Heat loss calculator: what insulation saves and when it pays back
Enter the areas of your walls, loft, floor, windows and doors and pick how they are built now and after the work. The calculator shows the heat lost through each part, the yearly saving and the payback time.
Where the heat goes now
A steady-state estimate: U-value × area for each part plus ventilation, times the degree days of an average UK year. Thermal bridges, solar and internal gains and how warm you keep the home are not modelled separately.
How the heat loss is calculated
Every part of the outside shell loses heat in proportion to its area and its U-value, the watts that pass through one square metre for each degree of difference between inside and outside. Ventilation adds 0.33 W per cubic metre of heated volume for each air change per hour. Together they give the heat loss coefficient of the home in W/K.
Over a year the coefficient is multiplied by the heating degree days, the sum of daily temperature shortfalls below 15.5 °C (DESNZ, 1991–2020 average for the UK). Default U-values come from the RdSAP 10 tables used for Energy Performance Certificates. The heat saved is turned into money through the efficiency and fuel price of your system.
Formula
- Heat loss coefficient H = Σ (U × area) + 0.33 × air changes × volume
- Heat lost a year (kWh) = H × degree days × 24 ÷ 1000
- Money saved = heat saved ÷ efficiency × unit price
- Payback = cost of the work ÷ money saved a year
Frequently asked questions
What is a U-value?
The rate at which heat passes through one square metre of a wall, roof, floor or window for each degree of temperature difference, in W/m²K. Lower is better: an uninsulated solid brick wall is about 1.7, a modern insulated wall below 0.3.
Which areas should I measure?
Only the parts between heated rooms and the outside or an unheated space. Leave out party walls shared with a heated neighbour. Measure walls without the windows and doors, which have their own rows.
Why does my real bill not match?
The model uses an average year and assumes the whole home is heated evenly. Homes heated only part of the day, or rooms left cold, use less; draughty homes and cold winters use more. The difference between before and after is more reliable than either total.
Can I enter a U-value from a survey or product sheet?
Yes. Choose “Custom U-value” in the list and type the figure.
Is my data sent anywhere?
No. Everything is calculated in your browser.
Where a 1970s terrace loses its heat, and what insulation saves
The calculator opens on a two-storey mid-terrace of about 85 m², built before 1976, with a condensing gas boiler. Because it shares both side walls with heated neighbours, only the front and back walls lose heat to the outside: 32 m² of unfilled cavity wall (U-value 1.5). Above is a 43 m² loft with 100 mm of insulation (U 0.40), below a 43 m² uninsulated solid floor (U 0.82). It has 10 m² of double glazing from before 2002 (U 2.8) and 2 m² of doors (U 3.0). The heated volume is 204 m³, with half an air change an hour.
Where the heat goes now
Multiplying each U-value by its area and adding the ventilation gives a heat loss coefficient of 168 W/K: the house loses 168 watts for every degree it is warmer inside than out. Ventilation alone accounts for about 34 W/K of that. Over the 2,060 degree days of an average UK year, that is 8,312 kWh of heat a year. The split, as the results panel lists it:
| Part of the house | Share of heat loss now |
|---|---|
| External walls | 29% |
| Ground floor | 21% |
| Ventilation and draughts | 20% |
| Windows | 17% |
| Roof or loft | 10% |
| External doors | 4% |
The two jobs in the example
The “after the work” column fills the cavity (U 1.5 down to 0.7) and tops the loft up to 270 mm (U 0.40 down to 0.16). Heat loss falls to 6,536 kWh, a saving of 1,776 kWh of heat. The boiler is 84% efficient, so that is 2,114 kWh less gas, worth about £168 a year at the 7.97p cap rate. The gas standing charge does not change, so it is not part of the saving.
The two jobs are not equal. Change only the wall and the saving is £120 a year (1,266 kWh of heat); change only the loft and it is £48 (510 kWh). The loft already has 100 mm, and the first layer does most of the work: in the RdSAP tables an uninsulated loft has a U-value of 2.3, against 0.40 with 100 mm. Adding more still helps, but each extra centimetre saves less than the one before. That is why the loft scores low here, even though loft insulation is often the first job people think of.
The list describes the house as it is now. Set the “Now” column to the improved wall and loft, and the order changes: the floor becomes the largest share (27%), followed by ventilation (25%) and windows (21%). The walls drop to 17%. The list shows where the heat goes, not whether the next job is worth doing: insulating a solid floor usually means lifting it, a much bigger job than filling a cavity.
Payback
The calculator does not guess what the work costs, because prices vary too much between houses and installers. Enter the quote you have and it divides it by the yearly saving. At £168 a year, a quote of £1,680 would pay back in ten years, and every £168 more adds a year. Grants or energy company schemes that cut the price you pay should be taken off before you enter the cost.
How the estimate works, and where it falls short
The method is a steady-state heat balance. Each part of the shell loses heat in proportion to its area, its U-value and the temperature difference. Over a year that difference is summed as degree days: how far, and for how long, the outside temperature falls below 15.5 °C. The base is 15.5 °C rather than room temperature because people, appliances, cooking and sunshine provide the last few degrees for free. The 2,060 figure is the 1991–2020 UK average from DESNZ Energy Trends; colder parts of the country have more degree days and milder ones fewer, so type in a local figure if you have one.
The model does not see thermal bridges, the gains from sun through the windows, or how you actually heat the house. It assumes the whole home is kept warm all season. For the same house, the heating comparison estimates 7,583 kWh from the median metered gas use for homes of that age (NEED), so this model’s 8,312 kWh is about 10% higher. Neither total is exact; the difference between before and after is the more reliable figure.
What to check in your own home
Your Energy Performance Certificate (EPC) usually records the wall type, loft insulation depth and glazing. You can also measure the loft depth with a ruler. Measure wall areas from outside, leaving out windows and doors, and leave out any wall shared with a heated neighbour. To sense-check the total, take the gas kWh from your annual statement and multiply by 0.84: if it is far below the calculator’s figure, you probably heat fewer rooms or for fewer hours than the model assumes, and the real saving will be smaller too.
If insulation is the first step towards a heat pump, the government’s Boiler Upgrade Scheme offers grants towards heat pumps and biomass boilers in England and Wales; check the current amounts and conditions there. A lower heat loss also means a smaller heat pump is needed.
Sources: U-values from BRE RdSAP 10, degree days from DESNZ Energy Trends table ET 7.1, boiler efficiency from BRE SAP 10.2, gas price from the Ofgem price cap.