The great escape
With growing evidence that air-tightness is proving a bridge too far for the industry, Stephen Cousins looks at the technical challenges facing builders and regulators.
A soon-to-be-published research paper by Leeds Metropolitan University reveals that poor design detailing, inappropriate materials selection and low-quality construction has led to unacceptable energy performance levels in a range of recently-built new homes, with some properties losing twice the amount of heat they were designed to.
The study, which collates evidence from tests on 28 different buildings built to the Building Regulations Part L 2002, 2006, 2010, the Code for Sustainable Homes level 3 and 4, Passivhaus and other enhanced performance standards, pinpoints common problems with cold bridging, poorly fitted vapour barriers, gaps in brickwork and discontinuities within insulation that allowed heat to flow through the fabric.
It found a consistent gap between design specification for energy consumption and actual energy consumption irrespective of what level of energy efficiency homes were built to, or the type of property, which suggests that the current drive towards stricter Building Regulations is not critical to ensuring better sustainable performance. Instead, the industry needs to put new emphasis on quality during the design, procurement and construction of housing.
“Some people think we just have to change the book and then everything will be okay, but in reality the level of complication in producing a typical house has increased enormously over the last 20 years, which requires more skill to get the energy performance right,” says Tony Perks, head of Building Control at Stratford-on-Avon District Council and chair of the CIOB’s Building Control & Standards Faculty.
Demonstration projects abound, such as these carbon light homes by Velux. But research suggests lessons learned have not been absorbed well enough
Effective product substitution
“There’s a history with our industry that we just sneak over the line with building performance targets, but thermal performance issues are much more significant than construction firms realise, which, among other things, comes down to education, joint ownership of the design, attention to quality and effective product substitution,” adds Perks.
The 28 buildings tested in the Leeds Met study included 26 new build detached homes, semi-detached homes and bungalows constructed using either timber frame, masonry and prefabricated building systems, plus a 1950s bungalow and a 1930s semi-detached house. All the properties were randomly selected — so house builders could not volunteer a house they knew had been built more carefully.
The buildings were subjected to whole house heat loss tests to measure the actual thermal performance of the building fabric as constructed. In most cases there was a thermal performance gap between expected design performance and that achieved in reality. “In some cases the gap was excessive and not within any level of acceptable tolerance,” says Professor Christopher Gorse, director of the Leeds Sustainability Institute, based at Leeds Metropolitan University, and one of the report’s authors.
Commonly observed construction faults had resulted from a failure to adhere to the design specification: ad hoc changes were implemented when a detail could not be built without considering the implications on performance and follow on trades; different amounts of materials were used than specified, for example, different quantities of timber were installed in panels, leading to cold bridging and a reduced amount of insulation between timbers. Meanwhile, value engineering sometimes meant that materials were substituted for an alternative without considering the impact on performance.
But most faults identified were the result of either bad workmanship or site-based quality control. They included: n insulation either missing, not fitted correctly, or not taped and sealed;
- discontinuities within the thermal barrier in difficult-to-access areas such as the eaves, around entry holes for services or around abutting beams;
- where services installation had not been properly sequenced, remedial construction work had led to punctures through the air and thermal barriers;
- gaps were observed through masonry that allowed air to circulate;
- Prefabricated panels were either installed incorrectly, meaning that additional structural members were required, or had been damaged in transit.
Thin joint masonry
Although the research was unable to identify a consistently better performing method or form of construction, effective buildings included a Passivhaus timber-framed home, built using modular panels, where considerable attention had been paid to the design and build process and the continuity of thermal and air barriers, and two prototype houses built using thin joint masonry and prefabricated structurally insulated panels.
“Our research shows that if buildings are designed and built with thermal performance in mind, they can work, but all too often not enough attention is paid to thermal performance on site and they fail to achieve the design intent,” says Professor Gorse. “In good performing buildings, proper consideration is given [in terms of thermal performance] to selection of products, design, sequencing of works, management, communication, workmanship and quality control on site so that targets are consistently achieved within a reasonable tolerance of the design values,” he adds.
As interest in the performance gap grows, the industry’s focus is now on understanding the scale of the problem. The results from Leeds Met are echoed in several other studies, the most recent being a “one-year-on” post-occupancy evaluation of the zero carbon homes development at Greenwatt Way, in Slough developed by utility company SSE with designs from architect PRP.
This research, carried out by partners BRE Trust, the NHBC Foundation and the University of Reading and due to be published this month, found that, on average, the 10 homes in the development used almost three times the amount of space heating predicted by SAP 2005 — the Government’s Standard Assessment Procedure for energy rating dwellings — and 3.4 times (336%) more when weather corrected.
At Greenwatt Way in Slough, residents enjoy their new homes but their space-heating bills run at three times the predicted level
Greenwatt Way’s energy centre integrates ground and air-source heat pumps and a biomass boiler
These Stewart Milne homes are part of the AIMC4 project
Unfamiliarity with systems
Possible causes for the increase include higher heat losses via ducting in the mechanical ventilation heat recovery system and poorer air tightness than expected.
Sometimes issues arose from tradesmen’s unfamiliarity with the systems, says Chris Wilford, associate director at PRP. “The high performance prefabricated timber panels came to site with seals and airtight detailing strips all attached, but through transport and poor manhandling many of these were ripped and damaged, leading to more taping up to achieve air tightness,” says Wilford.
The energy performance gap was also at the forefront of people’s minds during building of the first 17 homes under the AIMC4 project, set up last year to pioneer the volume production of low-carbon homes using “fabric-first” solutions.
The project, devised by a consortium of construction specialists including house builders Stewart Milne Group, Crest Nicholson and Barratt, is trialling a range of on- and off-site methods of construction, including thin joint mortar, SIPs, and closed and open panel timber frames, to find the most effective and economical designs capable of achieving Code Level 4 through the building fabric alone.
“Because the reasons for the gap are hard to pin down, we’ve tried to pay attention to every possible factor,” explains Chris Gaze, technical director at the Building Research Establishment, who is coordinating the project. “We wanted to set targets we felt could be replicated across thousands of homes, which meant setting realistic air tightness targets that could be met using appropriately simple details for volume house building. Air tightness detailing is still a bit of an art, you can’t just draw a detail and say ‘that will hit level 3 air tightness’. It’s not like Passivhaus designs, which have very specific thermally-modelled details.”
The team honed in on thermal bridging to determine specific values for every junction, in some cases redesigning details to simplify them and make them more robust. Housebuilder Stuart Milne and manufacturers, including H+H Celcon, also carried out workshops and tool box talks prior to work starting to ensure that contractors understood the systems and the factors that could impact on energy performance.
“The one thing I can take away from this now is the need to be absolutely certain that the designed system or material is then actually procured,” says BRE’s Gaze. “Too often things get substituted for equivalent products that fail to match the same thermal performance.”
Increasing evidence of the energy gap has prompted the Zero Carbon Hub (the government-backed body responsible for co-ordinating delivery of low and zero carbon new homes) to call for compliance to be based on “as-built”, as opposed to “as-designed” performance from 2016. Meanwhile, tighter regulations proposed under the revised Part L 2013 would aim to close down issues with air tightness by shifting focus to a fabric first approach to energy efficiency. It would also force developers to meet a Fabric Energy Efficiency Standard (FEES), setting out maximum energy allowable energy usage in kWh/m2/year, as well as an overall carbon reduction target.
Although detailing and site-based issues appear to have a major impact on the energy performance gap, it’s clear that they’re not the only factor in play. Low and Zero Carbon — Understanding the Performance Challenge, a report published this February by the NHBC Foundation and Zero Carbon Hub, outlines several additional areas where more research is needed, including the accuracy of SAP and other design assessment models, and the reliability of their input data — if thermal targets aren’t borne out in reality, the report argues you have to question the reliability of the design model.
Building materials and systems may not be living up to manufacturers’ claims, and it’s also argued that the complexity of some house designs could be placing an unreasonable burden on the construction team. In addition, the tests used to check the as-built performance of homes — including co-heating tests on heat loss in unoccupied dwellings — may not be providing accurate figures.
Nevertheless, there is clear agreement on the need to close the performance gap. “People are only just waking up to the fact that there’s a problem. We’re going to build a load of expensive houses against zero carbon standards and if we think that’s enough to solve the problem then we’ll have a nasty shock in 10 years when buildings underperform and require an expensive retrofit job,” says Richard Guy, senior strategy and operations manager at the Carbon Trust. “Getting it right first time is pretty crucial because many of the issues will be hard-wired into the buildings upon completion.”