Design Matters National Technical Advisory Note

Under Slab Insulation (NCC 2022)

Confusion has been caused by the construction of NCC 2022 and the need to flow through numerous parts of the document to arrive at the definitive understanding of specifications required to achieve regulatory compliance.

The requirement for under slab insulation for slab on ground floor construction in Climate Zones 6 & 7 is one of the items that has resulted in conflicting interpretations.

The pathway through the understanding of this element is provided here,

A key understanding is that there are two Deemed to Satisfy methods for demonstrating compliance of building fabric energy efficiency, being Elemental DtS and NatHERS DtS. The difference between Elemental and NatHERS approaches is that Elemental sets individual minimum standards for each component contributing to building fabric thermal performance whereas NatHERS allows for flexibility to trade off over performance of one element against under performance of another so long as the overall result is within the total energy allowance as well as the applicable heating and cooling caps.

13.2.6 Floors & Subfloor Walls sets the requirements for Elemental DtS compliance.

This includes 13.2.6 (5) Except for waffle-pod slab-

(a) in Climate Zones 6 & 7

1. Insulation with R-value greater than or equal to 0.64 must be installed around the vertical edge of its perimeter; and 
2. insulation with an R-Value greater than or equal to 0.64 must be installed underneath the slab; and 

Specification 42 – NatHERS Dts compliance - S42C4 (i) (d) 13.2.6(4), 13.2.6(5) and 13.2.6(6) for floor edge insulation  

Specification 42 House Energy Rating Software then clarifies with S42C4 Additional Deemed to Satisfy Provisions when using House Energy Rating Software.

Accordingly, when following the NatHERS DtS approach, it is necessary to specify R0.64 slab edge insulation for non-waffle pod slab on ground structures in Climate Zones 6 & 7 but there is no mandate for any under slab insulation if the necessary star rating plus heating and cooling caps are satisfied.