Thermal Resistance Conversion Calculator (R-value ↔ RSI)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
To convert a thermal resistance, multiply the US R-value by 0.1761 to get the metric RSI (or divide the RSI by 0.1761 to get the R-value). A US R-13 insulation therefore corresponds to an RSI of about 2.29 m²·K/W.
Explanation
An insulation's thermal resistance, which measures its ability to slow the passage of heat, is expressed in two different unit systems depending on the country: the "R-value", used in the United States and English-speaking Canada, is expressed in square foot-degree Fahrenheit-hour per BTU (ft²·°F·h/BTU), while the metric system used in France and most other countries expresses the same property as RSI, square meter-kelvin per watt (m²·K/W) — the SI unit directly derived from the definition of thermal resistance. The conversion factor between the two, about 0.1761, appears in most North American building technical documentation that gives a metric equivalent. The higher the R-value or RSI, the better the insulation: a common US R-13 insulation (used notably in wood-frame walls) corresponds to an RSI of about 2.29 m²·K/W, an order of magnitude consistent with the thermal resistances of common insulation in Europe. This calculator converts a known value between the two unit systems; to calculate the thermal resistance of an insulation layer directly from its thickness and thermal conductivity (R = thickness ÷ conductivity), see our insulation thermal resistance calculator, which stays in metric units, or our required insulation thickness calculator for the reverse question.
Example: US R-13 insulation to metric RSI
Inputs
Value: 13. Starting unit: US R-value. Target unit: metric RSI.
Calculation
13 × 0.1761101837 ≈ 2.2894 m²·K/W.
Result
This R-13 insulation corresponds to an RSI of about 2.29 m²·K/W.
Frequently asked questions
Why do the United States still use the R-value rather than RSI?
Because the United States never officially adopted the metric system for everyday and technical use, with the R-value in imperial units remaining the reference on insulation packaging, local regulations, and North American data sheets, instead of the RSI used in Europe.
Does a higher number always mean better insulation, in both systems?
Yes, in both unit systems a higher thermal resistance (larger R-value or larger RSI) means better insulation, since thermal resistance measures precisely the ability to oppose the passage of heat. A US R-30 insulation therefore insulates better than an R-13, just as an RSI of 5 insulates better than an RSI of 2.
Does this calculator directly give the thermal resistance of a complete wall?
No, it only converts a known thermal resistance value (usually that of an insulation) between the two unit systems. The total thermal resistance of a wall or roof combining several layers of different materials is obtained by adding the thermal resistance of each layer, a calculation distinct from the simple unit conversion handled here.