Facts last checked 17 September 2026
The short answer
This is an illustrative worked example, not a real customer's project: a typical 98m² 1930s semi with empty cavity walls, 100mm of loft insulation and a gas boiler, modelled step by step from published figures. Filling the cavity, topping up the loft and draught-proofing cut its heat loss by about 29% for roughly £3,050 at Energy Saving Trust typical prices, and cut the modelled gas bill by about £460 a year. A heat pump sized to the improved house needs about 5.6kW instead of 7.9kW, costs about £5,400 after the £7,500 grant at the national median, and runs for roughly £55 to £380 a year less than a new gas boiler, counting the gas standing charge you stop paying, depending on how efficiently it performs.
What is this example, and why a 1930s semi?
Homes built between 1919 and 1944 make up 3.68 million of England's 25.6 million homes, and semi-detached houses 6.3 million. Only 40% of interwar homes are rated EPC A to C, and the English Housing Survey's modelled average cost to bring one up to band C is £7,143. The Energy Saving Trust says homes built after the 1920s probably have cavity walls, and about 4.9 million cavity-walled homes in England still have no insulation in them. That combination makes the interwar semi the classic fabric-first retrofit.
If your home is from a different era, start with the five UK property types.
What does the example house look like before any work?
| Element | Assumption | Heat loss rate used |
|---|---|---|
| Size | Two storeys, 7m × 7m footprint, 98m² floor area (the survey's average semi is 100m²) | Volume 235m³ |
| External walls | Brick cavity, unfilled; 91m² after windows and doors | U 1.5 W/m²K (as built, 1930 to 1949) |
| Windows | Pre-2002 double glazing including the bay, 18m² | U 2.8 W/m²K |
| Doors | Two external doors, 4m² | U 3.0 W/m²K |
| Loft | 100mm mineral wool, 49m² | U 0.40 W/m²K |
| Ground floor | Suspended timber, uninsulated, 49m² | U 0.6 W/m²K (our assumption; your assessor calculates this) |
| Thermal bridging | Junctions around openings and floors | 0.15 W/m²K across all exposed area (RdSAP default for this age) |
| Draughts | Leaky: 0.8 air changes per hour | Our assumption |
| Heating | Old gas boiler, radiators | 85% seasonal efficiency (our assumption) |
The party wall with next door is ignored because both homes are heated. Heat loss for each element is its U-value (how fast heat passes through each square metre per degree) times its area. Draughts add 0.33 × air changes × volume. Added together, that gives a heat loss coefficient in watts per degree of temperature difference.
How much heat does the house lose, and where?
| Element | Calculation | W/K | Share |
|---|---|---|---|
| Walls | 91 × 1.5 | 136.5 | 40% |
| Draughts and ventilation | 0.33 × 0.8 × 235 | 62.1 | 18% |
| Windows | 18 × 2.8 | 50.4 | 15% |
| Thermal bridging | 211 × 0.15 | 31.6 | 9% |
| Ground floor | 49 × 0.6 | 29.4 | 9% |
| Loft | 49 × 0.40 | 19.6 | 6% |
| Doors | 4 × 3.0 | 12.0 | 4% |
| Total | 341.6 | 100% |
- Peak heat loss: 341.6 W/K × 23°C (21°C inside, a round -2°C outside) = 7.9kW. Real installers use a room-by-room calculation and a design temperature for your location.
- Annual space heating: 341.6 W/K × 2,061 degree days × 24 hours = about 16,900kWh. The 2,061 figure is the 1991 to 2020 average in DESNZ's Energy Trends weather tables, based on Met Office data, at a 15.5°C base.
- Adding an assumed 2,500kWh of hot water and dividing by 85% boiler efficiency gives about 22,800kWh of gas: £1,819 a year at the October to December 2026 price cap unit rate of 7.97p per kWh.
This is a simplified steady-state model. It does not capture how your household actually heats the home, or every detail of the building, so treat the absolute kWh as a rough guide. The percentages and the comparisons between steps are more reliable than the totals.
What happens at each step of the fabric-first sequence?
| Step | Change | Heat loss (W/K) | Peak (kW) | Modelled gas bill | Typical cost |
|---|---|---|---|---|---|
| Start | 341.6 | 7.9 | £1,819 | ||
| 1. Loft to 270mm | U 0.40 to 0.16 | 329.9 | 7.6 | £1,764 | About £600 (Energy Saving Trust figure for a top-up from 120mm) |
| 2. Cavity wall insulation | U 1.5 to 0.7 | 257.1 | 5.9 | £1,427 | About £2,200 |
| 3. Draught-proofing | 0.8 to 0.6 air changes (our assumption) | 241.6 | 5.6 | £1,355 | About £250 |
| 4. New windows and doors (optional) | U 2.8 to 1.4 and 3.0 to 1.4 | 210.0 | 4.8 | £1,208 | About £4,800 for windows |
Steps 1 to 3 cost about £3,050 and cut heat loss by 29%, saving about £464 a year on the modelled gas bill: a simple payback of under seven years. The cavity fill does most of the work on its own, which is why it is the first thing to check in a house of this age. Insulation and draught-proofing materials are zero-rated for VAT until 31 March 2027.
Step 4 is where the decision often goes the other way. Replacing windows that are already double glazed saves a further £147 a year here for roughly £4,800 or more. Do it when the windows are failing, not for the energy saving alone.
Why do the insulation work before the heat pump?
Because the heat pump is sized to the heat loss. Sized to this house as it started, it would need about 7.9kW. After steps 1 to 3, about 5.6kW. A unit sized before the insulation would be about 40% larger than the improved house needs. Size shows up in price: in the Boiler Upgrade Scheme statistics for 2025/26, the median air-to-water installation was £12,164 in the 6kW to 8kW band and £12,686 in the 8kW to 10kW band.
Radiators matter just as much. Approved Document L asks that a new or fully replaced wet heating system is designed to run at a maximum flow temperature of 55°C or lower, and the Energy Saving Trust notes you may need larger radiators for a heat pump. A lower heat loss means each room needs less output, so fewer radiators need replacing and the system can run at a lower, more efficient flow temperature. In our example, cutting whole-house heat loss by 29% cuts the total output the radiators must deliver by the same share, though the saving varies room by room.
What does the heat pump cost, and what does it save?
The median reported cost of an air-to-water heat pump installed under the Boiler Upgrade Scheme was £12,908 in April to June 2026, including the grant, with a median size of 8kW. The middle half of installations cost between £11,128 and £15,627. Taking off the £7,500 grant leaves about £5,408 at the median. Heat pumps are also zero-rated for VAT until 31 March 2027. The grant no longer needs an EPC, and your installer deducts it from your quote. Check your home with the BUS eligibility checker.
Running costs depend heavily on the seasonal performance factor (SPF): units of heat out per unit of electricity in. The Energy Saving Trust says heat pumps typically deliver three to four times as much heat as the electricity they use, so we show a range. Both options use the improved house (about 14,450kWh of heat a year including hot water).
| Heating option | Energy used | Annual cost | Difference vs new gas boiler |
|---|---|---|---|
| Gas boiler, 85% efficient | 17,000kWh gas | £1,355 + £108 standing charge = £1,463 | |
| Heat pump, SPF 2.7 | 5,351kWh electricity | £1,408 | £55 less |
| Heat pump, SPF 3.0 | 4,816kWh electricity | £1,268 | £195 less |
| Heat pump, SPF 3.5 | 4,128kWh electricity | £1,086 | £377 less |
Two honest caveats. First, this uses a standard single-rate tariff; the Energy Saving Trust notes that a heat pump time-of-use tariff can cut running costs significantly, while on a standard tariff a heat pump can cost slightly more to run than a new gas boiler. Second, Ofgem's current cap figures have no VAT on electricity from 1 October 2026 to 31 March 2027, so electricity will cost a little more after that unless the rule is extended. Our fuller comparison is in heat pump running costs vs gas.
What does the whole plan add up to?
| Route | Upfront cost | Modelled annual heating cost | Peak heat loss |
|---|---|---|---|
| Do nothing | £0 | £1,819 gas + £108 standing charge | 7.9kW |
| Fabric steps 1 to 3, keep gas | About £3,050 | £1,355 + £108 | 5.6kW |
| Fabric steps 1 to 3 and a new gas combi | About £3,050 + £2,695 (Smart Energie Advanced tier fixed price, which includes the Greenstar 4000) | Similar to the line above; a new boiler's gain depends on how inefficient the old one was | 5.6kW |
| Fabric steps 1 to 3 and a heat pump | About £3,050 + £5,408 = £8,458, plus any radiator upgrades | £1,086 to £1,408 | 5.6kW |
- Survey the walls, loft and floor, and check for damp.
- Loft top-up and cavity fill. The Energy Saving Trust says a cavity fill on an average home with easy access usually takes about two hours.
- Draught-proofing, keeping trickle vents, airbricks and extractor fans clear.
- Live with it through part of a heating season if your boiler still works, and note your gas use.
- Heat loss survey, then a heat pump sized to the improved house, with radiators checked room by room and a flow temperature at or below 55°C.
- Windows and doors when they are due, not before.
If your boiler is young and reliable, the fabric steps alone are a sound first move and the heat pump can wait for the boiler's end of life. If the boiler is failing this winter, the heat pump route and the insulation can be planned together. The retrofit planner sequences this for your own home, and our air source heat pump service covers the survey and design.
Common questions
Is this a real Smart Energie project?
No. It is an illustrative worked example built from published national figures and stated assumptions. A real home's survey will produce different numbers.
Do all 1930s semis have cavity walls?
Most houses built after the 1920s do, according to the Energy Saving Trust, but not all. Check the brick pattern and wall thickness, look at your EPC, and ask for a borescope inspection before planning around a cavity fill.
Should I insulate the suspended timber floor too?
It can help comfort and cut draughts through floorboards, but it is more disruptive and depends on access below the floor. In our example the floor is about 9% of heat loss. Keep underfloor airbricks clear whatever you do, because they keep the timbers dry.
Can I get the Boiler Upgrade Scheme grant without doing the insulation first?
Yes. For applications properly made from 28 April 2026 an EPC is not required, and there is no rule that loft or cavity insulation must be done first. The insulation is still worth doing for the reasons shown above.
Why use degree days instead of my real gas bills?
Because this example house has no real bills. For your own home, a year of meter readings is a better check on the model, and a good heat loss survey will reconcile the two.