When a new low carbon building doesn't perform as well as hoped, 'the design/performance gap' is often blamed. But is there really a gap between design intent and actual performance?
The evidence certainly shows that design calculations for energy use are much lower than the monitored data. But are we comparing like-with-like?
The design stage estimates are often based on calculations to demonstrate compliance with Building Regulations Part L as this is the only calculation that designers have to do.
It considers only the 'regulated' loads - heating, cooling and internal lighting - and excludes other loads such as small power (ICT), external lighting and lifts, that can be responsible for over a third of the energy use in an office building. It also assumes standard occupancy profiles (for example, occupied between 8am to 6pm five days per week), meaning that a building running for 24 hours a day is going to use considerably more energy than in the energy model.
The model also makes what may appear to be sensible assumptions, such as lights by the window being off during daylight hours, when in reality many buildings have the lights on all day. 'Blinds down, lights on' is a common sight.
This doesn't mean that the Part L calculations are wrong.
They are simply intended to provide a standardised calculation that allows designers to demonstrate compliance with the regulations.
If we want to try and estimate energy use in operation at the design stage, then we need to do a calculation that accounts for the likely occupancy patterns, control regimes and all of the end-uses in the building.
Then we can establish a range of energy performance scenarios and test out the sensitivity of changing some of the variables.
Not only will this help to close the gap between design and performance, but it will help us to identify where to put our efforts in saving energy.
Dave Cheshire is regional director of sustainability at AECOM.
You might also be interested in reading 'EPCs don’t tell the whole story' or 'Why do low energy buildings not always live up to expectations?'