I have a plain wall (no reveals) with 24mm deep p/t battens, between which will go 20mm celotex, then 20mm celotex & 9.5mm PB on top. That is the limit after removing architraves.
Do I have to maintain battens at 450mm centres when the insulation braces the PB?
Just happened to notice quite a few walls on the internet have obviously >600mm spacing with 12.5mm PB, relying on it seems the insulation to stop the PB flapping around & joints cracking.
There is a "new" specific foam for this - "insta foam". It is low expansion but high strength and very fast, but is gun only and tends to trash guns quite quickly.
A few hammerfix re obvious fire regs. With expanding foam you would need them quite quick :-)
I always skim, simply because wallpaper when stripped off plasterboard can be a right mess :-)
I will foil tape the joints, it is 2 layers of 20mm celotex so there will be quite a good vapour barrier. It is a box-room so limited moisture issues.
It just looked like a shed load of wood piled ready for the wall !! However the second layer of celotex will minimises the cold bridging of wood and 40mm PIR is a whole lot better than nothing in a "2 outside wall & prevailing wind" room.
With hindsight I would "bollocks to the spirit of BR" and use 40mm rough-cut eXtruded PolyStyrene, bond it to the wall, a few stainless "giant drawing pins" re fire, and directly skim the suface.
I only recently saw them.
It came down to what I could get cheaply as seconds for my mother. It gives her a "super-insulated bedroom" if the heating were to say fail or other work needs doing on her main bedroom.
PIR foam is quite a price, but seconds can be really cheap (=A34 per
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