How has the construction industry tried to tackle the performance gap so far?

A range of work has been done across the construction industry looking at reducing the performance gap. Unfortunately, for a variety of reasons, this work has tended not to have the widespread reach required to make consistent improvement occur across the sector. Despite awareness of the ongoing issues, the result is that performance gaps remain commonplace.

It might be expected that national building regulations would be written so as to avoid the possibility of performance gaps occurring. However, until the introduction of Part L 2021 and photographic evidence, there was little acknowledgement in regulations that the performance gap was an issue.

How did the Zero Carbon Hub try to reduce the performance gap?

During the 2000s and 2010s, organisations such as the Good Homes Alliance, the Carbon Trust and the NHBC Foundation investigated causes of the performance gap. Some of the most notable and well-publicised work was carried out by the Zero Carbon Hub, an organisation established in 2008 to support the delivery of zero carbon homes in England.

When the planned introduction of zero carbon new homes in 2016 was scrapped by the government, the Zero Carbon Hub’s funding was stopped. The Hub’s website remained live for a time, as an ongoing resource for people to use when looking to reduce the performance gap. However, that resource no longer seems to be available – though Hub documents have been archived on the Local Authority Building Control (LABC) website.

Useful documents produced by the Zero Carbon Hub included guides to thermal bridging and building services. Further highlighting the need to tackle junction details correctly, the Builder’s Book illustrated common details that are prone to being constructed poorly and making performance gaps more likely.

Although the Zero Carbon Hub is no longer active, one of its contributors – architect Tom Dollard, of Pollard Thomas Architects – has continued to champion better construction standards through his book Designed to Perform.

What lessons can be learnt from the Passivhaus standard?

While the work of the Zero Carbon Hub and other organisations has focused on ‘typical’ construction projects and build quality, the Passivhaus standard has forged its own path.

When buildings constructed using the Passivhaus methodology are monitored, they frequently demonstrate little or no performance gap. Fundamentally, they perform as designed, and where any performance gap does exist it is at a much smaller scale than in conventional construction.

There are many facets to the standard that produce this result. Arguably the best way to summarise it is that Passivhaus takes quality control very seriously.

For a building to be certified, it must meet the threshold performance for airtightness and surface temperatures. That takes collaboration and attention to detail at both design stage and during construction.

Those are lessons that are not hard to learn and apply to other projects, but still the Passivhaus standard is seen as relatively niche. It is often cited as adding cost to a construction project – something that is hotly debated, but which remains a negative perception even though the potential long-term savings stand to outweigh the upfront expenditure.

Have national building regulations ever tackled the performance gap?

The joined-up approach between building design and construction that characterises Passivhaus is not something that has generally been evident in the way building regulations are written.

A common criticism of regulatory requirements is that they are not joined-up themselves. Energy efficiency and ventilation standards have often been criticised as not working well together, leading to buildings that are flawed even though they are deemed to be compliant.

This is because national building regulations are a minimum standard to meet, whereas a standard like Passivhaus is voluntary and exceeds the minimum, with the intention of delivering a specific end result.

The main mechanism by which building regulations have tried to enforce as-built performance is through a second set of SAP calculations based on what has been installed on site. A dwelling can only be signed off once these calculations have been received by the Building Control Body.

However, these calculations may still feature a lot of assumptions – especially where the construction quality of junction details and linear thermal bridges is concerned.

A lot depends on how junctions and thermal bridges were detailed at design stage, what psi values were calculated or assumed, and how closely the detailing was inspected during construction to ensure continuity of insulation. If thermal bridges have not been constructed to a consistent level of quality then, ultimately, the building is likely to perform worse than the calculations predict.

Accredited Construction Details (ACDs) used to provide junction details with checklists to follow on-site, with the aim of ensuring they were constructed correctly. But ACDs have since been discontinued, and it was never a formal requirement to use them anyway.

In summary

Although national building regulations are taking steps to address performance gap issues, there is still a long way to go before the matter can be considered addressed. Good thermal bridge detailing remains a key tool in helping buildings to perform as intended.

As the work of the Zero Carbon Hub and the Passivhaus standard shows, reducing the performance gap starts with designing details that perform and which are buildable. The detailing then has to be followed through to site, making sure that details are built accurately and with attention to detail.

Unfortunately, the opposite often happens and junction detailing is seen as something that is too complex, too expensive, or both. As a result, the required psi values become harder to achieve in the finished building – even if the as-built SAP calculations are signed off.

BRS Technology has tried to address this by developing its online modelling tool, AutoPSI. Using AutoPSI, the calculation of psi values is quicker and easier at design stage, helping to produce details that can be translated to site.