Why do I need Slab Edge Insulation

Insulating the edge of your concrete slab is important because that’s where most of the heat (up to 80% according to BRANZ), escapes. We would recommend insulating the edge of your slab in all cases, however two cases where you definitely want edge insulation are:

  • If you want to achieve a Homestar rating above 4.

  • If you’re planning on using in-slab heating.

The New Zealand Building Code clause H1 (Energy Efficiency) [NZBC-H1] states that floors are required to be insulated to a minimum of R-value 1.3. Regarding concrete floors, it further notes that “The floor R-value is met by concrete slab-on-ground” i.e. no insulation is necessary under an unheated concrete floor that is laid directly on to a prepared ground surface. Nor is there any requirement for slab edge insulation.

(Note that with a heated floor the R-value rises to a minimum of R1.9, with the thermal resistance of the construction from the heating plane to the inside air to be less than the resistance to the outside air. A heated floor slab requires careful and specific insulation design so as to maximise the benefit of such a system.)

The normal “slab-on-ground” has an R-value very substantially below R1.3 depending upon concrete thickness, type of hardcore and earth below, height of water table, etc.

With the Building Code not requiring any insulation under a slab-on-ground, nor edge insulation, why is it that there is an emphasis on slab insulation when the industry and real estate market are happy to work with Code minimums for the thermal performance of walls, roof, and glazing?

 

Of course concrete floor slabs, and all aspects of a building, should ideally be constructed to greater than the minimum requirements of the Building Code, NZ Standards, etc. but so often they are not.

Heat loss through a slab to the outside (source: Design Navigator)

Thermal model showing  heat loss through traditional concrete slab (source: BRANZ)

Thermal model showing  heat loss through traditional concrete slab with slab edge insulation (source: BRANZ)