Insulation of old loft room (sorry long post!)

[wateringbury]

Hi All,
Bit of a lurker but thought this would be the best place to ask the questions.

Our old (1865) house has a loft room that has always been a room ( i suspect it was servant quarters once).
The room now has stairs direct into the room. The room itself has a pitched apex roof with 5ft dwarf walls leading to storage front and back. One side is a party wall ( bare brick) the other side (also bare brick with a window) is the side of the house.
The roof in the main part of the room has some sort of membrane, a layer of rockwool and plasterboard between the rafters.
The roof in the storage area has just the membrane. The dwarf wall is just a layer of plasterboard.
Need less to say its bloody cold or bloody boiling.

So my theory was to do the following.
In the main room

Rather than removing the rockwool and plasterboard i was intending on filling the joist gaps (above the plasterboard) with the foil based insulation and then fitting something like celotex or Kingspan across the rafter as i don't want to lose too much ceiling height.

1) Would this work?

2) would i need to leave a gap for the foam insulation? or does the foil, plasterboard, rockwall count as enough gap (I reckon from the top of the rafter to the membrane is about 100mm)

I was then going to just put foil based insulation in the storage joists and insulate the dwarf walls somehow (haven't got around to thinking of that yet.

Any thoughts or advice on the first bit of the project?

I appreciate that the proper way to do it would be to rip all the ceiling out and start from scratch but i would rather not do that if i can get a relatively good result without it.

Many thanks in advance and Happy new year to you all

 

[^woody^]

You can do that. The additional load of the extra plasterboard will need consideration, as will the messing about and efficiency.

For the storage area, you could instead insulate the back of the wall and floor

 

[wateringbury]

Thanks for the reply.
By weight consideration I presume you mean the weight on the roof/rafters? I presume i will be ok as the neighbour has a fully enclosed ceiling and there are signs of there once being a ceiling in ours.
I think the storage area already has insulation on the floor. I was going to do the walls but thought whilst i am doing the main room between the rafters i might as well do the storage room.

My main concern is whether i need to leave a gap between the celotex and the other insulation?
If i laid the celotex over the rafters and left the gap that is there (left by the recessed plasterboard) then there would be only celotex , a sheet of plasterboard and a layer of rockwool) but if the rockwool counted as a gap then i could add one or two layers of foil backed insulation.

thanks in advance