Cold flat roof help

[Roystone]

We're in the middle of building a cold dormer (added a couple of posts about it) and wondered if

a) Is a vapour control barrier is required (contractor says not) - there will be a house bathroom and an en-suite in the dormers.

b) We want to put some 5mm or 10mm thick Acoustic / Soundproofing Resilient Tape between the lower layer of PIR (or VCL if we're having one) and soundbloc F plasterboard - to isolate some sound (like the noise of rain - appreciate we can't get rid of it - but lowering it a little will help) - Does this make sense?

We had to go with soundbloc F as BCO required high fire rated plasterboard because of the closeness of neighbouring properties.

The picture is an example - ours is two layers of OSB, GRP finish.

 

[^woody^]

All flat roofs require a vapour check layer, it just alters as to where it goes.

Wet rooms certainly need it as there is increased moisture risk

 

[Roystone]

@woody, is the VCL not needed behind the plasterboard if using 25mm sheet of PIR on the underside of the joist (there will be 100mm Kingspan between the joists).

 

[^woody^]

As per your other thread, a vapour check layer can be the PIR if it is foil faced and taped