Insulation calculator (U-value)
See how much a layer of insulation improves a wall, roof, loft or floor: the new U-value and R-value, the thickness needed to reach a target, and a rough yearly heating saving.
Your estimate
New U-value
0.18 W/m²K
- R-value added by the insulation
- 4.57 m²K/W
- Total R-value after insulation
- 5.57 m²K/W
- Estimated saving per year
- £293
- Fuel saved per year
- 3,671 kWh
Estimates for planning only — not a quote, survey or financial advice.
Sources for the pre-filled values — United Kingdom
- Eurostat — Cooling and heating degree days by country, annual data (nrg_chdd_a), heating degree days · Average of 2021–2025 · accessed 2026-11-09 · Eurostat method: 18 °C minus the daily mean outdoor temperature, on days at or below 15 °C. National averages; your location can differ a lot.
- Ofgem — Energy price cap unit rates (Great Britain average, Direct Debit) · 1 October – 31 December 2026 · accessed 2026-11-06 · Unit rates only; daily standing charges are not included.
How this is calculated
Formula
- R before = 1 ÷ U before · R added = thickness ÷ λ
- U after = 1 ÷ (R before + R added)
- Thickness for a target = (1 ÷ U target − 1 ÷ U before) × λ
- Heat saved (kWh) = (U before − U after) × area × degree days × 24 ÷ 1,000 · fuel saved = heat ÷ efficiency · saving = fuel × price
Assumptions
- The current U-value is an example. Typical values depend heavily on how the element was built — an energy assessor or your energy certificate can give a better figure.
- λ values are typical examples for each material; use the declared value on the product. Compressed or gappy insulation, timber studs and thermal bridges make real performance worse than this calculation.
- The degree-day method is simplified: it ignores solar and internal heat gains and assumes the home is heated all season, so it tends to overestimate savings. Degree days are national averages — your location may differ a lot.
- Insulation changes how moisture moves through a wall or roof. Check vapour control, ventilation and fire rules, and get professional advice for solid walls, flat roofs and listed or older buildings.