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The performance gap - let's be rational about this

05/04/2012 15:21:01

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Cutland Consulting was recently privileged to write the NHBC Foundation's report "Low and zero carbon homes: understanding the performance challenge", ref. NF41.  The following is the Executive Summary of that report, reproduced by kind permission.  The full publication is available on the NHBC Foundation website.

If the energy consumption of an occupied home is greater than its designer predicted, then its carbon dioxide emissions will also be higher than predicted – this is the ‘CO2 performance gap’. There appears to be a growing body of research evidence that new housing is failing to deliver the anticipated levels of CO2 emissions, although there is relatively little understanding within the wider industry of what might be causing this.

Studies of housing schemes over the last 30 or more years have provided many useful insights, although they do not agree on the precise causes or the scale of the problem. For example, the design predictions for space heating energy at Milton Keynes Energy Park in the late 1980s showed extremely close agreement with measured consumptions. When low energy housing in Salford was revisited, the average energy consumption was found to be almost exactly the same as had been measured 20 years previously. More recently, co-heating tests at Stamford Brook, Altrincham, revealed that party walls did not have the zero U-value assumed in the standard design calculations; homes at Elm Tree Mews, York, had a higher heat loss than had been predicted, due to additional structural timber in the manufactured frame; and latterly a co-heating test on the Avante development in Maidstone measured the fabric heat loss as slightly better than predicted. There are seven key questions that need to be considered in order to understand and subsequently reduce the CO2 performance gap:

1.  Is the assessment model that was used to make the prediction accurate, and has it been correctly implemented in the software used by the designer?

2.  Is the model’s input data correct (and if not, is that due to the conventions or the user)?

3.  Is the home’s design overly complex, presenting unreasonable challenges to the construction team?

4.  Are there fundamental construction quality and skills issues?

5.  Do building materials and mechanical and electrical (M&E) systems perform as well in practice as laboratory tests predict?

6.  Do changes in specifications get properly communicated?

7.  Are the post-construction tests and checks appropriate and adequate?

There are ways of mitigating the effect of all of these possible causes. For example:

1.  The assessment model. BRE periodically validate the accuracy of the UK’s Standard Assessment Procedure(SAP) and re-calibrate the calculations. The Zero Carbon Hub has concluded that, subject to a number of technical enhancements, SAP should continue to be used as the carbon compliance tool for new homes.

2.  Input data. User errors can be reduced by employing an accredited On-Construction Domestic Energy Assessor (OCDEA). If monitored data on the performance of products or systems is not available, mandatory ‘safety factors’ could be introduced. The SAP conventions should continue to be regularly reviewed by BRE.

3.  The design. There are numerous benefits to making designs simple and buildable, and even low or zero carbon homes can be simple in principle. Designers should aim to produce designs which encourage site operatives to get the detailing right, which aim to eliminate the need for improvisation on site, and which make it easy for installers to route pipes and ducts. If challenging details cannot be avoided altogether, small sample sections could be built onsite and the workforce educated in new techniques.

4.  Construction quality and skills. Passivhaus-like approaches, which include an airtightness champion, clerk of works, photographic recording, etc, could clearly reduce the performance gap, but are unlikely to be workable in the mass market. Increased use could be made of existing industry publications, training schemes, good practice guides, etc, and new training techniques might also be tried.

5.  Building materials and M&E systems. New laboratory test methods could be used which better simulate real-world conditions. The performance of M&E systems could be tested as an installed whole rather than as a kit of parts.

6.  Communications. Simple systems could be put in place to improve inter-team, as well as intra-team, communications. Approved Document L1A now includes the requirement that SAP calculations are lodged before work starts as well as on completion. There is some interest in using building information modelling (BIM) for housing, as well as for larger projects, to improve communications.

7.  Post-construction tests and checks. It could be unwise to regard Building Control as an instant panacea. A greater emphasis on mass-scale monitoring would enable the performance gap to be systematically diagnosed, and the learnings  to be fed back to the housebuilding community. Any prescribed methodology must be pragmatic – for example, simple fuel consumption monitoring via smart meters may be more workable at scale than, say, co-heating tests.

A move towards simpler built forms and design features would not necessarily lead to architecturally uninteresting homes. Balconies, bays and projections can be designed as free-standing features which do not create thermal bridges. Simple built forms, if well-proportioned and detailed, can be visually pleasing.

It is clear that there are a multitude of possible causes of the performance gap, which between them span the period from the early predictions through to the occupation and operation of the finished home. In principle there are ways to mitigate the effect of each of the possible causes, and by applying a combination of these solutions a significant reduction in the size and extent of the performance gap could be achieved.

April 2012

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