Flat Roof and Fully Insulating the Void- Advice Needed!

[bosyuk]

I've started a project insulating a bedroom in my house. Its a dormer style room with a flat roof. I want to insulate the walls and roof.

The flat roof is currently felt deck which was redone by the previous owner a few years ago. There is a tiny bit of insulation in between the joists but it's so minimal it cant be doing much.

The joists run from the back of house to the front rather than side to side and as a result do not have soffit vents. The space to fill is 16.6 cm down to 11.2 cm.

I can only think my option is to fully fill the cavity and fit something like a thin Kingspan Kooltherm K18 board which has a vapour barrier in it under the joists.

Does anyone have any thoughts? How can I fully fill taking into account the varying size? Is this even a good idea?

I look forward to your comments as progress is on hold right now!

 

[freddiemercurystwin]

Full fill it with Rockwool.

 

[bosyuk]

Thanks.

Do you think I should put say 50mm of celotex angled from the high to the low end to improve U value. Then rockwool or just fill the entire void with rockwool?

 

[freddiemercurystwin]

Adding some Celotex or similar would be a benefit yes.

 

[bosyuk]

What sort of Rockwool would people recommend?

Its going to need to be easy for me to cut to get a perfect tight fit below the celotex given the way it slops upwards.

What about this

http://www.insulationexpress.co.uk/Multi-Purpose-Slabs/Rockwool-Flexi-Insulation-Slab.htm

 

[freddiemercurystwin]

I am not sure just how 'squashable' the insulation you have suggested would be, I would recommend something like this http://guide.rockwool.co.uk/media/73556/roll-sec.pdf

 

[bosyuk]

Ok that looks good.

So I will go with 50mm of celotex GA4000 pushed up against the underside of deck and nailed into place. Any gaps filled with expanding foam and sealed all round the edges with silicone. Then the Rockwool Roll cut to size below that to foam a tight fill of the joists.

Followed by the Kingspan K18 to insulate under the joists and ensure a vapour barrier.

Thanks!