Advice needed, insulating the soffit in a warm flat roof construction

[happyhero]

Hi I need some advice please on insulation. I am a retired electrician so I have some technical background when it comes to the makeup of house construction and I am quite hands on with doing stuff to the house.

I recently had a warm roof put on my leaking flat roof extension at the back of my house (6m x 4m approx.). I have a soffit at the back that overhangs about 300mm. This is insulated up to the facia in the top area part of the facia i.e. insulation level but not at the rafter level, basically the height of the rafters (7”) under this insulation/ply layer. I can easily see this from inside because I got the builder to leave the plasterboard off so I could do it myself after running cables etc.

Reading online I see this is a common weakness that lets the cold in and is often left unaddressed as they describe it so that it is insulated upwards but not behind the facia or on the soffit and now it’s getting colder you can really feel the cold coming in at those areas. I don’t want to put plasterboard up until I have addressed this cold problem, it seems to defeat the idea of having a warm roof.

The extension is a cavity wall with no insulation inside the cavity. The warm roof has been installed with no vents which I understand are not required on warm roofs. I understand that there can be problems where warm air meets a cold surface and condenses so I am considering this fact when I try to think how to insulate the soffit. I’m hoping to put recessed lights in the soffit so I don’t simply want to lay insulation on top of the soffit and at the back of the facia.

Also at the side of the roof both sides there is no overhang or about 1” inch if you count the facing PVC sheet. I have access to this area but the joists only allow me about 2” of access to squeeze anything in so I might be able to get some Celotex type insulation in there of about 2” thick with difficulty or alternatively fibreglass wool.

If I use fibreglass wool in the 2 sides would it be ok to completely fill the cavity over the wall up to the barge boards, I mean then warm air could not meet the back of the cold PVC, would that be acceptable, or terrible?

Where the 300mm soffit is could I put a sheet of 4” Celotex upright standing on the inner wall? Perhaps backing on to battens I could fix in place? Creating a cold box section but sealing the cold box section away from the internal joist section, would this be acceptable or terrible?

If I did the 2 insulation methods above I would have a complete insulated cap over the room, i.e. insulation from warm roof above and back and sides insulated with fibreglass packed in tight at the sides on top of the walls and Celotex closing the gaps into the soffit from inside the room.

It seems ok to me as there is nowhere for warm air to meet a cold surface, what do you think or what else should I consider?

Is it ok to totally fill a space with fibreglass, especially as there are no vents, do I need to leave any air space?

Could I use thinner Celotex and layer it i.e. create 4” of Celotex by layering 2 x 2” on top of each other or does that create new problems?

Any help or advice appreciated.