Home insulation schemes could be underestimating heat loss, new research finds

A paper published in the SCI journal Energy Science and Engineering called Whole‐House Thermal Bridge Modelling for Accurate Retrofit Energy Prediction says that seemingly small weak spots in a home's insulation create a route for heat to escape. These crucial gaps, known as thermal bridges, which occur around walls, floors, roofs and windows, can make a significant difference to how much heat is lost, even after homes are insulated.

The researchers, from the University of Salford and Leeds Beckett University found that a carefully designed whole-house retrofit reduced the home's overall heat transfer coefficient by as much as 63% using external wall insulation and 61% using internal wall insulation, compared with the original building.

But they also warn that commonly used default figures for estimating heat loss are often not representative, particularly after homes have undergone piecemeal insulation retrofits. This potentially matters because energy models are used to predict the effects of improvements to Britain's housing stock and inform frameworks including Energy Performance Certificates and retrofit assessments.

Dr. Christopher Tsang, lead author of the study, from the University of Salford, said: “Retrofitting insulation into homes has become increasingly important to help meet climate targets. However, as buildings become better insulated thermal bridges can become a more significant proportion of the heat loss.

He added: “Rather than simply asking how much insulation should be installed more attention needs to be paid to how insulation joins together across a whole building to fill in the thermal bridges.

“As well as heat loss, poorly addressed thermal bridges can also produce colder internal surfaces, which in turn can increase the risk of condensation and mould. We found that whole-house approaches could potentially help mitigate surface-condensation risks as well as reducing overall heat loss.

“Our call to government and industry is that we need better data on where heat escapes from different types of homes. Using more realistic figures could improve predictions of how well insulation and other retrofit work will actually perform. 

“Assessments should also take better account of the benefits of improving the whole house together, rather than treating measures such as wall, roof and floor insulation separately,” said Dr. Tsang.

The study draws particular attention to the simplified assumptions used when assessing existing houses.

Under Reduced Data Standard Assessment Procedure, or RdSAP, bespoke calculations of thermal bridge heat loss are not normally used. Instead, a default value is applied across a property. The research notes that the default is applied irrespective of factors such as the age, type and insulation level of the property.

While that simplification may have practical advantages: individually modelling every building junction would be costly and time-consuming, and the precise construction of an older home is frequently unknown. But the researchers argue that reliance on generic assumptions becomes more significant when modelling the effects of fabric retrofits.

Their proposed alternative is not necessarily bespoke computer modelling for every British home. Instead, the study calls for a more comprehensive and representative database of thermal bridge values covering common building types and PAS 2035-compliant retrofit scenarios, which could make energy-performance predictions more reliable. 

The team who carried out the project were Dr. Christopher Tsang, Energy House Labs, School of Science, Engineering and Environment, University of Salford, Greater Manchester, Felix Thomas, David Glew and Matthew Brooke-Peat, all Leeds Sustainability Institute, School of Built Environment, Engineering and Computing, Leeds Beckett University, Leeds.

The work was funded by the UK Department for Energy Security and Net Zero.