Energy comparisons of buildings are meaningless unless we define our terms very clearly


Ant Wilson

There seems to be an increasing amount of press now about the “performance gap”. Yet it appears to me that many people do not know what they are comparing when they are discussing the “gap”.

For instance, are they looking at the difference between predicted and actual carbon emissions or are they looking at energy? People may be measuring energy but reporting carbon, carbon dioxide or carbon dioxide equivalent, adding to the confusion. A low energy building should have a low carbon rating but so can a high energy consuming building – for example if it is fitted with biomass boilers.

There is also scope to improve the accuracy of our design stage calculations and the input data used in these calculation programs. Then there is also a concern that energy performance certificates (EPCs) do not really show how good a building is in operation. The calculation bases these on carbon dioxide emissions and compares these to a building of the same size and form. This is a bit like saying you can have an “A” rated Mini or a “G” rated Mini or an “A” rated Aston Martin or a “G” rated Aston Martin.

Display energy certificates (DECs) do not show how well a building will perform either. This is evident by examining how various schools may perform. Each year there will be a different energy consumption level based upon a number of variables including weather conditions and the number of staff and pupils in the school. It is also amazing the difference that a change in caretaker can make!

There are six areas that I believe we should focus on to improve the performance of buildings and I plan to look at these in greater detail in future blogs. They are:

1)     User error: wrong input parameters in design software

2)     Software error: the energy calculations are too simplistic

3)     Builder error: build quality of the building and tolerances

4)     Commissioning error: poorly commissioned building services equipment

5)     Maintenance error: Llack of maintenance of the building fabric and systems

6)     Recording error: the quality of the meter readings.

Good comparisons of energy must look at the real utilisation of the space. This could be in terms of useful hours of operation or the productive output of a space. Well-controlled buildings will have equipment that operates outside of occupied hours as keeping buildings at a constant temperature can be more efficient than switching oversized plant on and off in the mornings and evenings.

When designing a new building, we do not know next year’s weather data or the in-use occupancy density of any given floor in a building or the price of fuel. Therefore, it’s important to compare buildings using benchmark data as a sanity check. We must not forget that any building should be designed to be comfortable and functional.

Any thoughts on the six areas outlined above?

Ant Wilson, European Leader for Advanced Design, Applied Research and Sustainability, Aecom