Our energy assessor specified R5.0 ceiling insulation for a climate zone 7 build, but the builder insists R4.0 is enough because the roof has sarking. Who is right?

Neither figure can be defended until three roof inputs are fixed, because the governing provision, Table 13.2.3h for climate zone 7, is a matrix built on: roof ventilation (vented or standard), reflective under-roof insulation (yes or no), the under-roof insulation band, and the roof surface's solar absorptance. [1]

The builder's reasoning fails on its own terms: sarking is not one of the table's variables. It appears only in the ventilation notes, where a tiled roof without sarking can qualify as vented. Adding sarking to a tiled roof generally makes it a standard roof rather than reducing the required R-value, and even reflective sarking's R-value is expressly not counted toward the ceiling or under-roof insulation. [1]

What the zone 7 table actually says: [1]

• R5.0 is mandatory only where there is no reflective insulation, under-roof insulation below R1.0, and the lightest solar absorptance band.
• R4.0 is compliant in several other combinations, such as a vented roof with reflective insulation at darker roof colours.
• Values range from R3.5 to R5.0 across the matrix.

To settle the dispute, fix four things from the drawings: vented or standard per the table notes, reflective insulation yes or no, the under-roof insulation band, and the solar absorptance column. Then read the single cell that matches.

One further check before declaring a winner: if the assessor produced the R5.0 from NatHERS or a verification-method pathway rather than the elemental table, the figure comes from that modelling and the table does not directly govern. Ask which pathway the assessment used.

Tasmania is on NCC 2022 (Amendment 2) today, and the zone 7 values are identical in NCC 2025, so the 1 May 2027 transition does not move these numbers. [2]