PIR Insulation query

GreeBo

Hey,

I've just finished installing 50mm PIR board between my wall battens and I now have 12.5 + 20mm PIR plasterboard slabs to put on top.
Do I need a vapour barrier between the two of them?
The PIR board is all foil backed but its not air tight as there are slight gaps in places due to wonky battens, the slabs will be butt jointed and taped/skimmed but is this enough for a vapour barrier?

Also, where can I source foil tape to make the foil on the back of the PIR boards continuous?

Cheers lads!

RKQ

I would install a vapour barrier, like Pro-clima (or siga or Moy)

Use their sticky mastic and special tapes - as they permanently bond to block, plasterboard ceilings and conc floor.

Airtightness is very imortant. The skim tape won't really be air tight.

GreeBo

Thx for the reply (as always)

I have some NDC 500 gauge vapour barrier that I picked up just in case.
But I was planning on using good old fashioned staples to attach it (I did realise that this would penetrate the barrier but I guess so will my drywall screws etc.)

Is there are issue with having a vapour barrier between two layers of insilation?
It will still be on the warmer side (50mm v 20mm) but I dont want to be creating vapour behind the plasterboard either.

RKQ

GreeBo wrote: »

Hey,
I now have 12.5 + 20mm PIR plasterboard slabs to put on top.

Ideally the vapour barrier would be on top of all the insulation - but in reality I don't think your 20mm of insulation over the vapour barrier, is going to cause such a difference in temperature that condensation will form.

500 guage plastic is probably ok too. I used Pro-clima between 25mm and 50mm foil backed high density insulation - no condensation issue after 4 years. We stapled paper then taped all overlapped joints.

Pro-clima cells close in cold temperature to be vapour proof but it opens up in high temperature in summer to let any trapped vapour back into the structure where it can safely dry off.

GreeBo

RKQ wrote: »

Ideally the vapour barrier would be on top of all the insulation - but in reality I don't think your 20mm of insulation over the vapour barrier, is going to cause such a difference in temperature that condensation will form.

500 guage plastic is probably ok too. I used Pro-clima between 25mm and 50mm foil backed high density insulation - no condensation issue after 4 years. We stapled paper then taped all overlapped joints.

Pro-clima cells close in cold temperature to be vapour proof but it opens up in high temperature in summer to let any trapped vapour back into the structure where it can safely dry off.

Hmm sounds very cool. I'll look into that if/when Im renovating my "final, Im going to die here" house. For now its a 5 year plan deal.

Quick, somewhat related question.
When installing the insulation backed plasterboard, did you use a circular saw or can you still score/snap them?
I guess youd cut through the insulation side, then score the same side and then snap?

Cheers.