Performance Gap Failure Finding for Fading Energy Savings on Retrofit Study

A CAMBRIDGE UNIVERSITY STUDY found disappointing energy-saving outcomes following insulation retrofits.

The study, published in January – Assessing the effectiveness of energy efficiency measures in the residential sector gas consumption through dynamic treatment effects – shows that retrofitting insulation only reduces gas consumption for a few years.

The findings follow a 12-year study of 55,000 homes in England and Wales between 2005 and 2017.

The report’s authors – public policy, environmental and energy efficiency researchers – conclude the failure of retrofit insulation to secure long-term energy savings could be because installers have used products which don’t perform in real life as they do on paper.

It may show more positive results if it were repeated using homes in which the performance gap was addressed and a ‘whole house’ approach adopted, says insulation specialist Actis.

Architect and technical director at insulation specialist Actis, Thomas Wiedmer, said that not taking a holistic approach to retrofitting a property could be another reason why the fuel savings weren’t as long-lasting as hoped. Retrofit standards endorsing a whole house approach to energy efficiency – PAS 2030/2035 – were not even in place during the research data period.

PAS 2030/2035 is a guide for all government supported projects including those under the Energy Company Obligation ECO, but it can be used on a voluntary basis for private homes outside these schemes.

Thomas explained: “The Standard champions a matrix approach in which each individual measure is examined in the context of the whole house. This is because one action might have unintended consequences elsewhere in the property. It is important to look at each case on an individual basis.

“One of the unintended consequences which could occur if not looking at the house in its entirety is the appearance of moisture or condensation.

“For example, while on the surface it may seem sensible to insulate all external walls to the best possible U-value, this could have unintended consequences elsewhere. Where the ceiling joists meet the walls, in some scenarios, can create a weak point called a thermal bridge. This might lead to moisture build-up and mould growth so it may, counterintuitively, be better to use less rather than more insulation.”

Energy Savings

The Cambridge report revealed that insulating the lofts and cavity walls of the UK homes in its study only reduced gas consumption for the first year or two, with all energy savings vanishing by the fourth year after a retrofit.

Its authors said: “The disappearance of energy savings in the longer run could be explained by the energy performance gap, the rebound effect and/or by concurrent residential construction projects and renovations associated with increases in energy consumption.”

Performance Gap

Thomas agreed: “There is growing understanding of as-built energy performance and the existence of the performance gap. We are playing out part in this education process, through one of our RIBA-approved CPDs – Addressing the performance gap with reflective insulation – which looks at why the performance gap exists, the effects of external factors on the fabric efficiency of a building, air tightness and thermal bridging.

“Construction detailing is one of the biggest issues causing the performance gap. Tying projects up with specific details used is important to close the gap between designed and expected performance.”

“A building with insulation which is effective on paper can still see huge heat losses of 20 to 30% if thermal bridging occurs. The only way to avoid such a scenario is for construction details to be thermally approved and followed through on site,” he explained.

A set of Registered Construction Details created by the LABC in conjunction with Actis offers a combination of specific detail, good practice and points to watch, together with a range of modelled psi-values using different build ups. The RCD drawings and documents can be fed into specifications for projects.

Fabric First

“Energy efficiency standards should always be based on reducing the need for energy first and in particular limiting the heat loss through thermal elements – that is through achieving excellent U-values, reducing thermal bridging and improving airtightness – the fabric first principle,” said Thomas.

“It is important to ensure the materials used do perform in real life as they do in the paper calculations – that is that the insulation qualities of some products don’t decline with age and are combined with air- and wind-tight layers to address phenomena such as wind-washing which can negatively impact the effectiveness of insulation.”

Human behaviour, another factor cited as a possible reason behind the not-very-encouraging figures includes the ‘rebound effect.’

“An example of this is buying a new economical car but then doing twice as much mileage – meaning the owner ends up using the same amount of fuel as before,” said Thomas.

“New homes come with a new house owners guide offering advice on how to minimise energy usage. In a lot of cases, it’s to do with educating home owners to change their mindset and approach.”

The report also points out that many retrofit insulation measures were carried out at the same time as home extensions or conservatories – thus the energy saved by the insulation was then shifted into heating the new addition to the home.

“The big theme overall for me is the whole house approach,” said Thomas.

“The study looks at properties with improved loft insulation or cavity wall insulation or both combined – but what about the windows and floors? Retrofit standards like PAS2030/2035 were not in place five years ago so it would be good to see how this changes. It certainly will.  In fact, low carbon retrofit studies from the Passivhaus Trust and Sustainable Traditional Building Alliance prove this would work.

“Also, a retrofit co-ordinator – a responsible and competent professional – should oversee retrofit works to bring together the whole house approach and combine measures which make an impact. Themes like airtightness and thermal bridging and detailing are therefore becoming more important too for retrofit projects.”


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