Domestic energy efficiency - Jonathan Gregory's story

Jonathan Gregory is a climate scientist at the Met Office and the University of Reading. His home was built in 1873 and now emits no carbon dioxide (CO2). It is a Pioneer SuperHome – having been retrofitted to make it as energy efficient as possible, given the limitations of the original building. In this case study, we explore the work Jonathan has undertaken and the impact this has had.

Why retrofitting?

The UK Government target is to reach Net Zero by 2050, reducing greenhouse gas emissions in order to reduce the worst impacts of climate change. In the UK in 2022, CO2 emissions from residential properties made up 17% of all CO2 emissions¹ mostly from central heating by burning gas. A further 8% of all CO2 emissions come from generating electricity for domestic users (around 30% of all electricity²). Reducing CO2 emissions due to residential use is therefore critical in helping the country meet Net Zero targets. Using less fossil fuel energy by retrofitting energy saving measures and/or using renewable sources of energy is the way to do this.

Insulating to reduce space heating needs

Heat leaks out of houses by conduction through the roof, walls, windows, doors and ground floor. Reducing the ‘leakiness’ means we need to supply less heat to keep the house warm. A layer of thermal insulation can make a big difference to the heat loss of a home, but ~33% of lofts in Great Britain have no loft insulation and ~29% of properties with a cavity wall have no cavity wall insulation. Moreover, there are about 9 million houses that have solid walls because they were constructed before cavity walls became common in the early 20th century. Of these old houses, 90% have no insulation of their external walls³. 

Jonathan’s home is an old semi-detached house built with no cavity walls, so he took the step of adding an extra brick wall outside the side-wall to create a cavity between them, with a thick layer of insulation in it. “It looks no different from the outside, but the reduced heat loss can be seen in the thermal view of my house compared to my neighbours’,” explains Jonathan. “Other walls were also insulated internally, insulation was added underneath the floorboards on the ground floor, and double-glazed windows were fitted along with a new insulated and triple-glazed front door.” 

Graphic description: The cooler temperatures shown in the thermal imaging of Jonathan’s house on the left of the image compared to the neighbours’ house on the right, indicate the improved heat retention of the building.

Renewable energy

There are various renewable energy sources which can be considered when retrofitting homes, and Jonathan has chosen to fit solar photovoltaic (PV) panels for electricity, and a solar thermal panel for hot water. “There’s only one solar thermal panel, but that’s enough for all of our hot water needs during the summer months”, explains Jonathan. 

The electricity generated by the photovoltaic panels reduces the amount of electricity bought from the electricity company. When more is being generated than used, which happens during the day, especially in summer, the electricity company buys the excess. Over a year, the PV panels generate about as much electricity as is used for everything in the house except the central heating.

Jonathan replaced the gas boiler in 2020 with an air source heat pump (ASHP), which runs on electricity and is 2-3 times more efficient than burning gas for heat. “For the ASHP, we didn’t need to make any other change to the central heat system, such as replacing radiators. I’m pleased that at last we’re not burning any fossil fuel on the premises!” 

Image description: An array of solar photovoltaic panels and one solar thermal panel on Jonathan’s roof.

Cost and carbon savings

The exact savings to be realised from measures such as those taken by Jonathan will vary depending on the size of property, and the Energy Saving Trust has up to date estimates for different types of insulation and property size. For example, cavity wall insulation for a semi-detached house may cost around £2700, will result in savings of around £280 a year on energy bills and reduce household CO2 emissions by 650kg a year.

Grants are available towards the cost of some energy saving measures and details of the latest schemes can be found on the Government’s Energy Efficient Home website. These can make a notable difference to the initial outlay required and therefore the pay-back period.

Jonathan has been making improvements to his home for over 20 years, and has kept track of his energy usage during that time.

As is demonstrated in the chart below, the energy used in Jonathan’s home has dropped significantly as energy saving measures have been implemented, resulting in a reduction of more than 80% in the amount of energy it needs. Since this remaining requirement is electricity which he buys from a renewable provider, the CO2 emissions due to the house are now zero – a 100% reduction. 

That avoids around five tonnes of CO2 emissions per year. By comparison, flying from London to New York emits about 1 tonne of CO2 per passenger.

Graphic description: The above chart shows the gas and electricity imported to the home, energy generated by renewable sources, and energy use between 1998 and 2024. The total kilowatt-hours (kWh) of energy used between 2021 and 2024 was about 15% of that used on average per year between 1998 and 2000. By comparison, the graph shows the average energy use of residential properties in the country (130 kWh per year per square metre of floor area), and the maximum for space heating according to the PassivHaus standard (15 kWh per year per square metre of floor area).

“Older properties were not built to be energy efficient,” says Jonathan. “There are many ways to improve their energy efficiency which will save money and CO2. I described what I’ve done with my house, and why I did it, in a talk in 2023 for the British Library, which you can watch on-demand if you’d like to learn more.”

Find out what you can do to make your home more energy efficient

This article was originally published in April 2024 and updated in March 2025.

Sources:

1. https://assets.publishing.service.gov.uk/media/6424b8b83d885d000fdade9b/2022_Provisional_emissions_statistics_report.pdf 
2. https://assets.publishing.service.gov.uk/media/64c23a300c8b960013d1b05e/DUKES_2023_Chapter_5.pdf 
3. https://assets.publishing.service.gov.uk/media/64230bbd3d885d000cdadd20/HEE_Stats_Detailed_Release_-_Mar_23.pdf