Inkling Blog #2: What is the Building Performance Gap?

There has been a lot of discussion in recent months concerning ‘The Building Performance Gap’. However, on closer inspection it actually turns out that there are many different types of Building Performance Gap. These are caused by a wide spectrum of conceptual and quality issues ranging from a lack of clarity in the industry regarding building performance criteria to poor build quality on site.

So what exactly are these performance gaps?

Firstly there is a lack of understanding of the purpose and differences between the various compliance and rating criteria. For example:

–  an Energy Performance Certificate (EPC) rating is different to a Display Energy Certificate (DEC) rating, although both use an A to G scale

–  EPCs are assessed against a Reference building, Part L is assessed against a Notional building, these ‘baseline’ buildings are very different

–  two different buildings can both achieve 10% lower CO₂ than the Part L target but have very different EPC and DEC scores

As the purpose and basis of the calculations is different for Part L, EPC and DEC assessments, there is actually no reason why these should be consistent. This obviously causes confusion and potentially also a loss of credibility for the industry. A client, not unreasonably, may expect a building performing well under Part L to have a good DEC rating.

Secondly, although Part L compliance software tools were never meant to be used for design, they have frequently been used to predict future energy use in buildings. This is a gap in understanding of the purpose of the compliance tools, which are designed to only consider regulated energy loads under a limited range of standard conditions. Perhaps the incorrect application of compliance modelling tools is an understandable outcome considering the huge uptake in building thermal modelling associated with the addition of compulsory CO₂ emissions compliance calculations in 2006. Suddenly every building had an associated thermal model and as it would take time and money to recalibrate this model, it was generally used as it was far outside of its intended scope.

Thirdly there is the gap between predicted carbon emissions from a well calibrated thermal model (e.g. considering the correct hours of use and occupancy rates) and energy use in the real building. There are many reasons for this such as building management, buildings not being occupied as designed or system malfunctions.

Finally there is the gap between the building as designed and that constructed on site, for example in terms of U-values and air tightness.

Our next Blog Post will look at ways to address the Building Performance Gaps identified above.