The specification needs to fit!
Recently I came across a NatHERS rating provided to a builder who was asking for some explanation.
The walls in his construction were on plan as 90mm.
His assessor had noted that he required R3.5 polystyrene insulation!
This product is 180mm thick!
Construction knowledge goes a long way to providing results that work in the real world. Experience helps provide assessments that a builder can actually build.
For the record, R2.7 is the maximum in a glass fibre batt that will fit in a 90mm wall. Thicker solutions and higher R-values can be specified, but this needs to be communicated with the architect and designed to suit.
