I’m doing a renovation on an unusual roof.
The renovation is from the interior of an attic space room.

On the south side of the roof, there are solar PV tiles, with an air gap under them and a ridge vent. The ridge vent only currently vents the air space under the tiles, not under the deck.
Also on the south side the deck has an impermeable membrane (red line in attachment).

The beams are 2×6.
I want to obtain maximum R value but reduce the interior space as little as possible.
I have already removed the old gypsum board and fiber glass from interior.

I plan to use 1″ or 2″ of polyiso under the gypsum board to act as a thermal break between the gypsum and beams. More than 2″ would reduce interior space too much. This will give me about R-40 total (a vast improvement on the about R-10 for the original assembly).

My questions come down to: how should I fill the bays?

I think that the ideal is to create air chutes by leaving a 0.5″ air gap between the deck and the polyiso and have a foil face facing the air gap. However for an air chute to be effective I would also have to cut the deck at the ridge to allow the air to flow. Cutting a ridge vent from the interior is not so easy.

Q1> So should I leave an air gap under the deck?
Q2> Is a 0.5″ air gap enough for an 8ft distance?
Q3> Is it best to use a foil faced polyiso board facing the air gap (inside of fiber faced)?
Q4> Is there much benefit to leaving an air gap if I do not cut the ridge vents from the interior?

Q5> Importantly to cut the ridge vent I would have to work from the interior and cut on either side of the ridge board (which is a little lower than I want). Is it worth the extra effort to cut the ridge vent from the interior on either side of the ridge board?
(I could remove the ridge vent from the exterior but it is probably more difficult because of the attachment method on the south side, and means removing and refitting a perfectly good ridge)

My feeling is that it is better to have the air gap than not.
0.5″ is probably enough given the smooth surfaces.
It is better to use foil faced polyiso facing the gap.
If I cannot cut the ridge vent from the interior now, maybe later I could cut from the exterior another time.

Also any advice on trying to cut the ridge vent from the interior?

thanks
Mark