Small roof space insulation problem

Hi, this my first post. I am wanting to improve the energy efficiency of my 1987 built house. I had used a FLIR 1 camera in the past. But I wasn’t sure how to best treat the issues.
This post is about a roof cavity approx 150mm deep in the front bedroom.
We are getting condensation on the plasterboard, the cavity has old fibreglass insulation. What is the best way to deal with this issue?
I was considering boarding over with 25mm Kingsland and then plasterboard.

Photos are attached.(Hopefully!)

There are two potential issues here, probably both. There is a gap in the insulation at the junctions and probably air leakage in the same area. These need to be addressed separately.

Start by stripping off the paper on the sloping roof and strips on the ceiling and vertical walls so that your chosen depth of insulation doesn’t cover it.

Clean off any “mould food” on the exposed surface and then treat with airtightness paint (I would do the whole area) or suitable membrane and tape.

Cut rigid insulation to fit, remembering to taper the edges and then fix to the treated area., make good and decorate.

Thanks for the reply, the surface is painted plasterboard. Should I sand it down before applying the sealing paint?
Also I realised I might have posted this without a topic.

If it’s painted without wallpaper underneath, then you are lucky enough to skip two steps. Unless you have gloss paint you needn’t sandpaper, and even then it isn’t really necessary. Remove any flaking paint though.

If you really want to you could remove the tiles and deal with airtightness and insulation from above. That would enable a continuity of insulation with the loft/main roof space. If you should decide on that course of action then I can go over it in more detail.

For that particular area, I wouldn’t be able to do that work myself. It is too high for me to work on I have considered removing the plasterboard, but that would be disruptive.
The two larger bedrooms have similar issues, is it worth posting about them separately?

If the general shape is the same I think we can treat them all the same.

Potentially removing the plasterboard and redoing the insulation from the top of the wall to the apex would give a superior result but I would only go to the trouble if heat loss or damp are real problems and headroom is an issue if you add extra layers. I suggest adding a new layer with continuous insulation to cover all thermal bridges. You could finish with a new ceiling finish or a contrasting one where you have been working.

If you have the room, insulate as though there is nothing above. In other words 300mm Rockwool or equivalent in other materials.