Target insulation for pitched roof in extension

[Adrian80]

So another green question...

i think i want a cold deck insulation pitched roof. But not sure of the insulation thickness?

I have 6x2 (150x50mm) rafters and wanted to insulate between them.

with 100mm celotex i think i would need to counter batten under the raters to keep an air gap.

From my reading it seems you need the following specs?
1. tiles.
2. battens
3. felt
4. 50mm air gap
5. insulation (not sure how thick ?)
6. 40mm air gap
7. vapor control layer
8. plaster board

also is 100mm celotex enough it give me a u-value of 0.29 according to their calculator?

what do you'all think?
cheers

A

 

[noseall]

You are likely to need another layer (say 40mm) across the rafters and there is no need for an air gap on the warm side of the insulation.

I presume there is a room in the roof?

 

[Adrian80]

Technically its a room in a roof, it is a two storey extension with a low pitch. so not bothering with a horizontal ceiling...

Good to know about not needing an air gap.

I guess you just batten them on to the rafters which leaves a bit of threading room for cables..

cheers
A

 

[freddiemercurystwin]

I guess you just batten them on to the rafters which leaves a bit of threading room for cables.

Eh?

You fit your insulation between the rafters then fit a continuous layer beneath the rafters. The fit foil backed plasterboard so no need for a separate vapour barrier either.