Gabled roof - internal insulation

[jimmyriverbank]

Hello,

Total beginner here! I bought a 2nd (top) floor flat in a converted house last year, and the bedroom has a gabled roof. We soon found out that its really, really cold in winter so I want to insulate internally before winter sets in properly again.

I'm looking for a cheap, quick fix, and insulated plasterboard seems to be the way to go (correct me if im wrong!). I would like to attach this to the existing plasterboard walls.

Can this be easily done? If so, what plasterboard should I use?

I think the max. depth of board we could use is around 3-4cm... would that make any difference?

Any advice would be very much appreciated!

Thanks
Jim