Vapour control barrier needed?

[amsie]

Hi all,

My vaulted pitched roof is made up of 200mm rafters. Tiles on top with a breathable membrane, 100mm airgap, then 100mm foil backed GA4000. I was going to put Celotex PL4000 underneath. Do I need to add a vapour control layer in this setup?

Thanks in advance

Paul

 

[freddiemercurystwin]

You should have a vapour barrier and it should go between the plasterboard and the insulation.

Many types of Celotex have a foil back, which when the joints are sealed with foil tape forms the vapour barrier. However I don’t think PL4000 has a foil back so you should either use a separate vapour barrier membrane or use foil backed plasterboards, which saves faffing about fixing a membrane to the roof before you attach the boards, the foil being the vapour barrier.

Alternatively use an alternative to the PL4000 with a foil back.

 

[amsie]

Thanks. The other option put to me was 50mm Celotex under the rafters then foil backed plasterboard. Would this work and be stable?

Cheers

Paul

 

[freddiemercurystwin]

That would be fine though if the Celotex that is being used is foil backed there is no need for the foil backed plasterboard.