Bar shows how much of the wall’s heat loss the insulation removes. Steady-state estimate; real savings are often lower because of thermal bridges and heating habits. Rates checked 22 October 2026.
Formula and breakdown
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This external wall insulation cost calculator estimates the price of insulating a solid-walled home from the outside, then works out the heat and money it saves and how long it takes to pay back, for homeowners and landlords in the UK. You enter your wall area, the prices on your quote and your heating type, and the tool shows the new wall U-value, a building regulations check and the payback on your own figures.
Quick answer: An external wall insulation cost calculator multiplies the insulated wall area by the installed price per m², adds scaffolding and extras, then compares heat lost before and after. With example prices, 82 m² at £130 per m² plus £2,300 of extras costs £12,960 and saves about £636 a year with gas heating.
What Is an External Wall Insulation Cost Calculator?
An external wall insulation cost calculator is a tool that turns a quote into a cost per square metre and a payback period, using the change in the wall’s U-value to estimate savings. In plain words: cost equals insulated area times price per m², plus scaffolding and extras, plus VAT. The U-value after the work is 1 divided by the sum of the old wall’s resistance (1 / U) and the new insulation’s resistance (thickness / conductivity).
Heat saved each year equals the drop in U-value times the area times heating degree days times 24, divided by 1,000 to give kWh. Divide by your heating efficiency and multiply by the fuel price to get the yearly saving. The Wikipedia article on thermal transmittance explains why a structure’s U-value is the reciprocal of its total resistance.
How Do You Use the External Wall Insulation Cost Calculator?
- Measure each outside wall to be insulated, multiply width by height and add them up.
- Enter the area of windows and doors in those walls so they are taken off.
- Enter the installed price per m², scaffolding and extras from your quote, or keep the labelled examples for a first look.
- Choose the VAT rate that applies to your installation date, and enter any grant.
- Enter the wall’s current U-value, the insulation thickness and its conductivity from the product datasheet.
- Set heating degree days for your area, pick your heating type, then read the cost, savings and payback.
If you have a quote, type its total into the quote box: the tool compares it with the itemised estimate and uses it for the payback.
How Much Does External Wall Insulation Cost per m²?
There is no current national price list from a government body. Bath and North East Somerset Council’s retrofit guidance quotes BEIS data giving an estimated £55 to £180 per m² for external wall insulation, without a date, so the external wall insulation cost calculator uses £130 per m² only as a labelled example. Replace it with the rate on your quote.
The table applies the example prices to four example wall areas, with gas heating at 85% efficiency, 7.97p per kWh and 2,000 degree days, insulating a 2.0 W/m²K wall with 100 mm of board at 0.032 W/mK.
| Insulated wall area (example) | Estimated cost | Heat saved per year | Gas saving per year | Simple payback |
|---|---|---|---|---|
| 35 m², small flat or end wall | £6,850 | 2,897 kWh | £272 | 25.2 years |
| 50 m², mid-terrace front and back | £8,800 | 4,138 kWh | £388 | 22.7 years |
| 82 m², semi-detached | £12,960 | 6,786 kWh | £636 | 20.4 years |
| 120 m², detached | £17,900 | 9,931 kWh | £931 | 19.2 years |
Fixed costs such as scaffolding weigh more heavily on small jobs, so payback is longest on small walls. Each square metre in this example saves about 83 kWh of heat, worth about £7.76 a year with gas.
How Much Can External Wall Insulation Save?
In the external wall insulation cost calculator, savings depend most on how poor the wall is now and how you heat the home: the same 82 m² wall saves about £636 a year with gas, but about £1,786 a year with direct electric heating at 26.32p per kWh. A heat pump, at an example 300% efficiency, cuts the saving to about £595, because each unit of heat costs less.
A wall already at 1.0 W/m²K saves less than half as much as one at 2.0. If you do not know your wall’s U-value, an EPC or energy survey is the best source; the U-value calculator can build one from the wall layers. Degree days vary by region and by how many hours you heat; see the heating degree days calculator. To compare running costs across heating types, the heat pump running cost calculator and the electric heater running cost calculator use the same October 2026 prices.
Does External Wall Insulation Need Planning Permission?
Often it does, because it changes how the house looks. The Planning Portal insulation guidance says planning permission is not normally required for fitting insulation where there is no change in external appearance, and that owners of listed buildings or homes in conservation areas should consult the local planning authority. A new render or cladding finish is a change in appearance, so ask your council before signing a contract.
Listed buildings need extra care: Bath and North East Somerset Council, for example, states that Listed Building Consent is required for external wall insulation. Building regulations also apply when you renovate a wall. Table 3.3 of the 2026 edition of Approved Document L, which takes effect in England on 24 March 2027, sets an improved U-value of 0.30 W/m²K and a threshold of 0.70 for walls insulated internally or externally; the external wall insulation cost calculator checks your result against both.
Is VAT Charged on External Wall Insulation?
No, not if it is installed by 31 March 2027. HMRC’s VAT Notice 708/6 zero-rates the installation of energy-saving materials, including wall insulation, from 1 May 2023 to 31 March 2027. From 1 April 2027 the rate returns to 5%. The VAT switch in the external wall insulation cost calculator applies either rate, or treats your prices as VAT-inclusive.
What Affects the Price of External Wall Insulation?
Wall area and access set most of the price, because labour and scaffolding scale with the size and height of the walls. Other factors:
- Insulation type and thickness: thinner, higher performance boards cost more per m².
- Finish: render, brick slips or cladding.
- Details: window sills, downpipes, meter boxes, vents and satellite dishes all need moving or extending.
- Wall condition: repairs and damp must be fixed first.
- Moisture design: the Energy Saving Trust advises a moisture control strategy from an experienced specialist and a 25-year guarantee, for example through SWIGA or an insurance-backed scheme.
Grants change often; the home energy grant checker lists current schemes. If you would rather keep the outside unchanged, compare with the internal wall insulation cost calculator.
Worked Example: Semi-Detached House With Gas Heating
Example: a solid brick semi has 100 m² of outside wall to insulate, with 18 m² of windows and doors. The quote uses an example £130 per m², £1,500 scaffolding and £800 of extras, installed before April 2027. The wall is an example 2.0 W/m²K, the board is 100 mm at 0.032 W/mK, and the gas boiler is 85% efficient.
- Insulated area: 100 minus 18 = 82 m².
- Cost: 82 x £130 = £10,660, plus £1,500 and £800 = £12,960, with 0% VAT. That is £158.05 per m² overall.
- New U-value: 1 / (1 / 2.0 + 0.1 / 0.032) = 1 / 3.625 = 0.276 W/m²K, which meets the 0.30 improved value.
- Heat saved: (2.0 minus 0.276) x 82 x 2,000 x 24 / 1,000 = 6,786 kWh.
- Gas saved: 6,786 / 0.85 = 7,984 kWh x 7.97p = £636.31 a year.
- Payback: £12,960 / £636.31 = 20.4 years. Over a 25-year guarantee period the household is about £2,948 ahead at today’s prices.
Running the external wall insulation cost calculator with 60 mm of a 0.038 board instead gives a U-value of 0.48, which misses 0.30, so the board choice matters as much as the price.
Frequently Asked Questions
How much does external wall insulation cost for a 3-bed semi?
It depends on wall area and the installer’s rate. With an example £130 per m², 82 m² of insulated wall, £1,500 scaffolding and £800 of extras, the total is £12,960 with zero VAT. Council guidance quoting BEIS data gives £55 to £180 per m².
How do I calculate the wall area for external insulation?
Measure the width and height of each outside wall to be insulated and multiply them, then add the walls together. Subtract the area of windows and doors. Include gable ends up to the roof line if the installer will cover them.
How much will external wall insulation save on my bills?
For an example 82 m² wall improved from 2.0 to 0.28 W/m²K, about 6,786 kWh of heat a year: £636 with gas at 7.97p and 85% efficiency, or about £1,786 with direct electric heating at 26.32p.
Do I pay VAT on external wall insulation?
Not if it is installed by 31 March 2027. HMRC Notice 708/6 zero-rates the installation of insulation for walls from 1 May 2023 to 31 March 2027. From 1 April 2027 the reduced rate of 5% applies.
Do I need planning permission for external wall insulation?
Possibly. The Planning Portal says insulation does not normally need permission where the outside appearance does not change, but external insulation usually changes it. Ask your local planning authority, especially for listed buildings, conservation areas and flats.
What U-value should external wall insulation achieve?
Table 3.3 of the 2026 edition of Approved Document L, which takes effect on 24 March 2027 in England, gives an improved value of 0.30 W/m²K for walls insulated internally or externally, with a threshold of 0.70. Your building control body confirms what applies.
Checked October 2026 by the Solaxyra Editorial Team. Sources: HMRC VAT Notice 708/6, Approved Document L (gov.uk), 2026 edition Table 3.3, Planning Portal: Insulation, planning permission, Energy Saving Trust: Solid wall insulation, Bath and North East Somerset Council: Externally applied solid wall insulation, Ofgem: Price cap October to December 2026, Wikipedia: Thermal transmittance, Wikipedia: Heating degree day.