Our Perth renovation adds 20 downlights to an insulated ceiling. The energy assessor says we now need thicker insulation to make up for the gaps. Is that a real requirement, and how is the loss worked out?

The assessor is describing a real rule, but it is not automatically the rule for your renovation. Under the deemed-to-satisfy provisions, where ceiling insulation is reduced for operational or safety reasons, the loss must be compensated for in accordance with Table 13.2.3w, which lifts the R-Value of the insulation that remains. [1] [2] For a Perth renovation, which path applies is the open question.

The trigger. Downlights that need a clearance gap are the classic case: the explanatory information to the table names recessed downlights alongside exhaust fans and flues. [3]

How the loss is worked out. Two steps:

1. Total the uninsulated ceiling area (the clearance gaps around every fitting, not only the new downlights) and express it as a percentage of the ceiling area.
2. Read Table 13.2.3w: find that percentage band, then the column for the R-Value otherwise required by 13.2.3(1) and (3). The table gives the adjusted minimum. Interpolation is allowed, and "x" means not permitted. [2]

Selected values from the table: [2]

• 0.5% to less than 1.0%: R2.5 becomes R2.8, R3.0 becomes R3.4, R4.0 becomes R4.7
• 1.0% to less than 1.5%: R2.9, R3.6 and R5.2
• 1.5% to less than 2.0%: R3.1, R3.9 and R5.8
• 2.0% to less than 2.5%: R3.3, R4.2 and R6.5

The bands run up to 4.0% to less than 5.0%, and higher starting R-Values drop out as the gaps grow: a base of R4.0 is not permitted from 2.5% upwards. [2] Your base figure comes from the table for your climate zone and roof type.

What 20 downlights looks like. The explanatory information says 0.5% of the ceiling area of a 200 m² house would permit 2 bathroom heater-light assemblies, a laundry exhaust fan, a kitchen exhaust fan and either about 20 downlights with 50 mm clearance to insulation, 10 with 100 mm, or only 3 with 200 mm. [3] On simple arithmetic, 0.5% of 200 m² is 1 m². So 20 lights at 50 mm plus the usual fans sits around the first band, while the same 20 at 100 mm or 200 mm clearance pushes you further up the table. The clearance matters more than the count.

Avoiding the uplift. Compensation only applies where insulation is reduced. [1] NatHERS guidance, not the code, recognises fittings that can be covered over by ceiling insulation with no side clearance. [7] Confirm that against the fitting's own instructions before relying on it.

The renovation question. A WA note to Section H says that for existing buildings undergoing alterations or an extension, Part 2.6 and Part 3.12.5 of NCC 2019 Volume Two Amendment 1 may apply instead of Part H6. [4] An existing building is a Class 1 building that was not required to comply with the energy efficiency requirements of NCC 2022 or any later edition. [5] The explanatory information adds that a Performance Solution may suit existing buildings better. [4] So for an older Perth house, Table 13.2.3w may not be the governing test at all.

Next step. Ask the assessor which edition and path they are using, what uninsulated percentage they calculated and from which fitting clearances, and which base R-Value they adjusted. Have the building surveyor confirm how the energy provisions apply to this work.

WA edition note. WA is on NCC 2025 from 1 May 2026, with WA modifications in reg 15C, and an applicant can still elect NCC 2022 (Amendments 1 and 2) for applications made before 1 May 2027. NCC 2022 has the same clause, table figures and explanatory information, so the numbers do not change. [6] No WA variation to 13.2.3(5) or Table 13.2.3w was found.