Pitched roof thermal insulation

[white_noise]

I am looking to insulate a pitched roof above a bedroom in a 1930's house.
The construction is tiles on battens, no sarking, 100mm rafters and lathe and plaster.
The section is 1.3m long and I can get access to the 1.3m long letter box from both the loft and the eaves. The problem is the gap between the rafters are now even all the way up.
I need to leave 50mm gap below the roof tiles for ventilation, so its a question of how to insulate the remaining 50mm
I have tried with 50mm rigid board, but even with the most square rafters the are gaps at the edges.
So unless anyone else has any ideas my options are;
- insulate with rigid board but have large gaps at rafters, bodge job
- use a 50mm wool that will compress into the uneven ratfer gaps (if 50mm is available).
- remove the lathe and plaster ceiling create a increasd rafter thickness to 150 mm and insert 100mm rigid insulation, and plasterboard over.

Thoughts
WN

 

[oldbutnotdead]

A and B will be tedious. As long as you're prepared to bin the ceiling then;
Ceiling down
50mm rigid (or rockwool slabs) between the rafters- you already know about 50mm air gap, that must be maintained.
50mm rigid below the rafters (fixed to the face of them)
vapour control layer (make sure you tape all the joints on your face fixed boards)
Plasterboard or batten and plasterboard
Cutting the rigid to fit between the rafters is really tedious- you need a snug fit to gain the full benefit.

 

[white_noise]

Thanks obnd, my overall feeling was for the ceiling removal as the best option, but didnt know if I was missing a trick.
Looks as though it's going to get messy!
WN

 

[noseall]

Get some 80mm Celotex, some foil backed plasterboards and some long (150mm) screws and fix everything below the existing ceiling.

 

[chappers]

Depends what you want to achieve really. current regs for refurb would involve 90mm between the rafters and 50mm insulated plasterboard underneath, which would involve backing out your rafters.
But if you already have the 50mm then 50mm between and 50mm insulated board under would make a massive difference, even just the 50mm between would probably make a noticeable difference.

 

[catlad]

He's probably already bashed the ceiling in Nose

 

[white_noise]

Cheers for the replies, the ceilings are not down at the moment as its going to have to be carefully scheduled. I am half thinking to save the faff and the mess that 50mm of insulation above the ceiling and an insulated plasterboard below.

 

[tony1851]

If you use an insulated plasterboard, ensure it has an integral vapour barrier, otherwise you'll end up with brown streaks down your ceiling