Rafter PIR ins & vapour barrier - where service gap?

whosedaddy?

Hi.
Maybe a rehash question , but never really saw it asked that way.
I’m insulating attic between and below rafters with vapour barrier inbetween.
Insulation between rafters is flush with underside and I’m fixing the vapour barrier to the rafters and perimeter walls/ floor.
But would it be better to fix service battons first and then insulated plasterboard (with appropriate thickness of insulation)
Or put Layer of PIR straight under the barrier and then service battons and then plasterboard?
Would the airgap between the vapour barrier and the insulation below be bad idea?
And is 1x2 battons best size (for running cables)?

Cost of Insulated PB vs PB + sheets of PIR aside.

Thanks,
/WD

Calahonda52

can you itemise the layers as you see them please as its hard to follow.
The normal way to do it have the service gap outside everything and make sure any holes for cables etc are 100% airtight

The other point is that if the PIR has alufoil on it and you tape all the joints, it is 100% impermeable i.e. non breathable so VB is useless

whosedaddy?

Thanks. So if service gap should be outside everything as in just behind PB.
I was thinking of 120mm PIR between rafters (with 50mm vent gap to felt)
Vapour barrier below , and PB with 50mm insulation below.
I’d i get insulation PB I can Put service gap only between two layers of insulation (unless I pull it apart) but then I might just buy insulation slabs and PB separate.
I get you in the VB being useless between two layers or PIR.

But there is useless and then there is counterproductive.
And if not all taping is perfect and it’s not actually impermeable , then it might have some effect.
But I get the drift not to buy insulated PB.