Flat/pitched roof insulation queries

[Homerdoh]

I'm in the middle of some alterations on my bungalow and have a few dilemmas, here's one of them!

The back of the bungalow has a 7.5m x 4m roof which is has a 4.5 degree pitch and is topped with rubber over non breathable old style felt over plywood. I Want to insulate it but am not sure if I'll need a moisture barrier or not!
The rafters are 100mm deep so I was going to use 50mm celotex flush with bottom of rafters then fix foil backed plasterboard before skimming, I'll drill holes through the rafters to allow cross ventilation and fit circular soffit vents wher I can. There is also a 4.8m glass roofed utility room across part of this room so I'll only have 2.7m length of soffit to vent!

So based on this set up will I need a moisture barrier?
Will there be enough ventilation?

Any advice appreciated

 

[StevieV1]

Celotex between the rafters is OK. Drilling holes to allow airflow will weaken the rafters - Im not an expert but this seems wrong to me. Perhaps someone will more knowledge can expand on that.

Instead of foil backed pb, you can get get a moisture barrier which is like a thinner damp proof membrane which is what goes into floor slabs - Its cheap and you can put it up with a staple gun. I expect normal pb with some of the vpc would be cheaper than foil backed pb.

Just my thoughts