Hello, I am trying to detail the final makeup for the insulation and which membranes to use for the wall and roof of my extension.

But I am not sure how to choose the best membrane so would like some guidance if anyone here has some experience to share!

My wall make up is plasterboard, service cavity, 50mm PIR, 140mm mineralwool 032, ply sarking, membrane, 25mm air cavity, Plywood layer with metal standing seam roof. Or hardiplank cladding on the walls.

I have half a roll of Cromar vent 3 classic left over from building my shed, but I have also been looking at Permavent APEX or Tyvek Housewrap or something even more expensive like one of the insulated options but other than the price I cant see much difference on the spec sheets, but I am a first timer!

Thanks

For the walls, because you will have a batten space before the Hardiplank, I would go for a fire resistant membrane.

The good news is that with taped foil PIR on the warm side and ply outside the mineral wool, almost any decent breather membrane will do the job technically, so the choice comes down to where it's going and what the cladding needs.

Walls: use a proper wall membrane rather than roof underlay. Tyvek Housewrap is made for exactly that. Mr Punter's point is a good one though, so ask your building control officer whether they want a fire rated membrane behind the Hardieplank, especially if that wall is anywhere near a boundary. If they do, that decides it for you.

Roof: Permavent Apex or the Cromar will be fine on the ply sarking, but check the standing seam maker's installation guide before you buy anything. Many of them want a structured underlay or separation layer directly under the metal on the top ply deck, and some won't honour the warranty without it. That matters more than which brand goes on the sarking.

Make sure the inside layer is the tight one. Tape every PIR joint and service penetration with foil tape so the ply outside is more vapour open than the inside, and the wall can dry outwards.

I'd skip the insulated membranes. With 190mm already in there, the extra money buys very little.

Lap everything shingle fashion, tape the laps, and dress it properly into the window openings before the battens go on.

I spoke to building control and he didnt request a fire resistant membrane he was happy that the cement based cladding will be enough of a fire break.

Thanks @JpManning the metal seam manufacturer will supply the membrane for directly beneath the metal on the top deck. I will use what I have for the sarking layer. I cant see it doing too much it will just deal with some condensation and ventilation. I will use what I have on the roof and add the money saved to a different pot!