Vapour barrier query with insulated plasterboard

[wallis1905]

Hello. Looking for a bit of advice. I have reached this stage of renovating a dormer bedroom. Note that all joints are about to be taped with foil tape, so all timbers will be covered at that point. My plan was to finish with insulated plasterboard, but i have bought the wrong type i believe. They type i have is foil backed, whick will make a double vapour barrier from what i understand, but they will be more or less flush against each other. Is this a no no? I guess paper backed is the way to go here? Any advice appreciated.

 

[Ivor Windybottom]

Looks great. Your vapour barrier needs to be on the room side of all insulation. The foil on the back of the plasterboard is probably adequate.

I had a similar situation. But I used 50mm separate PIR board, with plasterboard over. But I did thoroughly foil-tape all the joints in the PIR before plasterboarding, so had an airtight seal at this point, right behind the plasterboard. You'll have a thin gap between your foil-backed boards. So the boards will keep 99% of moisture out, then your taped foil layer will keep out anything that gets past it, to prevent any theoretical risk of it finding its way into the wood.

It's all probably fairly academic anyway, as whatever you do you then stuff a load of screws through it all.

It will be absolutely fine, and will have been done vastly more thoroughly than any loft conversion or building company would ever have done.

 

[wallis1905]

Ok great thanks for that. I guess i'm worried that any moisture that makes it past the first imperfect barrier (imperfect because i cannot tape the joints between foil on the back of the plasterboard sheets, and as you say, screw holes etc), would have nowhere to go when it meets the 2nd, more complete vapour barrier formed by the pir boards and taped joints, and may condense in there. Probably worring about nothing though! If i was starting again, i would do as you suggest though, seems a cleaner way to go. And tbh, i'd replace the pir with rockwool too, as its a lot less work to install.

 

[Ivor Windybottom]

PIR's vastly superior. I wouldn't use rockwool anywhere other than an unoccupied loft.

Yes, there's a theoretical risk of condensation from the room squeezing through that tiny gap then living in there for eternity. You'll be about 1000 years old by the time this becomes an issue. In reality, unless you're having a kitchen or bathroom in there it will be irrelevant.

 

[wallis1905]

Brilliant, thanks for that. Will drive on with what i have.