SBEM under scrutiny
Can SBEM (the Simplified Building Energy Model) and Part L be integrated to deliver real improvement to building performance? Tom Malina has been investigating.
The narrative surrounding the Simplified Building Energy Model (SBEM) has been a turbulent one. Since its introduction in 2006, the prevailing opinion of SBEM has become increasingly critical — to the point where the ‘simplified’ portion of the acronym is said with great irony in certain corners of the building-services industry. Many have derided it as being overly complicated, ineffectual and consequently unable to deal with the ‘energy performance gap’ (an issue is addressed in the CIBSE guide TM54), and there are some prepared to denounce SBEM as a failure.
Now, with the impending release of the latest iSBEM software coinciding with the April 2014 revised Building Regulations, which will lower the previous target of CO2 emissions reduction, SBEM and Part L are under increasing pressure if there is any hope of achieving the Government’s goal of zero carbon for new buildings by 2019.
However, the necessity and the value of something like SBEM, a tool that intends to drive efficiency targets as established in the European Performance of Buildings Directive, is clear. Therefore, it is important to consider what could be done to get people back on board. In looking at the common criticisms, it appears there are a number of factors contributing to a culture of confusion and disdain towards SBEM, as well as some significant misunderstandings as to what the tool is actually for. These need to be addressed first and foremost before any teething problems can be solved.
‘SBEM is not a design tool, categorically and adamantly. Anyone who says it is quite frankly doesn’t know what they are doing,’ said Janet Beckett, CIBSE low-carbon consultant and director of Carbon Saver UK.
A frequent misconception regarding SBEM is the idea that it should be used to design buildings, when this is not actually the case. The Introduction section of the SBEM user guide clearly outlines that it is a compliance procedure intended to evaluate the energy use of buildings, in accordance with the Part L guidance of the England and Wales Building Regulations. It does not claim to be a design tool. In fact, the user guide even encourages the use of other programs, stating: ‘It is prudent to use the most appropriate modelling tool for design purposes.’
‘At eye level, SBEM addresses a market need. I think that for the budget and resources available to develop it, it does a reasonable job,’ said Richard Hipkiss, chairman of the Energy Services & Technology Association and director of I-Prophets Energy Services. ‘Where it does tend to fall over is when people try to use it as a design tool. A lot of the issues people face with SBEM are down to the application of the user and not necessarily the functionality of the software.’
By its own admission, SBEM alone cannot ensure good design, but its involvement in the design process is required by law. Effectively, this suggests that engineers are supposed to utilise it concurrently with one of the commercially available modelling tools, like TAS or Hevacomp. But this begs the question: how well does SBEM work with these other models?
‘In my work, I end up creating two different models — one with SBEM to work with Part L and then the second model for a good M&E design, which is something not everybody can afford or choose to do,’ said Susie Diamond, building physics consultant and co-founder of Inkling LLP. ‘For me, one of the biggest frustrations with SBEM is that the database it uses is very limited. Sometimes, the two models will have different characteristics, so when you run the M&E design through SBEM, it might not be given a pass.‘
This touches upon the tool’s primary issue. SBEM operates under the National Calculation Methodology set out in the Building Regulations, and incorporates a series of complex databases, yet it has proven to be rather inflexible in numerous instances where the design realistically should comply with the Part L guidance.
A well-known example of this, which arose a few years ago, was where one commercial building that was ventilated with full air-conditioning units received a pass from SBEM. Meanwhile, around the same time, another building that used natural ventilation failed the Part L certification, despite clearly having the more efficient ventilation system of the two structures.
It should be noted, though, that in its original incarnation, SBEM did not appear nearly so difficult to work around. In fact, the initial idea was based around comparing against a set of compliant design examples for each type of commercial building — at least at face value, it seemed like an elegant system.
‘When SBEM first started out, it was pitted as a benchmarking exercise, which I think is a good thing and something to be applauded. There weren’t as many different permutations, so you put in a reasonable assumption and as an engineer, you might choose to work around some things,’ said Janet Beckett.
However, with so many different possible variables in a modern commercial building, is it even possible to account for everything? Without more standardised designs or greater communication between architects and building service engineers, perhaps not. Therefore, one of the factors necessary for a more successful SBEM would be to have more comprehensive joined-up thinking within the industry.
For now, SBEM sits in an awkward position. The 2013 revisions for the Part L guidance provide a greater number of notional buildings, including top-lit warehouses and side-lit office buildings, each with specifications to work from. While most would agree that this is a step in the right direction, there are still many calling for a long-overdue rise in standards of training, in that people should require a certain level of understanding of how to correctly use SBEM.
‘I see a lot of people who have been trained up on an SBEM course for a few days, and they just know how to tick boxes without really knowing what it all means,’ said Janet Beckett. ‘I have witnessed first-hand some highly inaccurate assessments go through to Building Control, but often they don’t understand it either, so it gets passed. Really, we just need more qualified people who know what they are doing.’
This highlights the other chief issue currently affecting SBEM. Although all assessments are sent for verification via Building Control, a combination of a lack of thorough understanding and a lack of resources means that, as of now, there is no vigorous enforcement for SBEM, and there is a growing concern that people are beginning to take advantage of this.
‘The problem with SBEM is that it’s just not policed, and people eventually cotton on to the fact that it’s not policed and they take shortcuts,’ said a building-services engineer going by the pseudonym of Ductulator Dan. ‘In my opinion, it’s just too complicated. Nobody understands SBEM apart from the people who push the button, and unfortunately the rest of the design and construction team has no idea what goes into an SBEM calculation. If Building Control pulled over some people that go into the model, it might make a difference. Unfortunately, providing the evidence for what goes into the model is a lot of hard work.’
Evidently, if SBEM is to be taken seriously on a broad level, there needs to be radical changes to the way it is policed, whether it is achieved by pumping more resources into Building Control or by performing random spot checks through a peer review system.
It does go to show that, although it has become fashionable to blame the tool itself, there are problems that go much wider which need to be addressed — such as the infrastructure surrounding SBEM and the level of understanding amongst industry professionals. Again, a tool like SBEM is necessary and potentially very valuable, so it is essential that we work to improve it, rather than abandoning it too hastily.
Tom Malina is a freelance journalist and researcher for Energy Solutions Associates