Dan Jestico, a member of the CBxchange Steering Group, argues that the energy consumption data from assessment tools as they currently stand should be seen as a compliance calculation only, not a prediction of actual performance.
The compliance software used to assess the CO2 emissions of buildings for Part L was never intended to be used as an energy consumption prediction tool. This is one of the many reasons why the performance gap between compliance and actual energy consumption data exists. However, the application of Part L results for planning policies and certification schemes means that compliance software is increasingly being used as a design tool, which can restrict innovation.
Many factors affect how buildings actually use energy and these are notoriously difficult to predict. Compliance calculations utilise fixed occupant usage patterns and therefore only represent an assessment of the specification of fabric and systems. This is necessary to provide a level playing field for assessment by Building Control.
Thus, the energy consumption data from assessment tools as they currently stand should be seen as a compliance calculation only, not a prediction of actual performance.
Debate continues about whether Part L compliance tools will ever be able to accurately predict building energy usage and indeed, whether they should even aim to. The Zero Carbon Hub have stated that from 2020, 90% of all new homes should be able to meet or perform better than the designed energy / carbon performance, so clearly, from the domestic side at least, there is pressure to close the performance gap.
In order to achieve sign-off from Building Control, it is only necessary to pass the CO2 emissions assessment, surpassing the minimum compliance standard. Yet planning policies and BREEAM schemes require buildings to exceed these standards, so designers are encouraged to achieve significant pass margins over the minimum compliance level. This means that the compliance tool has become a design tool, adding to the confusion over whether simulated energy consumption figures are an accurate prediction or just a compliance calculation.
As assessment tools need to be consistent across platforms and users, and applicable to all types of buildings, they’ve reverted to being the lowest common denominator of modelling strategies. The low carbon benefits of naturally ventilated buildings, buildings with free cooling or buildings with innovative mechanical systems are not always recognised by the current assessment methodology and this needs to change as the zero carbon future becomes the zero carbon present.
If compliance software is going to be used as a design tool, it should therefore be capable of providing meaningful feedback on building design, incorporating the benefits of innovation into the assessment methodology.
Conversely, the methodology needs to retain simplicity for assessing uncomplicated buildings, providing a cost-effective route to Part L compliance where suitable.
To assist in closing the performance gap, assessment tools could have an option to enhance basic compliance calculations with intended usage patterns to provide more accurate predictions of eventual energy usage.
This all points to a need to develop Part L assessment methods to suit a range of approaches, with options for more complex buildings, better predictions of energy usage and credit given to natural ventilation strategies. With Part L 2016 rapidly approaching, now is the time for the new government to prove its green credentials, and develop existing assessment methodologies into useful design tools.