Unvented cold deck flat roof
located in SE Michigan.
2x8 rafters with OSB roof deck.
The roofer was supposed to put
3 inches of ISO board above the deck
and below the membrane on this new roof
but he screwed up and put  no ISO at all
or only fanfold in large areas.  These areas
now constitute"cold deck" conditions and yes
it is ultra sensitive to condensation.

This area is now being gutted from the inside
so its time to get this done right with a decent R
value.

Standard constuction 2x4 walls.  Plywood ext. sheathing.
Brick exterior in most places.  The brick are reclaim bricks
and are quite breathable.  One wall has a steep wood/shingle mansard
exterior instead of bricks.  Could possibly add insulation on the outside
in that wall.

Most of the rafters are going to be sistered, in some cases
even tripled due to structural issues.
This will double or triple the framing factor when computing the
R value contribution from the rafter bays.  The sistered rafters
might also provide additional pathways for airleaks. 

Sometimes the humidity can be hard to control in a multifamily
situation.   People like to save their heat in the winter and thus
tend to underventilate.  On a cold day, I can walk around the
exterior of the building and see frost in the soffit area where the
moisture is trying to breathe its way out.  Loosely built 1960's
construction.  Seems almost hopeless to seal up.

The rafter and stud cavities have old glass batts currently.
My current inclination to leave the batts in there and
cover the whole ceiling and walls with a layer
of ISOboard (no foil face)
Then a layer of horizontal wood strapping.
Then sheetrock with airtight techniques.
I would like to NOT have any plastic/foil vapor barriers.

I've talked sprayfoam, cellulose; nothing seems to adequately
make up for the lack of ISO on top of the deck. 

Whats the best way to insulate/build this ceiling?
Can it be done right using only batts, ISOboard
and NO plastic barriers?

I am assuming this whole area has a cathedral ceiling. The way I would deal with this is after you get all the structural problems dealt with on the 2*8 roof deck, leave a few inch gap, and actually frame a second "ceiling" using 2*6 or similar joists. It wouldn't have to be much, because it would only be holding the ceiling, not any of the roof load. Then you can put Foam boards in that gap area or fiberglass batts "sideways" in the area between the boards. Then, you get an extra R-19 from the lower ceiling joists. Basically you trade the strapping for a new place to add insulation. I hope the picture helps to explain what I mean. You will lose a few more inches of head height, but that shouldn't matter on a cathedral ceiling.

Thanks for the reply aardvarcus.

This is a totally flat roof.
In the same category as cathedral ceilings
and treated the same way it seems.

I like the independently hung ceiling 2x6 joist idea.

Not sure I have the headroom for 2x6 though.
I also thought about skipping the strapping
and putting ISO directly on the rafters and then
screw in the sheetrock directly against the ISO.
Same way for the walls. Wouldnt be able to hang pictures
that way and I'd be afraid of screws popping their heads thru
if someone leaned hard or fell against the wall but I still like the
idea.

I liked the strapping more as an aid to help with inward drying.
Drywall directly against ISO would dry slower than drywall separated
with strapping. Does it matter?

I'd also like to skip the ISO too and put cross layed batts as you described.
But then I'd have to do a perfect airtight drywall job.
That moisture must be kept from hitting that deck underside
and it needs to be done without poly plastic vapor barriers.

Plastic vapor barriers and flat roofs do not go together IMO.

Thanks for reading thru this.
Its a vexing problem and there are
some more angles that play into it but
I gotta try not to ramble too much !