Retrofitting homes for energy efficiency

Retrofitting energy-efficiency measures can help the UK take a huge step towards its net-zero carbon targets. But how can it do so at the speed and scale required?


  • Jon Warren

24 October 2019

The UK's piecemeal approach to installing energy-saving measures isn't fit for purpose. A radical solution is needed – one that reduces the need to heat by so much that clean energy generated and stored on site will suffice to meet it. The technology now exists to reduce a home's energy demand by 80 per cent by combining retrofit products, which represents a crucial step towards meeting the government's new ambition of net-zero carbon emissions by 2050. But how can we retrofit enough homes in time?

We need a whole-house approach that works at scale, and this means a product built in the factory. A terrace of homes may all look the same but in practice they all have slightly different measurements, therefore panels have to be individually designed to achieve a tight fit, where 10mm gaps between standard one-size panels would be too big to achieve airtightness.

By creating high demand for volume and moving assembly into factories, we get bespoke measures that are quick to install while at the same time reducing costs and making products more energy-efficient. At present the target market in the UK is social housing, as this provides the fastest route to creating the necessary demand.

Reports from the Institute of Engineering and Technology as well as environmental think tank Green Alliance have already identified Energiesprong's standard as a viable retrofit approach that can be readily scaled up. What particularly appeals to social housing providers is Energiesprong's long-term guarantee of affordable net-zero energy use that also ensures everyday warmth.

Energiesprong, Dutch for 'energy leap', is a standard for whole-house refurbishments and new builds that requires contractors to achieve affordable, year-round comfort for three decades at no extra cost to households. To meet this standard, a retrofit must provide a 21°C year-round temperature in the living room and 40 minutes of hot water a day, plus adequate clean electricity to run appliances. How these standards are achieved is left to the contractor, but Energiesprong's Dutch experience has shown that on-site works can typically be completed in ten to 15 days.

The goal is net-zero energy use, so each refurbished home must produce enough clean energy to meet its heating, hot water and electricity needs. Money that would normally be spent on bills and maintenance pays for the works, so there is no additional cost to the occupants or housing providers. Success is judged in terms of affordable comfort in the long term.

Smaller, forward-thinking construction enterprises such as Melius Homes have already embraced net-zero energy as a badge of excellence, with managing director Rob Lambe seeing a shift in sentiment: 'I think the industry has woken up to the need to embrace smart design, and the need for better performance.' Recent Construction News, Ashden and EU Sustainable Energy Awards support his optimism; Melius has attracted government funding towards the cost of setting up its first Nottingham factory, and has started rolling out retrofits that will bring the Energiesprong standard to 155 homes in the city with support from the European Regional Development Fund.

EU funding has so far been essential for introducing, adapting and demonstrating Energiesprong in the UK. The National Energy Foundation led the pioneering EU Horizon 2020-funded Transition Zero project, which was designed to establish appropriate market conditions for the wide-scale introduction of net zero energy homes across Europe, and has now seen this through to demonstration phase. These adopters are two pilots, both funded by the Interreg NorthWest Europe E=0 project, which interested stakeholders have been able to visit to understand how Energiesprong works in practice.

The first of these is a new 17-home pilot now under way, led by Nottingham City Homes with Melius carrying out the work, and prefabricated roof and wall panels were placed by crane in June. There is also a five-home pilot in Maldon, Essex being carried out by multinational utility company ENGIE for housing association Moat, which is nearing completion.

The latter bears witness to some of the challenges involved in net-zero energy retrofits in the UK. These pairs of semis all looked similar, but no two are exactly the same. Two pairs were fully retrofitted, while one side only of the third pair was retrofitted. This will test the impact of thermal bridging where an unimproved right-to-buy home sits next to a retrofitted housing association-owned property, and extra monitors will be installed in the party wall for example. The local planning authority required that the street scene be maintained, entailing compromises such as keeping chimney stacks and dormers even where they shade solar panels. ENGIE's regional managing director Simon Lacey concedes that 'retrofits can be highly complex. We created bespoke measures for each of the five properties, providing the same Energiesprong result.'

An original semi-detached house on the left and a retrofitted house on the right

One of the surprising findings from the Maldon pilot is that even a home with an average energy performance certificate (EPC) rating that would make it appear to be a lower priority for energy efficiency improvements can be expensive to heat. This is partly because these pilot homes are highly exposed to the wind, near the coast and with open land in front of and behind them. Moat's director of property services Jason Amos says: 'These properties had an EPC rating of a low D, but bills were particularly high. The retrofit has allowed us to build a highly insulated house around the existing property. The aim is for these homes to achieve an A rating.'

ENGIE has wrapped a super-insulated and airtight shell around the existing structures and installed triple-glazed windows. This will dramatically reduce heat loss and also limit road noise. Mechanical ventilation will pre-warm fresh air using residual heat from the stale air it expels. Roof and wall panels with 200mm of mineral wool insulation were produced off site by Mauer and installed using a crane. The wall insulation extends below the damp-proof course, and glued polybead insulation has been blown into the 300mm void below floors to prevent any through-draughts.

ENGIE opted for lightweight acrylic brick-effect external wall insulation panels from Mauer, colour-matched to the brickwork being covered. It also replaced the dormer windows to the front, with the new windows being triple-glazed and letting in plenty of light.

The chimney has been capped and sealed and high-performance doors have been added, which has kept the look of the facade consistent with the other homes in the street as the planning authority required.

A compact, fully integrated Factory Zero energy module, which contains an air-source heat pump, hot water tank, solar photovoltaic (PV) inverter and mechanical ventilation, has been connected to the back of each property, and the integrated monitoring kit will keep the energy supplier, landlord and occupant informed about performance with real-time measurements. Equipment in the module can be maintained by a third party without the owner needing to be at home. Battery storage will also enable electric energy to be stored for use when needed rather than exported to the grid, which does not offer much return. The final energy performance will not be quite zero, given that the whole roof could not be covered with PV, but the energy performance of the homes is guaranteed for 30 years.

Although the cost per individual retrofit is currently high and requires grant funding in order to be viable, the Dutch experience proves the price can fall rapidly as the work scales up. Energiesprong UK head of team Emily Braham says: 'This pay-as-you-save model works because of the performance guarantee.' Nonetheless, to ease this initial stage and limit costs, net energy consumption of up to 1,500kWh per year is currently allowed rather than the net zero that is desired.

Braham adds that there are 11m older, hard-to-heat homes in the UK that could be retrofitted to the net zero standard with existing technology. Today the cost is too high, but once 40,000 retrofits are in the pipeline – which is the plan by 2030 – costs can halve. It would then be entirely possible to self-finance high-volume, low-carbon makeovers from energy and maintenance savings.


Related competencies include: Sustainability

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