Should I insulate the sloping roof or not?

Hello,

I am currently renovating an old house.

In the bedroom under the roof, I have the following situation: Knee wall quite high: 1.22 m [good] Roof slope: approx. 3.5 m wide and 1.7 m high. Construction from outside to inside: roof tiles, battens and ventilation gap approx. 5 cm, rafters and between them filled with tuff stones approx. 10 cm thick and finally lime plaster approx. 1 cm. [not so good]. My life expectancy: 20-30 years (important for economic viability :)) [not good]

My ideas are: - do nothing -> I suspect: cold in winter, hot in summer. Or does it not weigh that much because of the small area? - Cover with plasterboard. Small improvement, cost-effective. Does not look so good with moisture formation according to internet U-value calculator - Plasterboard with 4 cm polystyrene underneath. Should be better regarding dew point and moisture formation. - Make the effort: remove the tuff stones between the rafters, raise the rafters with boards, add rock wool in between, plasterboard on top. More work, but only one room.

What do you think? Any other suggestions?

Thanks and regards Sicky

Well, I'm not really into half measures. For me, the tuft stuff would come out, insulation would go in - if necessary, raise the boards, put on drywall, fill, paint, enjoy. I would do that even if I only had 1 year left to live and it was completely uneconomical - just to feel good. :)

Thank you kaho674,

I have another variant with wood fiber boards about 40 mm on tuf stones from the inside.
According to the internet, it is supposed to be very good for heat protection, and vapor diffusion would also be okay.
At the moment I am just between the two options: remove the tuff + insulate properly and the one with the wood fiber boards.
Regards
Sicky