WillKen

Thank you for the thorough explanation. Wall is already painted. Not sure on paint type but will check. Thanks. It’s pretty well ventilated due to a lot of gaps everywhere! But when I have the soffits done, I’ll get them to add some vents, as well as close up the gaps that shouldn’t be there…

So then why fit a vapour barrier ever? I like the thinking. My only issue is I have a dozen sheets of PIR on the drive already! Can you think of a way to use it? What about foil taping the stud to the plasterboard then installing the PIR? Or even foil taping the whole rear face of the plasterboard…? i may just have to cut my losses and try to sell the PIR.

I’m currently insulating the first floor of my 70s chalet bungalow. There is one wall which is essentially a stud wall which opens up to the loft above and eaves at the back. I can fully access these walls from behind, and am planning not to disturb the plasterboard. I’ll instead insert 100mm PIR from behind. The issue I have is that there’s no vapour barrier on the current plasterboard/wall. It’s ordinary paper backed board. Am I safe to add PIR behind? My instinct is that foil taping the PIR to the studs at the rear is a bad idea as it traps any moisture that forms on the inside of the wall. Should I add the PIR and foam gaps, but not tape? Or should I try to retrofit some kind of vapour barrier inside the studs?

Thanks How significant would that thermal bridge be? Sorry if I wasn’t clear, but for 1, the 50mm excess would be on the backside of the wall, so plasterboard would still be flush to the front of the studs.

In my haste I of course meant: ”any problems with 1? It would be cheaper and easier”

I’m currently renovating my 70’s built chalet bungalow style house. The first floor rooms all have dormer windows/walls which are studs with plasterboard on room side, but open to the eaves on the other. Therefore there’s no wall insulation on the external walls. I need to upgrade that. I plan to remove the plasterboard and insert PIR insulation, then replace plasterboard in front of the studs. My heat loss calculations for our heat pump install are based off 150mm of PIR insulation. Given the space on the cold side of the wall I can either: 1. Install 150mm of PIR between studs, with approx 50mm sticking out behind the 100mm studs; or 2. Install 100mm PIR between studs (and flush at the back) with 50/12.5mm insulated plasterboard in front. Which is better thermally? Are there any problems with 2? I think 2 would be cheaper and faster and of course I won’t lose 50mm in each room (no big deal though…). Thanks!