Pole Barn Ceiling Insulation

[lobbster]

I'm at the insulation/drywall stage of converting half
of my 1700sq ft pole barn garage/shop into an apartment for
my ailing mother. The apartment is separated from the garage/shop by a double wall insulated on both sides.
I want to finish the vaulted ceiling with drywall.
Between the trusses are 2x6 purlins 24"OC, above those
is vinyl-backed fiberglass batting and over that is the
metal roofing.
Can I remove the vinyl backing (as I did for the walls)
and add fiberglass insulation between the purlins and
then drywall up against that? the fiberglass would be packed up against the metal roofing. I know there is some venting up to the ridge through the corrugations in the metal.
My concern is about venting and condensation. Another
idea I had was to nail 2x4's 16"OC across the purlins
and then the drywall. This would serve 2 purposes - one
to better attach the drywall to prevent sagging, and
two, to create a channel for air to circulate from the
top of the wall to the peak of the ceiling.

Anyone out there have some suggestions.

You can view a few pics here:
http://www.michaellobby.com/pbapt.html

Thank you for your help!

 

[jklingel]

fg left open (not dense packed, like in walls) is pretty poor insulation. cellulose will work far better. you need ventilation air over the insulation, and to AIR seal in the conditioned space. google ADA (airtight drywall approach). unless you are in zone 7 or 8 (very cold) a vapor barrier is probably not recommended. glad you are helping mom out!