I am renovating a single storey stone cottage and after removing the ceiling in one room and having 2 velux windows fitted to allow light in, I now need to replace the ceiling but leave part of the ceiling 'vaulted' to allow light down into the room.
The plan is to leave the ceiling joists in place and clean them up, so I'm making no structural changes in that regard.
So, I need to insulate between rafters and board so this can be plastered leaving me a kind of vaulted ceiling with fully exposed ceiling beams so the window light can come through.
The roof has been completely re-slated - so I have 5x2inch rafters (125mm depth at approx 500mm centres) - above which is 18mm sarking board, new felt, and new slate.
I know I need to leave a 50mm air gap under the roof, so I plan to place battens on the inside of the rafters to ensure any insulation does not push into this 50mm gap. So that leaves me with approx 70-75mm gap I can insulate.
Anyway, the questions I hope someone can assist me with:
1. I assume I can put 70mm Kingspan TP10 in between the rafters, then Vapour Barrier, then 12.5mm plasterboard fastened to the rafter faces. Is this enough?
2. Should I also fasten insulation across the face of the rafters (I read somewhere that if I do not do this I might get marks coming through from the rafters?) - if so, what thickness do I need to use?
3. If I do need to insulate UNDER the rafters as well as between, can I just then fix normal plasterboard straight to the kingspan ready to be plastered?
Any guidance really appreciated. As always, on a tight budget...! :D