Insulation Calculator — R-Value, U-Value & Heat Loss
Pick an insulation material (or enter a custom λ), set the thickness and area, enter the temperature difference, and instantly see the R-value (thermal resistance), U-value, and steady-state heat loss in watts — with an automatic quality rating.
Insulation material
mm
m²
°C
°C
Higher R-value means better insulation
- 1
Thickness in metres
100 mm ÷ 1000 = 0.1 - 2
Thermal resistance (R = d ÷ λ)
0.1 ÷ 0.035 = 2.86Higher R-value means better insulation.
How does this calculator work?
Divide insulation thickness (in metres) by thermal conductivity λ (W/m·K) to get the RSI (R-value). Flip it for the U-value. Multiply U × area × temperature difference (°C) to get heat loss in watts. Higher R = less heat lost. The calculator converts SI R-values to imperial automatically.
Formula
How this is calculated
Insulation is rated by its thermal resistance, the R-value (called RSI in SI units). The formula is R = d/λ, where d is the insulation thickness in metres and λ (lambda) is the material's thermal conductivity in W/m·K. A lower λ means a better insulator: closed-cell spray foam achieves around 0.026, mineral wool sits near 0.035, and standard fiberglass batts are around 0.044. Doubling the thickness doubles the R-value; using a material with half the conductivity does the same.
The U-value is the reciprocal of the R-value (U = 1/R) and describes how quickly heat flows through one square metre of the assembly per degree of temperature difference. Building codes typically specify maximum U-values (e.g. U ≤ 0.20 W/m²·K for walls in many European climates). To find the steady-state heat loss in watts, multiply U × area (m²) × temperature difference (°C): Q = U × A × ΔT.
The imperial R-value is the SI RSI multiplied by 5.678, so RSI 3.52 equals approximately R-20 in North American usage. The λ values used here are reference values measured at 10 °C per EN ISO 10456 (2023); actual site performance can vary ±10–20 % depending on moisture content, installation quality, and material age. This calculator covers conductive heat loss only — real buildings also lose heat through thermal bridges, air leakage and ventilation.
Frequently asked questions
Requirements vary by climate zone and local building code. In temperate European climates, exterior walls are commonly required to reach RSI 3.5–5.5 (about R-20 to R-31). In cold climates like Canada or Scandinavia, roof/attic insulation of RSI 7–9 (R-40 to R-50) is typical. Use this calculator to experiment with thickness and material until your R-value meets the local minimum.
This calculator gives steady-state conductive heat loss through the insulation only. Real buildings also lose heat through thermal bridges (studs, window frames, slab edges), air infiltration, and mechanical ventilation — and receive heat from solar gains and occupants. The result here is useful for comparing materials and thicknesses, not for precise energy billing.
They are reciprocals. R-value (RSI) measures resistance to heat flow — higher is better insulation. U-value measures conductance — how fast heat passes through — so lower is better. If your building code states a maximum U-value (e.g. 0.18 W/m²·K), the minimum R-value required is 1/0.18 ≈ RSI 5.6.
Also known as
TG we-Calculate Editorial Team. (2026). Insulation Calculator — R-Value, U-Value & Heat Loss [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/insulation-calculator
TG we-Calculate Editorial Team. "Insulation Calculator — R-Value, U-Value & Heat Loss." TG we-Calculate. 2026. https://we-calculate.com/calculator/insulation-calculator.
TG we-Calculate Editorial Team, "Insulation Calculator — R-Value, U-Value & Heat Loss," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/insulation-calculator
@misc{wecalculate_insulation_calculator, title = {Insulation Calculator — R-Value, U-Value & Heat Loss}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/insulation-calculator}}, year = {2026}, note = {TG we-Calculate} }
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