Thermal Resistance Calculator (Insulation)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
Thermal resistance is calculated as R = thickness (m) ÷ λ (the material's thermal conductivity). For 20 cm of glass wool (λ ≈ 0.035 W/m·K), the thermal resistance is about 5.71 m²·K/W.
Explanation
Thermal resistance (denoted R, expressed in m²·K/W) measures a layer of material's ability to resist the passage of heat: the higher R is, the better the insulation. It's calculated by dividing the material's thickness by its thermal conductivity (λ, lambda), an intrinsic property of each material indicating how fast heat passes through it — the lower λ is, the better an insulator the material is at equal thickness. Polyurethane, with a particularly low thermal conductivity (about 0.022 to 0.028 W/(m·K)), achieves a given thermal resistance with a noticeably smaller thickness than a standard mineral wool (glass wool or rock wool, around 0.030 to 0.045 W/(m·K)), which explains its preferred use in configurations where available thickness is limited. French thermal regulations generally set minimum resistance values to reach depending on the area concerned (attic, walls, floors); this calculator gives the resistance of a single layer of insulation, to compare against the regulatory requirements applicable to your project.
Example: 20 cm of glass wool
Inputs
Thickness: 20 cm. Material: glass wool (λ ≈ 0.035 W/m·K).
Calculation
Thermal resistance = (20 ÷ 100) ÷ 0.035 = 0.20 ÷ 0.035 ≈ 5.71 m²·K/W.
Result
This layer of insulation provides a thermal resistance of about 5.71 m²·K/W.
Frequently asked questions
What thermal resistance should I aim for when insulating an attic?
Regulatory requirements vary depending on the type of work (new construction or renovation) and change over time; they commonly sit around 7 m²·K/W for unfinished attics in renovation projects, but you should check the thermal regulations in force at the time of your project rather than relying on a figure that may have changed.
Why choose a low-conductivity insulator over a cheaper one?
An insulator with lower thermal conductivity (like polyurethane) reaches the same thermal resistance with less thickness, a decisive advantage when available space is limited (interior insulation with a small footprint, for example). In exchange, these high-performance insulators are generally more expensive per square meter than standard mineral wools, a trade-off to weigh against your project's budget and space constraints.
How do I calculate a multi-layer wall's total thermal resistance?
The total thermal resistance of a wall made up of several layers (insulation, partition, render) is obtained by adding up the resistance of each individual layer. This calculator gives the resistance of a single insulation layer; for a complete wall, you need to calculate and add the resistance of each material the heat passes through.