Slab insulation (Class 10a)

Created by Vicki Marshall, Modified on Tue, 29 Jul at 4:30 PM by Vicki Marshall

State: All, particularly CZ 6, 7 & 8

Assessor question:

Can I please ask for clarity on slab insulation for a garage (Class 10a)?

1. If doing a NatHERS assessment, is slab edge insulation still required for the entire perimeter including the garage?
2. If doing an elemental DtS, is the garage required to be insulated with both under slab and slab edge insulation? NCC 13.2.7 says that a 10a building must have an external fabric that achieves the required level for a class 1 building (so I would assume yes for both), or be separated from the Class 1 building with construction having the required level of thermal performance for the class 1 building (so would this mean internal wall insulation between the garage / residence and slab edge insulation between the garage and home slab?

TechLink Response:

A stand-alone unconditioned class 10a garage does not have any requirements for building fabric energy efficiency.

If the garage is part of/adjoining the class 1a building, then it may be considered to need the same level of performance as the dwelling.

Thermal transfer out of the dwelling slab is best resisted at the junction of the dwelling to the garage.

This is especially so if the dwelling slab is heated.

In that case a thermal insulation strip should separate the two slabs and non-conductive reinforcement (GFRP) should be used.

If not thermally isolated at the junction, then it would be appropriate to insulate the garage slab with the same insulation as the dwelling.