Closing the gap between design and as-built performance

As the house building industry recovers from recession, it faces a different set of challenges.

How do we meet the projected demand for more homes? How do we address the increasingly complex technologies and systems required to meet the ever-increasing regulatory requirements? How do we ensure that completed homes delight new owners, not just on day one, but for many years to come?

Not least among this jigsaw puzzle of pieces is the issue of delivering homes that meet the expected energy performance, and more specifically the Zero Carbon Hub’s recommendation to Government, in 2011, that by 2020 at least 90% of all new homes should meet, or perform better than, their designed energy/carbon performance. Since making this recommendation the Hub have been working with the housebuilding industry to understand how this ambition might be realised.

At the 2014 EcoBuild exhibition the Zero Carbon Hub released their latest report on the evidence of a performance gap between designed, or expected performance, and as-built performance (Closing the gap between design and as-built performance, Evidence review report). In the context of the Hub’s project, the as-built performance is taken as the predicted performance based on the constructed home, rather than the measured performance with an occupant in residence. Their findings highlight problems with skills and knowledge, communications and responsibility for the final energy performance of the dwelling.

Previously, the Hub published an interim report in July 2013, Closing the Gap between Design and As-Built Performance that drew on the experience and expertise of over 140 professionals in the house building industry that identified many possible sources and causes of the performance gap. From these ideas the Hub derived a list of over fifty issues that needed researching and evidencing to determine whether they occurred systematically or randomly, and to what extent they caused or influenced the performance gap. The Hub team devised a research plan that included an extensive review of over 100 published and un-published reports, an end-to-end review of the housebuilding process on development sites volunteered by housebuilders and an audit of SAP assessments. Much of the evidence gathered and reviewed is not currently in the public domain and was only made available to the Hub and its researchers to help inform the findings.

Perhaps the most enlightening element of the work has been the findings from the end-to-end review. This has sought to systematically collect data at each stage of the housebuilding process, starting with concept design, moving through detailed design, procurement and construction. Semi-structured interviews with all the project teams were conducted in stages, design information was reviewed and site inspections made.

These inspections have been timed to allow the greatest number of active plots to be reviewed on a development, from early stage plots just coming out of the ground through to those with final decorations being completed. Whilst to date a relatively small number of major housebuilders’ developments have been reviewed and reported on, the project is on-going with the scope widening to include smaller, regional housebuilders. The Hub team are aiming to review the different procurement strategies that are adopted throughout the industry including the volume approach, the small one-off project approach and a contracted approach.

From all this evidence the Hub has identified fifteen areas where the industry and government need to work together to develop solutions that will help meet the Hub’s recommendation and 2020 ambition. A further seventeen areas are identified where more evidence gathering and research is needed to fully understand their impact in the context of the performance gap.

What do these highlighted issues begin to tell us?

• That we struggle to communicate the intended energy performance for the design from the earliest stages, and that we have ongoing problems with communicating the design intent throughout detailed design.
• That we also don’t communicate back from site what is, and what is not buildable. This greatly influences architectural detailing issues and site practices that may have been acceptable 20 years ago, no longer meet the required standards.
• Finally, that there is nearly a complete absence of engineering rigour around the design and installation of the services for our homes.

To move forward and make buildings better, the housebuilding industry needs to engage in understanding what their clients (homeowners, social and private landlords) really want, to update its methods of communicating and sharing information and adopt a system of verifying performance that covers the whole process from design to construction.

NB: In November 2015, BSRIA announced that it would back a four-month feasibility study to develop a prototype UK scheme intended to deliver the standard of energy performance specified in client briefs by adopting a ‘design for performance’ approach, first pioneered in Australia. See BSRIA support study into Australian solution to performance gap for more information.

This article originally appeared in the May 2014 edition of BSIRA’s Delta-T magazine. It was written by Ian Orme, Team Leader Sustainable Construction Group. It was posted here by --BSRIA 07:55, 7 December 2014 (UTC)

[edit] Related articles on Designing Buildings

• BREEAM.
• BREEAM: Value multiplies while costs plummet.
• Building performance metrics.
• Building use studies (BUS).
• Code for sustainable homes.
• Domestic ventilation systems performance.
• Leadership in Energy and Environmental Design.
• Performance.
• Performance gap.
• Performance of exemplar buildings in use: Bridging the performance gap FB 78.
• Soft landings.
• Sustainability.
• University of East Anglia - case study.
• Zero carbon homes.
• Zero carbon non-domestic buildings.