Roof void insulation

[davedevon]

Hi,
We have a very old stone built house with a rag slate roof with no felt or battens. I've lagged the main roof space but the bedroom & bathroom at the back of the house have sloping ceilings because they are partly built into the roof space and are very cold because there is no insulation in the ceiling. I'm having a new roof put on next month and will have the chance to fit some insulation without having to rip the ceilings down. I'm going to fit some Celotex between the rafters but have had conflicting advice on this. ie. don't let the Celotex touch the plasterboard, leave a 50mm gap between the Celotex and the new felt, leave a 10mm gap etc. I've got 4" rafters so what is the maximum thickness of Celotex that I can put in between the rarters to allow adequate ventilation and can I bed it onto the plasterboard.

Regards
Dave.

 

[noseall]

You will need an air gap on the cold side of the insulation i.e. a gap between the celotex and the felt, usually 25-50mm.

To get the most from your insulation you will need to fit about 140mm thickness. This is best done in two layers - one between the rafters and one slung under (under draw) prior to fitting the plasterboards.

The preferred way is to maximise the insulation between rafters, fit 100mm between, leaving say 40mm underdraw as to make fitting the plasterboards easier.

If you only have 100mm deep rafters then you will need to fix a knock-on batten to the rafter bottom (say 25mm x 50mm) in order to achieve an air gap above, should you wish to fit 100mm celotex between.

 

[davedevon]

Thanks Noseall,

That makes things clearer. I was hoping to get away with not ripping the ceilings down. I've got plenty of headroom in the rooms so do you think it would be ok to put 75mm Celotex between the 100mm rafters and then if the insulation proves to be inadequate, rip down the ceilings and fit say a 50mm underdraw & plasterboard. The raked area of the ceiling is about 1.5m long in all of the 3 rooms (measured from the eaves up to where it meets the flat part of the ceiling).