Case study: energy efficient homes in Tonbridge and Malling

Tonbridge and Malling Borough Council worked on a exploratory project to improve the energy efficiency and carbon output of social housing homes in their community.

Working with partners in the private and public sectors, they wanted to prove how cost effective it could be to retrofit existing hard-to-heat homes.

The problem they faced

New homes are built to increasingly efficient energy standards to minimise carbon output and maximise energy efficiency.

However, most social housing is made up of older buildings not planned or designed to the same standards. In many case homes built in different eras will be built to different specifications.

As they started to think about what measures will be suitable and cost effective for older properties, more practical data was needed to help decision makers make informed decisions.

What they did

Tonbridge and Malling Borough Council do not own any social housing, so they worked with a local partner Clarion Housing Group.

Clarion Housing Group identified two suitable social housing schemes. One in Tonbridge and one in Fenland, Peterborough.

The Social Housing Decarbonisation Fund Demonstrator (SHDFD) project was launched with the aim of sharing learning accross different areas and property types.

The project saw 110 homes across the two local authority areas retrofitted to gain practical experience, and learn what barriers or challenges might need to be overcome in the process.

The homes chosen for the project had Energy Performance Certificate (EPC) ratings of band D or E. The target was to lift each to an EPC band C rating and to reduce the amount of energy needed to heat the home by 50 kilowatt hours, per square metre, each year.

A key part of the project in Fenland was installing air-source heat pumps to provide central heating and hot water. Air sourced heat pumps are the most efficient energy source for houses off the gas network.

In Tonbridge and Malling the focus was on improving 60 mass-produced homes built after 1945 with a range of improvements, including improved insulation, energy efficient windows and solar panels. The properties in Tonbridge and Malling have only recently had new gas boilers installed so it wouldn't have been efficient to replace these.

The project was co-funded by a £4.5 million grant from the Department of Business, Energy & Industrial Strategy (BEIS), match funded by Clarion.


The partners were delighted with the results of the retrofits.

  • The target of all properties meeting being EPC band C was exceeded, with all ranging between EPC band B and A.
  • All properties reduced their heating demands by at least 50 kilowatt hours, per square meter, per year (kw/m2/year).
  • Each home will save about £500 per year on their energy bill (based on the UK Government's Standard Assessment Procedure model) and reduce their carbon output by 2.5 tonnes each year.

The following lessons were learnt during the project that will help housing providers in future, larger retrofit projects:

  • The key factor is to understand the potential impact of cold bridging. This is where there is a gap in insulation, for example at roof and wall junctions, where there is greater risk of condensation.
  • It  is essential for architects or retrofit designers to carefully detail external wall insulation to avoid cold bridging, and external wall insulation now often must be taken below ground level.
  • Items already attached to the external wall such as windows or doors need extra thought as these may need to be moved to accommodate the thickness of external wall insulation.
  • Advanced retrofit is still a new area for the construction supply chain (particularly the requirements detailing air tightness) and the industry needs to make use of existing transferable skills when training the workforce.

The future

All the homes improved by the project will have their energy performance, air quality, and inside temperatures monitored for 3 years. This data will make sure the improvements are sustainable for each housing type.

The partners are now working together to expand the project to another 450 properties. These will be in Fenland, Tonbridge and Malling, and a new area, the London Borough of Merton.

Get involved

If you have any questions about our case study, or have any ideas for future trials we could run, email