Let me run my thoughts by you for my old solid wall insulation that forms part of the extension (cavity walled).
Solid brick wall > 50x50mm treated wooden battens > 50mm celotex between battens > VCL > plasterboard > skim
This OK? If not, what to do differently
Solid brick wall > 50x50mm treated wooden battens > 50mm celotex between battens > VCL > plasterboard > skim
This OK? If not, what to do differently