Thought leadership
11 . 09 . 17
By Kev Coulin, Bristol
The often discussed and much experienced performance gap is what’s at work when you get a much larger energy bill for your new building than you were expecting. For owner occupiers, involved throughout the design and construction process, this can be particularly frustrating and for those whose revenue budgets are increasingly under pressure, realising your new building is going to cost much more than you had anticipated can be a painful experience.
At the heart of the issue is how you arrive at an expectation of in-use energy consumption. Historically, designers didn’t typically consider in-use energy consumption in the process of developing a building design. This wasn’t necessarily because they didn’t want to, but more often because they hadn’t been asked to. So, there must be a question around what any expectation is based on – if there was no consideration of in-use energy consumption, how can we determine if actual energy use is excessive?
Irrespective of whether the expectation is formed on a solid basis, the fact remains that it exists, and, it is frequently not met. This has, not surprisingly, often left clients feeling disappointed with this aspect of their new building. The issue has been compounded by industry rarely reviewing new building performance in a thorough way; post-occupation, thus failing to effectively learn lessons which could improve their next project.
Although the issue was first properly identified in the 1990s, it is true to say that it is only relatively recently that industry has started to grapple with the performance gap issue in a meaningful way.
A strategy to close the gap
One approach to tackling the issue is to utilise the Government Soft Landings (GSL) process to deliver the project; ensuring that the entire process, from briefing to post occupancy evaluation, is adopted. Although GSL addresses issues other than the performance gap, Section 5, Environmental Management of GSL includes a requirement to develop an estimate of in-use energy consumption as part of the design process.
Industry guidance, Technical Memorandum 54 (‘TM54’), sets out a method for evaluating in-use energy consumption at design stage. Although there were some examples of similar work being undertaken previously, they were few and, since the launch, momentum has been steadily building in requiring design teams to produce estimations of in-use energy consumption as part of the design process.
Don’t leave it all to the designers
In order for the TM54 (or any other method) of estimation to be as accurate as any future prediction can be (TM54 specifically recognises the uncertainty of making predictions), it is essential that the client is fully engaged with the process.
Understanding how the building will be used (i.e. levels and periods of occupation) and the equipment which the users will bring into the building is essential to the development of an estimate of energy use and only the occupier of the building can meaningfully consider these issues.
Consider the evolving design
Similarly, in order for the work to be useful, timing is key. Carry out the assessment at several points in the design process as well as immediately after building completion (so that commissioning data can be taken into account), and, for those iterations during the design period, ensure that they are used as an integral part of the design process rather than something which is separate to the main work of the design team. The ‘as-built’ assessment should be used in the building log book and be used as the yard stick for measuring performance.
Make sure it works
There’s no doubt that designing and constructing buildings is a complex process and, in most cases, there will be a period of bedding in when the building and its systems should be fine-tuned. This period will almost always last at least 12 months, not least because the building needs to experience a full year of weather variation to allow systems to be optimised.
It is essential that the project define and include aftercare activities immediately after occupation and during the first year to review performance and identify where there are deviations from the estimates developed during design – the performance gap. If the process has been implemented effectively, this should hopefully be somewhat smaller than otherwise. This work should focus on making adjustments to the building in order to close the performance gap to an acceptable level, thus reducing energy consumption, running costs and carbon emissions.
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