two thirds rule: the VDR has one third of the cavity insulation to its warm side, two thirds to the cold side." so i think i should be all set with the R-21 fiberglass to the cold side and the R-10 XPS to the warm side.
if there are any comments or feedback on my plan - whether to let me know i'm headed in the right direction, or am way off base - it would be much appreciated.