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Enhabit Blog » How’s it performing?

How’s it performing?

By Akta Raja , Tuesday 11th December, 2018

At a recent project meeting with some Council Employees, I was describing the advantages of Passivhaus compared with other standards, in that it does not suffer from the ‘Performance gap’ i.e. it actually performs as it was designed to.   I was interrupted with the statement that “no-one cares about the performance gap”, and was compelled to write this blog. 

Why should we care about the Performance Gap?

The performance gap has been in our language for some time now, but really came into the limelight in 2010 when the, now defunct, Zero Carbon Hub were commissioned by Government to undertake a review of energy modelling for new homes and produced ‘Closing the Gap between Design and As-Built Performance’, March 2014.  The report focussed on how to improve the performance gap, there was no question that it exists in the UK.   There is plenty of evidence that houses are not performing as they were designed, that energy bills can be up to five times higher than planned.[1]  However this doesn’t really answer the question, ‘why should we care about it?’ Clearly this is a big oversight, if the people who are the gatekeepers and financiers to our new buildings just don’t see the point.   

The drivers for improved building performance are limited now that the ‘in-use’ zero carbon targets have been scrapped.  The Hackett review (into building regulation sand fire safety post-Grenfell) will direct close scrutiny to the construction industry and may help improve onsite quality and building control procedures.  In the mean-time it is up to energy professionals to explain the unintended consequences of the performance gap to anyone that will listen.  So why should we care about the performance gap?


Clearly the energy performance gap is going to affect the energy bills of a property.  Let’s be clear that occupant behaviour can have a big impact on the energy bills of any home.  This can be seen in studies of houses that have been built to exactly the same standard with high quality design and stringent onsite quality controls, with an experienced design team and contractor who are well versed in avoiding the energy performance gap.  There are still things we can do as an industry about this and that is to educate occupants of our houses and much as we can, and design user friendly buildings.

Energy bills may be higher due to poor installation (or specification) of insulation and excessive draughts where they shouldn’t be.  In addition to this are in efficient heating, hot water and ventilation systems (due to poor design, installation and commissioning).  So why does it matter if we have lower energy bills? Well at the moment, for the majority of the population it’s not high on the list of household costs as energy is still a relatively low-cost living expense.  Of course this could change in the future with an increasing number of properties being built with all electric services, and rising electricity costs.   For social landlords this is problem, as a significant portion of their tenants are in fuel poverty.  In this case the performance gap can mean having to choose between paying the rent, eating or keeping warm.


Health & Well being

A building with a performance gap, and lack of quality in construction, is more likely to see unintended consequences, which means poor air quality, uncontrolled humidity levels, cold spots and ultimately mould growth. 

The newly formed UK Centre for Moisture in Buildings hasrecently released a report demonstrating the link between excessive moisture (or lack of) in buildings and the health of the occupants[2].  Mould growth is shown to be associated with over 17 different health conditions including asthma and eczema, and even some mental health conditions. In new houses, the increasing airtightness combined with the frequent failure of ventilation systems, typical of those buildings with a large performance gap, is a particular concern. The report quotes as yet unpublished research from DCLG and the Zero Carbon Hub that over 95% of new homes fail to reach the required ventilation rates.

The long hot summer of 2018 has bought the issue of overheating to the fore – Kostas Mourtos research on overheating and the performance gap Loughborough University 2017

Well being is the state of being comfortable, healthy or happy.  In poorly built houses, reduced comfort is common with occupants feeling too hot or too cold.  Cold spots on walls or around windows can make us feel colder when we stand near them. Buildings that completely miss their air tightness targets expel warm air from the building, costing money, and bring in cold air in unwanted areas.  More external air entering into the building and bypassing the filtered ventilation system can also be a serious concern for our health in cities where the air pollution levels are high.

The performance gap is also prevalent in retrofit.  BBC Radio 4 recently reported on poorly designed and installed external wall insulation that was funded under the Green Deal or ECO schemes. These were causing thousands of cases of condensation, damp and mould across the country. The new PAS 2035 PAS 2035:2018 Specification for the energy retrofit of domestic buildings strives to improve the quality of retrofit and to reduce the performance gap and unintended consequences.

Overheating caused by uninsulated pipes



Ultimately, if a building is put together badly in the firstplace, then maintenance is likely to be higher in the long term.  For example, mould problems are a maintenance nightmare, especially for social housing where these problems are most prevalent.

Building ventilation and heating systems that have been incorrectly installed and commissioned will complex controls will not be working efficiently and are likely to lead to a performance gap.  As well as this, the systems will be working under stress and therefore maintenance periods will be shorter, and a higher probability of system failures can be expected.


In summary, the performance gap is not just about energy, it is the effect of poor quality design and construction across the board, that is unfortunately embedded into our construction industry. When people talk about standards such as Passivhaus and the AECB Building Standard, that do not exhibit the performance gap, the benefits are not just lower energy bills, but far wider reaching.  It’s not these standards themselves that result in these buildings performing as designed, it’s the discipline required to meet the standard that counts. Inherent in these standards is design rigour and onsite quality control which result in not only a beautifully comfortable living environment, but also lower maintenance costs, and a standard of workmanship that extends way beyond energy efficiency.

Enhabit works to ensure our new builds and retrofits do not exhibit the performance gap.  We use basic principles of building physics in our modelling and design work, and make sure that as many of our project as possible are monitored post-completion.  If you would like to learn more about our work, please see our case studies, or contact

[1] Closing the Gap, Carbon Trust, July 2011



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