Do I need a VCL in this wall build-up?

[michal95]

Hello, as in the topic - is VCL needed in the below wall build-up?

100mm brick
50mm cavity
50mm glass wool
100mm concrete block
50mm rockwool - net in yet
PB - not in yet

It's freezing cold in winter with what is there currently.

Cannot have insulated plasterboard or PIR due to fire safety.

 

[Notch7]

Yes

I presume the 50mm rockwall is going in between studwork, so fix VCL to studwork, then plasterboard over top.

You need to stop vapour on warm side reaching cold materials on damp side

 

[michal95]

Thanks, yes will have metal studs. Won't VCL create a potential issue of trapping moisture if not installed 100% airtight? Weighing the benefits vs potential downsides here.

 

[^woody^]

Not on masonry walls.

 

[wessex101]

Technically you should get a dew point calculation, some insulation manufacturers will do it for free. My gut feeling is the dew point will be in the outer skin of brickwork so a vapour barrier might not be necessary.
Having said that I would probably put a layer of polythene behind the plasterboard just to be on the safe side. If the cavity wall insulation is not 100% you might get cold spots and sweating on the inner face of the blockwork leaf.
Moisture won't get "trapped" as the wall construction is vapour permeable to the outside although you might get some minor interstitial condensation.