Unventilated cold flat roof - insulation

There's a lot written on this! I have a top floor bedroom with a roof above it that is in two sections - one side is pitched with felt and tiles, the other is 'flat' with boards and felt. The tiled side appears to be ventilated, the 'flat' side is unventilated and cannot be ventilated.

I have removed the layer of plasterboard beneath the joists. I will replace this but am contemplating heat insulation.

Already I can see droplets of condensation on the bottom of the felt in the tiny gaps between the boards.

There is no insulation between the boards and the felt.

I am on a low budget with this one.

What is the best economical solution?

I am considering just putting 2 layers of plasterboard back up to marginally improve the insulation as I am concerned about the condensation issues with an insulation layer. Maybe do insulation at a later stage if and when I replace the 'flat' roof from above?

Please advise, bearing in mind the low budget!!

Just fill it with rigid insulation pushed hard up against the boards. Its no different to many a garage conversion roofs. Its not what you'd do if building from new but it'll be fine.

Adding an additional layer of PB is pointless.

Thanks!
Would that rigid insulation need to be the full 100mm depth of the joists, to completely fill the void, or could it be, say, only 50mm with the gap then between the plasterboard and the insulation? Would it be neccesary to use foil backedplasterboard, and if so, do I join these by taping the non foil side (underside) before plastering?

If you used insulation that was 100mm instead of 50mm that would be considerably better. If you want to save cash in the short term but spend more on heating in the long term then use 50mm, with a gap between the insul and the PB.

Use foil backed plasterboard, forget about taping the joints, its not necessary, just skim the lot as normal.

BTW rockwool or similar is inappropriate for this situation as was recommended by someone on your other identical thread running in the Building section.

Sounds like good advice, out of the myriad of discussions on the subject. Yes I did multi-thread as I wasn't sure which sections.

Haven't even mentioned the plan to put in fire rated (enclosed) downlights yet! I guess I would enclose the section between joists with a couple of bits of 2 x 4 wood sealed with silicon to the joists and the plasterboard to stop air getting in?

Thanks.

BTW would it make sense to seal the long thin sections of hard insulation into the joists with building silicon?