so the upstairs wall of this bedroom faces into the
attic? this is acessable from the garage?

use a 1" xps foam/foil sheating board to cover
the wall from top plate to bottom plate.
install with button cap nails, and caulk at top
to 2x and bottom to 2x. tape seams with foil tape.
face the foil into the attic space to reflect
heat out of the wall.

if you can continue the foam board to the sheetrock of
the ceiling below it will stop the air movement
between floors.
this too should be caulked to seal.

we do this quite often at change of ceiling
heights (kneewalls) and walls of upstairs rooms
that are shared with attic space.

if you can stop the air movement then the
insulation will perform.
the foil works as a radiant barrier to keep
heat from transfering through the wall.

do you have baffles in the rafter bays
to keep insulation out of soffits?
are soffits free and clear so that air
can move?
this sounds obvious..but is the ridge
vent open? I've seen attics with ridge
vent installed but plywood and or felt
paper was not cut back at peak of roof.

best of luck.

Thank you for your very thoughtful response. I was concerned of possible code violations. Responses to your questions and some follow up follows:

"So the upstairs wall of this bedroom faces into the
attic? this is acessable from the garage?"

Actually, the attic is accessible from a first floor ceiling hatch.

"use a 1" xps foam/foil sheating board to cover
the wall from top plate to bottom plate.
install with button cap nails, and caulk at top
to 2x and bottom to 2x. tape seams with foil tape.
face the foil into the attic space to reflect
heat out of the wall."

Not familiar with 'button cap' nails. Can you elaborate? Aren't there foam boards that interlock precluding the need for foil tape?

"do you have baffles in the rafter bays to keep insulation out of soffits? are soffits free and clear so that air
can move?
this sounds obvious..but is the ridge
vent open? I've seen attics with ridge
vent installed but plywood and or felt
paper was not cut back at peak of roof."

Yes, there are baffles and they are all open (light enters). Likewise, the ridge vents are open. Original ridge vents were made of perforated metal, but were replaced with Cobra type vent. Neither metal nor Cobra vent allowed any air passage.

The soffitts are perforated aluminum. I once started to calculate the actual free air space, but got way-laid.

I take it that I need not be concerned about the foil faced board trapping moisture between it and the sheetrock. Winter time indoor humidity usually runs about 40% max.

Thanks again for your help.

V

http://www.acehardwareoutlet.com/ProductDetails.aspx?SKU=998009895

interlocked foam board like dow blue board
or corning pink board does not have
reflective coating. also, as you cut the
boards to fit, you can lose the interlocking
sides. the interlocking sections are easily
broken.
and--its harder to cut as razor knife gets
dull. pita to change or snap off blade in attic.

best of luck.