What this calculator models
Insulation is the single most cost-effective retrofit measure for almost every UK home, but the maths varies dramatically between measure types. Cavity-wall insulation often pays back inside 2-4 years on suitable stock; external wall insulation (the most transformative for solid-wall homes) can take 18-30 years on private-pay terms but flips to under 5 years where ECO4 or HUG grants cover most of the capital cost. This calculator surfaces both scenarios so eligibility for grants becomes the central question rather than a footnote.
Inputs
- Property type (terraced, semi, detached, mid-floor flat, top-floor flat)
- Wall construction (cavity, solid brick, solid stone, system-built, timber-frame)
- Existing insulation status (loft depth, cavity-fill, floor)
- Floor area (m²)
- Current heating fuel and tariff
- Annual heating consumption
- Likely grant eligibility (postcode + benefit status + EPC band proxy)
- Insulation measure(s) considered
Outputs
- Annual heating saving per measure
- Capital cost (private-pay)
- Capital cost net of likely grant contribution
- Breakeven year (private-pay)
- Breakeven year (grant-funded)
- 25-year cumulative saving
- Carbon-saving estimate (tCO2e over 25 years)
- Compatibility / sequencing notes (e.g. ventilation upgrade required)
Typical payback band: Cavity-wall: 2-5 years private-pay, 0-1 year grant-funded. Loft top-up: 1-3 years. Internal solid-wall: 12-25 years private-pay, 0-3 years grant-funded. External solid-wall: 18-30 years private-pay, 0-3 years grant-funded.
Best for: Homeowners and landlords assessing whether to fund insulation privately or wait for grant eligibility, plus those weighing which insulation measure to do first when budget is constrained.