Hi. We're undertaking a fairly majoy refurb. Will be doing an extension, re-wiring. Also looking to make the house more snug, so looking at improving the overall insulation.

External wall insulation is not an option as it's a listed external structure, so we're going to slab internally.

The ceilings in our upstairs rooms are slanted at the wall, we have pretty high ceilings, about 10 ft. What this means is that in order to insulate the slanted part of the ceiling, we'd need to do it from within the bedroooms. I've drawn a pathetic representation of this, where the red indicates this section of insulation. The black lines are the walls, roof, and ceiling. We have some rock wool insulation in the attic at the moment, but I don't know how well it works, the wind whistles through the attic - old house, no membrane below the 1920s roof tiles.

Now I know the standard approach to insulation would be to insulate the attic properly/better, represented by the yellow colour in the attached image. But given the draightiness of the attic, I don't know if this can be done effectively without putting in rigid insulation, or some sort of membrane over the rock wool. I'm presuming that rock wool is not performing well because of the excessive draught.

I am wondering if it is an option to do the insulation from inside the bedrooms, seeing as we will be doing the slanted bit from the inside anyhow? This would be represented by the purple colour. We're already going to be insulating the internal walls (green in the pic), so may be worthwhile considering?

Would insulation from within the upstairs rooms be a good idea, would it work, would it potentially perform better than insulating in the attic? If it could work, would it be done like dry lining, fixing the insulation/slab to the joists in the attic?

If it could be made work, obviously the cons would be that we'd need to skim the insulation board, and we'd be losing ceiling height. But we'll be rewiring and need to replace/centre lights, so all the ceilings will need patching anyway, and they are not in great shape already, so maybe an overall improvement. We'll also need to make good where the slanted bits insulation meet the ceiling

Really just want to understand why this approach might be a bad idea, or why sorting the attic insulation might definitively be a better approach.

Apologies for the long (first) post and for what is perhaps a really stupid question.