We are all familiar with the BCA’s stringent Section J requirements – particularly those concerning car park soffits which are most popularly insulated with the under-soffit installation of rigid PIR boards. However, what if we told you that there was another way to satisfy Section J without using under-soffit insulation?

The solution is deceptively simple – in-slab insulation. Traditional under-soffit methods of insulating car parks have been the norm for a long time. While the material itself is relatively cost effective, under-soffit insulation has many inherent issues, such as systemic and sustained disruption to other trades working in the same area during installation, adherence to strict fire standard and long term maintenance to name a few.

Figure 1 – In-slab is the vastly superior option for satisfying BCA Section J requirements on insulating conditioned spaces.


A far better solution is to design for insulation to be included in the floor slab of the first conditioned space – effectively creating a concrete sandwich panel. The bottom slab of the panel would remain as it would if under-soffit insulation were to be used. A layer of XPS, such as Composite Global‘s rigid Goldfoam board, is then be laid over the bottom slab, and an upper, reinforced wearing slab is poured on top of the insulation, forming the upper sandwich panel. While obviously not appropriate for established buildings, if planned during the drafting stage it is relatively easy to design for.

Figure 2 – If a connection between concrete panels is required, THERMOMASS patented connectors are available from Composite Global Solutions


While the in slab insulation solution would entail additional reinforcing costs, the benefits over the existing under-soffit method are hard to ignore:

  • Insulation and services can be installed concurrently, meaning no more scheduling conflicts between services and insulation installers.
  • As a result, the efficiency of the installation process is greatly increased, saving both time and money.
  • As the insulation is contained within the slab, stringent fire specifications relating to under-soffit insulation no longer apply.
  • Under-soffit insulation is easily damaged, requiring considerable expenditure for ongoing repairs, while in slab insulation is maintenance free.
  • In slab is by far the most aesthetically pleasing insulation solution.

While under-soffit insulation is the best solution for buildings already past the design phase, if planned correctly, in-slab insulation is by far the best option. If you’re interested in finding out more about how our products can be used for in slab insulating of car parks, visit our site, or call our Technical Team on (03) 98248211 to find out more.