greenhomesums
Insulation Savings Calculator UK
Cavity wall, loft, and underfloor insulation — annual bill savings, payback time, and ECO4 grant impact. UK-specific U-values, current energy prices, and straightforward payback maths.
Interactive insulation savings calculator
Worked example
£1,700/yr gas heating bill (65% heating share), modelling cavity wall insulation + loft top-up, no ECO4 grant:
- Annual saving
- £381
- Upfront cost (net of grants)
- £1500
- Heating portion of bill
- £1105/yr
- 10-year net benefit
- £2312
You'll probably want this next
Insulating gets your EPC rating up fast
Cavity-wall or loft top-up often moves an EPC from D to C in one step — the threshold MEES is targeting in 2028.
Model an EPC upgrade →How the savings are modelled
Every UK home loses roughly 35% of its heat through walls, 25% through the roof, 10–15% through the floor, 20% through glazing and another 10% through infiltration. Insulating one of those elements reduces its share of loss by a predictable percentage — 70% for a good cavity fill, 85% for a fresh 270 mm loft install, 40% for topping up a 100 mm loft to 270 mm. We multiply the saved heat fraction by your heating bill to get £/yr saved.
It's deliberately blunt. A proper SAP or RdSAP assessment will be more precise and will account for thermal bridges, ventilation trade-offs and regional degree days. But for a homeowner asking "is it worth insulating?" the blunt maths gets you to within ±20% of the right answer — and that's all the precision the decision needs.
What v1 doesn't model
- Solid wall insulation. Completely different maths — £8–15k upfront, 20–30 yr payback. See the EPC upgrade calculator for solid-wall modelling.
- Carbon savings. Real, but not shown. Rough rule: 1 kWh of gas avoided = 0.18 kg CO₂.
- Ventilation risk. More insulation without more ventilation can cause damp. Newer installs usually bundle trickle vents; older retrofits sometimes don't.
- Supplier-specific grants. GBIS, Warm Homes Discount and local schemes sometimes stack with ECO4. We only model ECO4.
- EPC rating movement. Insulation often moves you from D to C — use the EPC upgrade calculator for band-specific modelling.
Frequently asked questions
- What savings can I actually expect from cavity wall insulation?
- Energy Saving Trust puts it at £180–£690/yr on a gas-heated semi, depending on current condition. Our defaults assume a 75% reduction in wall heat loss — roughly U=1.5 → U=0.45 — which matches post-2002 bonded-bead installs. Actual savings depend on how much of your bill is heating (typically 65–70% in a UK home) and on the unit rates in the current [Ofgem price cap.
- Is loft insulation top-up worth it if I already have 100 mm?
- Yes, but less than installing from scratch. Going 100 → 270 mm cuts roof heat loss by roughly 40% — usually £30–£80/yr in savings. Fresh install (0 → 270 mm) cuts it by ~85% and saves £250–£400. The marginal kW matters most if you're also sizing a heat pump — feed the improved fabric into the [heat loss calculator.
- What's ECO4 and does it pay for insulation?
- ECO4 is the current Energy Company Obligation, funded by a levy on energy suppliers. It pays for whole-home retrofits — insulation plus heating — for low-income households, private renters with EPC D–G, and some benefit recipients. If you qualify, cavity wall insulation is usually 100% funded; loft insulation always is. Check eligibility via TrustMark or an ECO4 installer. Landlords facing the 2028 MEES deadline should line ECO4 up alongside the [MEES landlord compliance calculator.
- What's left out of this calculator?
- Solid wall external and internal insulation (very different maths — ~£8–15k upfront). Floor insulation (difficult retrofit, variable savings). Window upgrades (small share of heat loss — covered in our [EPC band upgrade cost calculator instead). Ventilation and airtightness trade-offs (more insulation often means you need MVHR to avoid damp).
- Should I insulate before or after installing a heat pump?
- Before. Every kW you save in heat loss saves roughly £500–£1,000 on the heat pump install (smaller unit, smaller radiators, less emitter upgrade work) and several hundred £/yr on running cost. A heat pump sized for an uninsulated house is oversized after you insulate — and oversized heat pumps cycle inefficiently. Model the after-insulation system cost in our [heat pump vs gas boiler calculator and claim £7,500 off via the [Boiler Upgrade Scheme grant.
Insulation and heat pumps go together
If you're considering a heat pump, do the insulation maths first. Every kW of heat loss you remove shrinks the heat pump you need — and a 7 kW pump costs less than a 9 kW one, needs fewer radiator upgrades, and runs at a better efficiency. Model the before-and-after in the heat loss calculator .
If you're a landlord facing MEES 2028 (EPC C minimum), insulation is almost always the cheapest path to compliance. Use the MEES compliance calculator to see what band your property lands in after.
Related calculators
Ranked by topic overlap with this tool.
- Boiler Upgrade Scheme Grant Calculator UK — Check eligibility for the £7,500 BUS heat pump grant and see your net install cost.
- MEES Landlord Compliance Calculator UK — Landlord-focused MEES compliance check with cheapest-first measure stack and grant offsets.
- EPC Band Upgrade Cost Calculator UK — Cost of raising your EPC to band C, B or A — measure stack, grants, resale uplift and green-mortgage saving.
- Holiday-Let EPC Upgrade Calculator UK — Holiday-let EPC upgrade cost, occupancy-weighted running-cost saving, comfort-premium revenue uplift, and FHL capital allowance notes.
- Heat Pump vs Gas Boiler Calculator UK — Compare running costs and 15-year total cost of ownership against keeping a gas boiler.