MSC Quarterly: The Evolution of Slab Edge Insulation from NCC 2022 to NCC 2025

Slab edge insulation has been one of the more confusing residential construction items under NCC 2022, particularly for projects using NatHERS or house energy rating software.

Under NCC 2022, the requirements around concrete slab-on-ground insulation were tightened, especially in cooler climate zones such as Climate Zones 6, 7 and 8. For conventional concrete slabs, this often meant builders and designers had to consider both slab edge insulation and under-slab insulation, unless an excluded system such as a waffle pod slab was used.

The main issue was not just the requirement itself, but how the NCC wording was applied. Under the house energy rating software pathway, NCC 2022 referred broadly to the slab insulation clause. This created confusion about whether under-slab insulation was also required, even where the dwelling had already achieved compliance through its NatHERS rating.

NCC 2025 provides useful clarification. The updated wording narrows the additional requirement so that, when using house energy rating software, the focus is specifically on slab edge insulation rather than automatically calling up the broader under-slab insulation requirements.

In practical terms, NCC 2025 helps resolve the industry uncertainty created by NCC 2022. Slab edge insulation remains an important consideration for thermal performance, particularly in colder climates, but the compliance pathway is now clearer. Conventional raft slabs, waffle pod slabs and projects with in-slab heating should still be reviewed carefully against the applicable NCC pathway, climate zone and project-specific construction details.

For builders, designers and energy assessors, the key takeaway is simple: NCC 2022 introduced stricter slab insulation expectations, while NCC 2025 refines the wording to make the slab edge requirement clearer and more practical to apply.

For further information or enquiries, feel free to contact the team at MSC.