Rigid loft insulation and ventilation

[Anak]

I am putting up a new ceiling in a room and would like some advice about insulation.
The majority of the ceiling is pretty standard, and I will lay insulation between the ceiling joists in the loft space (as it was before I dropped the original ceiling).
However at one side of the room, the rafters drop below the flat ceiling level so the ceiling in the room slopes down - the sloped section runs the length of the room and is about 50cm wide before levelling off at 'ceiling' height
Previously this section had no insulation, so I feel I should do something about this.
There rafters are 100mm deep.
I was planning to add 50mm rigid insulation and leave a 50mm gap between the felt (non-breathable) and the insulation for ventilation.
On the other side of the insulation will be plasterboard (fixed to the rafters).

I want to see if this sounds reasonable or...
Should leave a gap between the plasterboard and insulation (however any gap created here would reduce the size of the gap between the felt and the insulation).

Also the insulation I have is foil backed on one side, I believe this side should face inwards towards the room, can someone confirm.

 

[freddiemercurystwin]

Yes all good to go, leave a 50mm air gap between the felt and insulation, the foil should face into the room. You are right to avoid a gap between the plasterboard and insulation.