How to insulate a pent roof from warmside ?

[conks]

Hi,

What is the best way to insulate a pent roof.

Obviously, I can't lay insulation externally so will have to tackle from the underside/warmside.

I am aiming to take down the existing T&G boarding to the ceiling in the single storey lean to and will proceed to insulate, vapour barrier and then plasterboard.

I can't figure which is the best insulation method & product to use.

Do you fix to the joists or battens (using rigid sheeting insulation) or, is there any method for laying wool insulation etc. ?

Thanks.