Need to determine best way to insulate single slope parallel chord roof. Chords are 24" in depth. Standing seam metal roof with sturdy vent baffles between the 24"OC trusses. Ceiling will be either T&G pine or ship-lap pine. Plan is to blow in fiberglass. Question, what is the best way to create the vapor barrier on the interior side? We have looked at 6 mil poly, MemBrain, or 1/2" foil faced foam.

Install spacers against the roof sheathing, then install plywood or foam board to create a 1-1/2" air space. Once that is done, blow in your dense pack fiberglass, cover with Mem-Brain and strapping, then the T&G. DO NOT penetrate the membrane with nails. If you are using nails of a length that will go through the membrane, then use thicker strapping. Poking holes in the membrane will create air leaks which can be a disaster.

I hope I can pile on...

I have a similar roof already assembled (20 years ago) so there is 6 mil poly behind the drywall already..since I can't add insulation into the cavity can I add 4 inches (or more) of either EPS or XPS to the interior...bonding it onto the drywall (or nailing/screwing if needed), then T&G boards as above..

I'm also in zone 7 (feels like zone 700 this morning... -32c)

then install plywood or foam board to create a 1-1/2" air space.

I'm curious - what purpose does this non-vented air gap serve?

Won't you get nail holes thru the membrain from the strapping? I already have 2" spacing against the roof sheathing (Plastic baffles)and it is vented at soffits on low and high side of single slope roof.

I assumed it was vented; it should be. Depending on the membrane, some have tapes that you use where you'll be nailing, but most will seal well enough from the pressure of the strapping.
Kenora: Use EPS which has some capacity to breathe.

Bob:

Forgive my basic questions. I have gotten so many different opinions! My contractor said just staple up 6 mil poly, blow in the insulation and put your boards on.

So, the membrain is stapled to the bottom of the chords, then the strapping goes over the staples? What exactly is the strapping material?

I thought I could just install the membrain, tape the staples, and then nail the T&G.