Pitched roof needs insulating

[Harry028]

Hi.

I have an extension with a pitched roof and exposed rafters with skylights but it gets very cold outside of summer. It has a fibreboard ceiling that bows and has cracks in places (see pictures) and I want to insulate it as best I can.

Firstly, are the fibreboards worth keeping or should I replace them with plasterboard.

Secondly can I simply insulate using insulation and plasterboard that fit in between the rafters or is there more to it than that?

What would be the best solution??

Thanks!

 

[Tigercubrider]

What is above the current boards?
Do y ou wish to keep the look of the existing ceiling? (Exposed rafters)

 

[Harry028]

It's just a slate roof:

and yes it would be good to keep the rafters exposed if possible.