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
0.028 BTU/h·ft²·°F
- R-value added by the insulation
- 24.7 ft²·°F·h/BTU
- Total R-value after insulation
- 35.7 ft²·°F·h/BTU
- Estimated saving per year
- $98
- Fuel saved per year
- 1,937 kWh
Estimates for planning only — not a quote, survey or financial advice.
Sources for the pre-filled values — United States
- U.S. EIA — Monthly Energy Review, Table 1.11, population-weighted heating degree days (United States, base 65 °F) · Average of 2021–2025 · accessed 2026-11-09 · National population-weighted average; your climate zone can differ a lot.
- U.S. EIA — Natural gas prices, residential price (U.S.), $15.34 per thousand cubic feet · 2025 annual average · accessed 2026-11-06 · Converted at ≈1,036 Btu per cubic foot (1,000 ft³ ≈ 303.6 kWh).
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.