Slab insulation for thermally modelled dwellings in climate 6/7/8

Created by Vicki Marshall, Modified on Tue, 29 Jul at 10:21 AM by Vicki Marshall

State: Climate Zones 6, 7 and 7

Assessor enquiry:

Can I get clarification on the slab insulation requirements for the thermally modelled pathway- do we need to always model under slab and perimeter insulation for raft or conventional slabs?
Secondly, is perimeter insulation requirement triggered where waffle pod is specified?

The talk in the NatHERS community chat isn't clear on the correct approach for this and I want to double check.

TechLink response:

The confusion arises out of the common mix up between the NatHERS Deemed to Satisfy and Elemental Deemed to Satisfy compliance pathways.

Under slab insulation is required in CZ6 when compliance is confirmed with an Elemental Deemed to Satisfy assessment.

It is not required if compliance is confirmed with a NatHERS Deemed to Satisfy assessment when the required Star Rating plus Heating and Cooling Cap limits are achieved.

Slab edge insulation is required whether compliance is confirmed with either the NatHERS or Elemental DtS methods.

The insulation does not need to be placed under the edge beams and ribs.

2. NCC Vol2 clause 13.2.6 (5) is the item required for clarification of the question and the word EXCEPT is critical.

(5) Except for a waffle-pod slab—
(a) in climate zones 6 and 7—
(i) insulation with R-Value greater than or equal to 0.64 must be installed around the vertical edge of its perimeter; and
(ii) insulation with an R-Value greater than or equal to 0.64 must be installed underneath the slab; and

This is then qualified by Specification 42C4 (1) (d) where it is only slab edge insulation that is required when using a NatHERS rating.