Energy Modelling and Building Air Leakage in Australia

The Green Building movement in the past few decades brought in many changes to the construction industry and has also changed the mindset amongst design professionals. One of the key changes is the dramatic increase in the use of thermal and energy modelling packages in the design decision making process. This evidence based decision making process has contributed to the construction of some highly energy efficient buildings. Unfortunately some of these energy efficient measures have not equated to a buildings consuming less energy (Bell et la, 2013).

There are many variables contributing to the discrepancy between the modelling energy performance and the “actual” energy performance. Some of the main issues include the following:

• User behaviours, (Leaving heating or cooling systems higher than normal or longer than normal.

• Changes in occupancy patterns, (24 Hour operation)

• Poor commissioning efforts, (etc. over pressurized parts of the building )

• Air leakage is a seriously overlooked issue, and is not modelled correctly.

The most popular energy simulation software packages all include the ability to input air infiltration, however the representation of the impact can only be described as at best, basic. These models treat air leakage as a steady volume of outside air leaking in and out the building envelope thus increasing the load of the HVAC systems. This assumption does not correlate to real world scenarios.

There are two main issues in Australia’s energy modelling practices:

1. Incorrect assumptions on the typical air infiltration rate

2. Simplistic methodology used in modelling the energy impact.

Buildings in Australia have been shown to be 2 to 5 times leakier than UK buildings based on current research. Poor performing buildings have been modelled with typical air infiltration rates from the UK construction industry. The figures attributes are 0.5ACH@ambient in perimeter zones as described in Green Star modelling guidelines. As a very rough indication, this would be around 2m³/h/m² at ambient pressure, which in turn is roughly equivalent to 8 m³/h/m²@50pa.  This clearly understates the issues in Australian construction.

Test results collected in Australia have shown a typical permeability rate of 15 to 40 m³/h/m²@50pa. The modelling assumption for air leakage is likely to be grossly underestimated especially for the retrofitting of older buildings. In the latest version of NCC (2016) it has now been updated with a more realistic assumption of 1 ACH @ Ambient for perimeter zones which will help align modelling results with reality.

The effects of wind and stack effect are often ignored by the energy/thermal simulation packages. A Number of these software packages adopted the constant infiltration method which assumed a constant ACH for the building. As stated above, even the current Australian building code requires an infiltration rate when the HVAC system is on and a different setting when the system is off. The infiltration modeling Guidelines from DOE (DOE 2009[1]) published the results of an analysis which indicates models that take wind and stack effect into account, significantly increased the predicted infiltration heat exchange.

• The ASHRAE 90.1-1989 method uses a constant infiltration method.

• The DOE2 method accounts for wind speed and indoor/ outdoor temperature difference.

• The BLAST method includes wind and stack effect in the calculation.

 The following chart shows the results for a building in Minneapolis(USA). There is a significant variation between methods.

A seven fold difference between the constant infiltration method and the BLAST method.  These variations can play a significant role in underestimating return on investment, and putting in place a plan to improve building performance.

[1] K Gowri, et al, Infiltration modelling guidelines for commercial building energy analysis, DOE