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I'm talking about a heritage-style solid wall installation with no vapour control layer.
I realise that - the image was meant to illustrate the ventilation / airgap point, not about the vapour control aspect.
And I recognise the faffiness of it all - we're in the same boat, wondering how to insulate cold old walls with wood sash windows.
On the wall that we have done, an offset stud with a gap was easiest, as it is a blank wall.
For the rest, wood wool board & sheep wool may be the way forward.
NotThamesWater
@JonoMarshall
@TW I'm talking about a heritage-style solid wall installation with no vapour control layer. So a fully permeable insulation system using wood-fibre board and lime plaster that would work with the walls thermal mass (not against it).
A non-permeable insulation layer with gap would be pretty standard, but also leads to overheating in the loft, trickier bridges/returns (around sash window boxes) and might mean the wall/timbers achieve 20% humidity in poorly ventilated areas regardless.