Insulation detail cathedral ceiling.

[portlouis]

I am looking to get the correct details for the cathedral ceiling insulation.

I have 150mmx50mm rafters with a breathable membrane, and roof tiles above it( mono pitch cold roof).
I have insulated it with 100mm Celotex in between, and filling the edges with expanding foam and taping all the joists faces with insulation tape.

I need to finish another layer of insulation (40mm) under this, and then plasterboard to meet the U values. How should I fix this? Is VCL required and if so how should this be fixed.

Should I attach 40mm with nails and then put some battens on top for plasterboard,
or, should I run an VCL layer first then insulation and batten then plasterboard.

Any help appreciated.

 

[catlad]

Tack it up with some 50mm aluminium clout nails and then when you plasterboard you can fix straight through into the rafters. buy some 100mm foil tape for the joints to create a vapour barrier.

 

[portlouis]

Tack it up with some 50mm aluminium clout nails and then when you plasterboard you can fix straight through into the rafters. buy some 100mm foil tape for the joints to create a vapour barrier.

Thanks Catlad. So a separate vapour barrier is not required like a plastic sheet. I was led to believe that plasterboard will still allow condensation and to avoid Interstitial condensation into the roof space need a VCL, or is it the case only for the flat roof.

 

[IT Minion]

Thanks Catlad. So a separate vapour barrier is not required like a plastic sheet. I was led to believe that plasterboard will still allow condensation and to avoid Interstitial condensation into the roof space need a VCL, or is it the case only for the flat roof.

Normally the PIR has a foil backing, that acts as the VCL if you tape it properly.

Possibly stating the obvious, but don't forget to mark up the joist locations as you out up the 40mm sheets.

 

[Mr Chibs]

You can also use insulation washers to pin up insulation/plasterboards.
They have a larger diameter than a screw, and might be handy.

when I did my roof, I used 100mm celo in between and 50mm on top, all taped on each layer, with no other vcl.

 

[mikeey84]

I did as suggested above, but also used foil backed plasterboard as a belt and braces