Calculations use Ofgem price-cap rates for 1 July – 30 September 2026: electricity 26.11p/kWh, gas 7.33p/kWh.
1 Insulation type
2 Your property
3 Energy prices
4 Installation cost
Your estimated insulation savings
You may qualify for free insulation under ECO4
Based on your benefit status, you may be eligible for free or heavily subsidised insulation under the ECO4 scheme. Contact your energy supplier or check with your local authority to apply.
Run the full ECO4 calculator → · Check ECO4 eligibility in detail → · View all open grants →
Typical savings by insulation type (medium 3-bed home, gas heating)
| Insulation type | Typical cost (2026) | Annual saving | Payback | ECO4 available? |
|---|---|---|---|---|
| Loft insulation (none to 270mm) | £300–£600 | £150–£280 | 1–3 years | Yes (if eligible) |
| Cavity wall insulation | £700–£1,500 | £100–£200 | 5–12 years | Yes (if eligible) |
| Solid wall, internal | £5,000–£15,000 | £200–£500 | 15–40 years | Yes (if eligible) |
| Solid wall, external | £8,000–£25,000 | £200–£500 | 20–50 years | Yes (if eligible) |
| Underfloor (suspended floor) | £600–£1,200 | £50–£120 | 8–18 years | Sometimes |
Source: Energy Saving Trust typical savings data, adjusted to the Ofgem price cap in force from 1 July 2026. Payback depends on existing insulation level, property type, and fuel costs. Figures assume you are starting from no insulation, see the top-up caveat below.
How we work out the payback
Three numbers drive everything on this page. Here they are, so you can check the result against your own quote.
Annual saving = baseline saving × (your fuel price ÷ capped fuel price)
Payback years = installed cost ÷ annual saving
10-year position = (annual saving × 10) − installed cost
The baseline saving comes from Energy Saving Trust figures for each measure at each property size, for a gas-heated home going from no insulation to the current standard. If you enter a fuel price above or below the Ofgem cap, we scale the saving by that ratio. Electric, oil and LPG heating are scaled again, because the same kilowatt-hour of avoided heat loss is worth more when the fuel costs more.
A worked example
Medium 3-bed semi, gas central heating, bare loft, insulated to 270 mm for £400. Baseline saving is £220 a year. At the July 2026 cap rate the price ratio is 1.0, so the saving stays at £220. Payback is 400 ÷ 220, or 1.8 years. Over ten years you are £1,800 up.
The number that catches people out
Payback is only that fast when you start from nothing. Most of the heat loss goes on the first 100 mm, so a top-up saves far less than a first install. The Energy Saving Trust's July 2026 figures put a bare loft taken to 270 mm at around £360 a year for a detached house, £190 for a semi and £140 for a mid-terrace. Topping up an existing 120 mm to 270 mm in the same houses saves roughly £35, £17 and £13.
That changes the answer completely. A £400 top-up on a semi pays back in about 24 years, not two. It is still worth doing when the loft is open for other work, but it is not the quick win the headline numbers suggest. Set the "current insulation status" field honestly and the calculator will reflect it.
What is not in the figure
- Standing charges. Insulation cuts the units you burn, not the daily charge, which is 29.04p for gas and 57.19p for electricity regardless of use.
- Comfort taken as warmth rather than savings. Plenty of households turn the thermostat up after insulating and bank the comfort instead of the money.
- Future price moves. Ofgem sets the cap quarterly, and the October 2026 figure is published by 26 August 2026.
- Any effect on your EPC band, which matters if you let the property.
What 270 mm and "full fill" actually mean
Loft
Approved Document L sets a U-value target rather than a depth. For a roof insulated at ceiling level the limiting value is about 0.16 W/m²K. Standard mineral wool at λ 0.044 W/mK hits that at roughly 270 mm; higher-performance wool at λ 0.035 gets there at about 250 mm.
It goes down in two layers. Around 100 mm sits between the joists, then about 170 mm goes across them at 90 degrees so the timber itself is covered. UK loft rolls are combi-cut, meaning one roll is partly perforated so it can be split to fit between joists or laid full width across them. As a working figure, Knauf Loft Roll 44 covers about 13.89 m² per roll at 100 mm and about 8.01 m² at 170 mm, so a 48 m² semi loft needs roughly four rolls of the first and six of the second before cutting waste. Check the coverage printed on whatever pack you actually buy.
Keep the eaves clear. A cold loft has to stay ventilated or moisture builds up behind the insulation, and Approved Document C is met on a cold pitched roof by following BS 5250, which wants continuous eaves ventilation equivalent to a 25 mm gap on shallow pitches and 10 mm above 15 degrees.
Cavity wall
Most houses built from the 1920s onwards have two leaves of brick or block with a gap of around 50 mm between them. Full fill means that gap is completely filled, usually with EPS beads, blown mineral wool or a polyurethane foam. A registered installer drills a pattern of small holes, injects the material and makes the holes good. It is a one-day job on a typical semi.
Use a TrustMark or CIGA-registered installer. The 25-year guarantee that makes cavity insulation worth having comes from CIGA, and an unregistered install leaves you with neither the guarantee nor a route to redress if the wall gets damp. A borescope check confirms the cavity is clear and the right width before anything is committed.
Solid wall
Pre-1920 houses usually have solid brick, so there is no cavity to fill and the insulation goes on one face or the other. Internal insulation is cheaper but takes roughly 100 mm off the room on every treated wall and means moving skirting, sockets and radiators. External insulation keeps the floor area but changes how the house looks, which is why it needs care in a conservation area. Both are the slowest-paying measures on this page and neither makes sense before the loft and cavity work is done.
Frequently asked questions
How long does loft insulation take to pay for itself?
Insulating a bare loft to 270 mm usually pays back in one to three years. A medium 3-bed semi costs around £300 to £600 and saves roughly £190 to £220 a year on gas heating, so the money is back inside two winters. Detached houses pay back fastest because they lose more heat through a larger roof.
Is it worth topping up loft insulation from 100 mm to 270 mm?
It is worth doing, but not for the payback. The first 100 mm captures most of the available saving, so a top-up in a semi saves around £17 a year against roughly £190 for a first install. On a £400 job that is a payback measured in decades. Do it when the loft is already open, when you are boarding, or when a grant covers the cost, rather than as a standalone money-saving project.
How long does cavity wall insulation take to pay back?
Typically five to twelve years. A medium 3-bed semi costs £700 to £1,500 and saves £100 to £200 a year. The spread is wide because it depends on how exposed the walls are, how many of them are external, and whether the cavity turns out to be a consistent width once the installer puts a borescope through it.
Does solid wall insulation ever pay for itself?
On the numbers alone, rarely within the life of the installation. Internal insulation runs £5,000 to £15,000 and external £8,000 to £25,000, against savings of £200 to £500 a year, which puts payback somewhere between 15 and 50 years. People do it for comfort, damp and condensation reasons, or because a grant covers most of the cost, not because the arithmetic works.
Which insulation should I do first?
Loft, then cavity walls, then draught-proofing, then floors, then solid walls. That is the order of cost per pound saved, and it is also the order that matters if you are thinking about a heat pump later. Fabric first means the heat pump can be smaller and cheaper to run.
Can I get insulation for free?
Possibly, through ECO4, which runs to 31 December 2026. It is aimed at households receiving Universal Credit, Pension Guarantee Credit, Pension Savings Credit, Income Support, income-based JSA, income-related ESA, Child Benefit or Housing Benefit. Tax credits are no longer on the qualifying list, so any page telling you otherwise is out of date. Councils also run ECO4 Flex on their own criteria, with a gross income cap of £31,000 on the main route. There is no published maximum grant amount for ECO4, whatever installer advertising says.
Does the payback change if I have a heat pump?
Yes, and usually in insulation's favour. Insulation savings scale with the price of the fuel you avoid burning. Electricity is 26.11p per kWh against 7.33p for gas, and although a heat pump delivers three to four units of heat per unit of electricity, the effective cost per unit of heat still tends to sit above gas. Select electric heating in the calculator and the annual saving rises accordingly.
Will cavity wall insulation cause damp?
Correctly installed in a suitable wall, it should not. Problems come from filling a wall that should never have been filled: exposed positions with driving rain, an existing damp problem or failed damp course, a cavity bridged by mortar debris, or a non-standard cavity width. A proper survey exists to rule those out, which is the main argument for using a registered installer rather than the cheapest quote.
Do I need to leave a gap around the loft hatch and downlights?
Yes. Keep insulation clear of any recessed light fitting that is not rated for insulation contact, or fit fire-rated loft caps over them, because covering a standard downlight traps heat. Leave the air path at the wall plate open so the roof void can breathe, and do not bury heavily loaded cables, which BS 7671 treats as a derating condition.
Assumptions and limitations
- Savings estimates based on Energy Saving Trust typical savings data for each insulation type and property size, adjusted to the Ofgem price cap in force from 1 July 2026 (gas 7.33p/kWh, electricity 26.11p/kWh).
- Baseline figures assume a first install into an uninsulated element. Topping up existing insulation saves substantially less, and the payback lengthens accordingly.
- Savings assume insulation is installed to current standard (loft: 270mm; cavity wall: full fill; solid wall: to current U-value standard).
- CO₂ savings based on UK grid carbon intensity and typical gas carbon factors (BEIS conversion factors 2025).
- Installation costs are typical market rates, get three quotes before proceeding. Costs vary significantly based on property size, accessibility, and installer.
- ECO4 eligibility is indicative only. Actual eligibility is determined by your energy supplier or local authority. If you qualify, installation is free or heavily subsidised, making payback near-instant.
Ofgem price cap from 1 July 2026, ofgem.gov.uk (accessed August 2026)
Approved Document L and Approved Document C, gov.uk (accessed August 2026)
CIGA cavity wall insulation guarantee, ciga.co.uk (accessed August 2026)
BEIS carbon conversion factors 2025, gov.uk
ECO4 eligibility: ofgem.gov.uk/eco
Reviewed by BetterHomeEnergy Research Team, August 2026. Editorial policy →
Doing it yourself? Rolls of loft insulation and draught-proofing strip are the cheapest quick wins from the table above.
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