Plasterboarding onto old eves

[Phyl1]

Hi, my house is 200 years old and I am working to a budget. The bedrooms on the top floor have sloping walls on two sides, which are the actual roof. The plaster is literally being held up by old chip paper. I have removed the paper and the plaster is ancient lath and plaster and needs completely replacing. There is no insulation and if I pull the plaster off, I will see just batons and roof tiles. The tiles on this part of the roof are kent pegs and on the top of the house is slate. I would like to insulate before using plasterboard. What should I use, bearing in mind I have little space between the tiles and the new plasterboard.
Any advice would be much appreciated.
Thanks,
Phyl1
By the way, the house is not Grade 2 listed

 

[noseall]

100mm celotex between the rafters and 40mm underdraw across the rafters.

It is likely you will have to fix a knock-on batten onto the underside of the rafters in order to maintain an air gap on the cold side of the insulation.

 

[Phyl1]

Thanks for this, really helpful. What is best to use for insulation?

 

[Phyl1]

Sorry, missed the first part of your reply.
That's great.
Thanks very much!