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Elemental Trading Part 3: External Floors

This article is the final instalment of a three-part series examining elemental trading in a Verification Method (JV3) Thermal simulation. The series discusses the effects that solar absorptance, insulation and thermal mass have on the performance of the building envelope. In this final article we’ll look at suspended floors. But first, let’s explain a few terms in case you’re not up to speed with all the lingo in the building industry.
Elemental Trading - External Floors
Solar absorptance is a number between 0 and 1 that measures how well a material’s surface absorbs solar energy. This is primarily (but not completely) affected by the colour of the material. Different colours of the same material have different values and a lighter colour means a lower solar absorptance. For example, 0.2 is brilliant white, while 0.9 is dark brown. Actual material solar absorptance values are a little more complex as they also include the material’s absorption in the Infra-Red and Ultra-Violet spectrums, which are invisible to the naked eye. So for simplicity, we’ll be referring to the different roof colours as just their solar absorptance numbers in this article.

Thermal mass is the ability of a building material to absorb and hold onto heat energy. A rough rule of thumb is that the heavier a material is, the more thermal mass it has. High density materials (such as concrete) have a high thermal mass, whereas lightweight materials (such as wood) have lower thermal mass.

We used DesignBuilder to model three different locations for the JV3 simulation: Darwin, Brisbane and Melbourne. These represent hot, warm/cool and cold climates. Buildings operated at other conditions will, of course, have different results. We modelled an office building measuring 30m × 15m × 3.5m with the longest-side facing North. We investigated the building envelope (roof, external walls and suspended floor), with varying solar absorptances, insulation and thermal mass. We also compared heat transfer (kWh) against the JV3 Reference Building – which is a basic structure that meets the DTS requirements of the Code.

You can see the annual total heat transfer (kWh) for different roofs in the graphs below. A positive number means that heat is transferring into the building, while a negative number means that heat is transferring out of the building. This heat transfer must be balanced by either artificial cooling, artificial heating or heat transfer in the opposite direction from other elements. We made all the graphs have identical scales to so you can easily make comparisons between locations.

Graphs of External Floor Colour vs Building Energy Consumption

Solar absorptance on different coloured floors


Solar Absorptance: You might not have expected this one – colour makes some difference to the heat transfer through a suspended floor! In all climates that we simulated (but not necessarily for every climate in Australia), darker colours perform better than lighter colours. This is likely due to the darker colours better absorbing the ground temperature, which in almost all situations is a good thing. Colour has a larger impact if the floor isn’t insulated. In the most extreme case in Darwin, changing from a light-coloured suspended timber floor to a dark-coloured suspended timber floor improved performance more than staying the same colour & adding R2.0 insulation.
Thermal Mass: If the floor isn’t insulated, you can see a significant variation between annual aggregate heat transfer of timber and concrete. Thermal mass obviously plays an important role in a building’s thermal performance. However if the floor is insulated, there’s little difference between timber or suspended concrete. Most of the beneficial thermal mass of a concrete floor is lost when it’s insulated.
Insulation: Increasing floor insulation was beneficial in all locations the simulated (but that doesn’t necessarily mean it’s true for every climate in Australia). Hot or warm climates require less insulation as this may aid the night-time cooling effect. Cold climates need some insulation to reduce heat transfer to the cold outside air and minimise their annual heat loss.

As always, the optimum performance of your specific building will depend on a combination of the most suitable solar absorptance, insulation and thermal mass of each element. Every building really is different!

Reference Building and Elemental Trading

In a Deemed-to-Satisfy report, elemental trading is not possible because every separate element needs to perform the same or better than the minimum Code requirements. However, to comply with a JV3 assessment, only the Annual Energy Consumption of the proposed building as a whole needs to be less than the Reference building. This lets you trade the benefits between elements – you can choose elements which perform better than the DTS minimum requirements in one area and use that to offset other elements that are less than DTS minimum requirements. For example, it’s possible to achieve a Building Solution by trading non-compliant DTS insulation for:

  • Better than DTS-compliant glazing
  • Better than DTS surface solar absorptance
  • Better than DTS in available thermal mass

Despite having a non-DTS complying element, the proposed building can still comply with NCC Performance Requirement JP1 with a JV3 assessment if your Annual Energy Consumption is less than the Reference Building. This means a Verification Method assessment can offer substantial savings in construction costs. In our experience, we frequently find that a JV3 assessment can provide a more economical Building Solution for your project than a DTS assessment.

If you liked this article, be sure to check out Part 1 which covers roofs, and Part 2 which covers external walls. And don’t forget to Subscribe to our Newsletter to be notified of future articles like these!

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