You can download the full report here

In November 2021 the UK Government published its Net Zero Strategy. The strategy seeks to reduce greenhouse gas emissions from residential heating by 1) seeking to have homes upgraded where possible to at least an EPC “C” rating by 2035; and 2) “phasing out” the installation of gas boilers from 2035. The Government is continuing to evaluate hydrogen as a possible fuel for residential heating. However, most energy analysts expect electrically powered heat pumps to be the technology of choice, since hydrogen from renewables will be at least as expensive as electricity, and heat pumps will be much cheaper and more energy efficient in operation than hydrogen boilers.

A variety of grants are available for home energy improvements. Current support for insulation is mainly available only for households on low incomes or including vulnerable people. The Boiler Upgrade Scheme is available more generally, and offers a £5000 grant towards the installation of a heat pump system. The Eco+ scheme announced in 2022 is expected to provide grants for fabric improvements to homes in lower Council Tax bands, with EPC ratings of “D” or below. Eco+ was to be launched in spring 2023 but is now anticipated later in the year, and will be administered by energy supply companies.

This report evaluates the energy and CO­2 savings that could be achieved by upgrading some of the home designs typical of Botley, in particular the 1930s and 1950s semis in Elms Rise and the Seacourt Road bungalows. The main measures considered are: insulating roofs, walls and floors; installing new double or triple glazing; and replacing gas boilers with heat pumps.

The table below summarises some of the main findings from the home assessments. Key points are that:

  1. Most of our lofts already have at least 100mm insulation. Where they do not, adding insulation will save nearly a third of fuel bills, paying for the installation cost within a year. Any gaps in existing loft insulation should also be filled.
  2. Topping up loft insulation to 300mm will save energy and CO2 emissions at a low cost, which will be recovered in reduced fuel bills within a very few years.
  3. Most of our cavity walls are also insulated. Where they are not, insulation will save about 20% of fuel bills, paying for itself within a very few years.
  4. Most of our 1930s semis and the Seacourt bungalows have at least some solid walls. Where external insulation is possible this can save as much as 40% of fuel bills in some cases. It is expensive and could take over 20 years to recover the costs, but Eco+ is expected to help with this and there are other major benefits from external wall insulation, including improving thermal comfort and reducing damp and mould.
  5. Floor insulation can save 5-7% of energy and CO2 emissions, but the approach required and hence the ease and cost of insulation depends on the type of floor, access to the space underneath, and groundwater levels.
  6. Replacing 20- to 30-year-old double glazing with high-performance triple glazing can save about 12% of energy and CO2 emissions. It might take 20 years to recover the cost of installation through reduced energy bills, but there are additional benefits in improved internal comfort, reduced noise, and prevention of damp and mould.
  7. Replacing a gas boiler with an air-source heat pump can reduce energy use and CO2 emissions by 65-75% but will not necessarily reduce running costs, as electricity is three times as expensive as gas.

Evaluation for a typical Botley 1930s or 1950s semi:

Current homeMeasureEnergy savingCO2 savingPayback timeInconvenience /disruptionCo-benefits
No loft insulationInstall 100mm insulation between joists30% or about 3000kWh/year30% or about 600kg/year< 1yearNeed to clear loftReduce draughts, mould
100mm loft insulationTop up to 300mm insulation5% or about 500kWh/year5% or about 100kg/year~ 4 yearsNeed raised platform to use loft for storage 
Uninsulated cavity wallInjected cavity insulation20% or about 2000kWh/year20% or about 400kg/year~ 4 yearsNoneReduce draughts, cold spots, damp, mould
Uninsulated solid wallExternal wall insulation40% or about 4500kWh/year40% or about 900kg/year~25 yearsMinor – requires scaffoldingReduce draughts, cold spots, damp, mould
Uninsulated solid wallInternal wall insulation40% or about 4500kWh/year40% or about 900kg/year~25 yearsMajor – need to clear rooms for workReduce draughts, cold spots, damp, mould
Insulated cavitiesExternal wall insulation18% or about 1800kWh/year18% or about 360kg/year~75 yearsMinimal 
Suspended timber floorInsulate below floorboards5-7% or about 600kWh/year5-7% or about 120kg/year~30 yearsMinimalReduce draughts, cold spots, damp, mould
Suspended concrete floorInsulate above or below concrete5% or about 500kWh/year5% or about 100kg/year~30 yearsSignificant for insulating overlayReduce cold spots, damp, mould
Standard double glazingHigh performance triple glazing12% or about 1200kWh/year12% or about 240kg/year~50 yearsModerateReduce draughts, cold spots, damp, mould
Typical current houseReplace gas boiler with heat pump, use existing radiators if possible65-75% or about
6500-7500 kWh/year (using electricity instead of gas)
60-70% or about 1200-1400kg/yearn/aMay require larger radiators so heat pump can work at high efficiency. Requires domestic hot water cylinder 
Typical current houseReplace gas boiler with air-to-air heat pump and point of use water heaters75% or about 7500kWh/year (using electricity instead of gas)70% or about 1400kG/year~40 yearsInvolves installing warm/cold air units instead of radiators.Heat pump can provide cooling in summer.
Typical current houseAll measures – insulation, triple glazing, heat pump80-90%80-90%~100 yearsMajorConsiderable improvement to thermal comfort, eliminate cold spots, damp, mould

Note: where Boiler Upgrade Scheme ore ECO+ grants are available, they will significantly improve payback times.

When making home improvements the best results are achieved by starting with measures like insulation and improved glazing, and then specifying the heating system to meet this requirement. This is known as a “fabric first” approach.

“Fabric first” is especially important when considering installing a heat pump because its efficiency (“coefficient of performance”) depends on the temperature at which heat is delivered to radiators, warm air systems etc. For a well-designed heating system this temperature will usually be in the range 35-45°C, compared with 60-80°C for a boiler system. Heat pumps are often specified with underfloor heating which is designed to operate at low flow temperatures.

If your radiators have been specified for an uninsulated house, adding insulation may mean that they can heat the house adequately even with a water flow temperature of 35-45°C. However, you might still need to install larger or more efficient radiators.

The findings in the table above will largely apply to older houses which have solid walls, and to houses built with cavity walls up to the early 1990s. Homes built since the mid-1990s were required to have insulated walls, roofs and floors. It was also normal to install double glazing although this was not required by building regulations until 2002. These houses are likely to achieve EPC “C” without improvements.  While they can be retrofitted to improve their energy performance, this will mostly involve relatively high-cost measures, adding to existing insulation or upgrading doors and windows. It may be appropriate to install heat pumps in newer houses without first improving the fabric.

There are several reasons why people may choose to adopt heat pumps without improving the fabric of their homes – summarised in the table below.

Fabric first: pros and cons

Avoid over-specifying the heating system, which would cost more and require more space

If installing a heat pump a lower heating requirement means the radiators can operate at lower temperature, which makes the heat pump more efficient

Permanent reduction in energy requirement and fuel/electricity costs

Shorter heating season

Improved thermal comfort (reduced draughts and cold spots)Reduced condensation, damp and mould

Quieter home interiorSome fabric measures (loft and cavity wall insulation) will pay for themselves in reduced energy costs within a year or two

Some fabric measures are expensive (e.g. solid wall insulation, replacing doors and windows)

Some fabric measures are disruptive (e.g. internal wall insulation)

Fabric measures need to be carefully designed and it can be hard for householders to find good installers

Grants may be more readily available for renewable and heat pump based heating systems than for insulation

Where the household has a limited budget and must choose, e.g. between a heat pump and solid wall insulation, the heat pump may deliver a greater CO2 emission reduction.

If you are considering any of the more expensive measures considered in this report – especially solid wall insulation, floor insulation, replacing doors and windows, or installing a heat pump – it is worth getting professional advice. Look for a retrofit assessor or retrofit coordinator with “PAS 2035” accreditation.