R-Value Thermal Resistance Calculator
Thermal resistance, commonly called R-value, quantifies how strongly a material or assembly resists heat flow. The formula is R = L / k, where L is the material thickness (m) and k is the thermal conductivity (W/(m K)). For a wall assembly consisting of multiple layers, the total R-value is the sum of each layer's R-value. A higher R-value means better insulation. This calculator finds the R-value of a single layer and converts between SI (m^2 K/W) and US customary (ft^2 F h/BTU) units, which are widely used by the US DOE and ASHRAE building codes.
R-value formula
R(SI) = L / k R(US) = R(SI) × 5.6783
L is thickness in meters (m), k is thermal conductivity in W/(m K). R(SI) is in m^2 K/W. To convert to US customary R-value (ft^2 F h/BTU): multiply by 5.6783. For layered assemblies: R_total = R1 + R2 + R3 + ...
Worked example
- 100 mm (0.1 m) of fiberglass batts with k = 0.04 W/(m K).
- R(SI) = 0.1 / 0.04 = 2.50 m^2 K/W.
- R(US) = 2.50 x 5.6783 = 14.20 (commonly written R-14).
Frequently asked questions
What is thermal resistance (R-value)?
R-value measures a material's resistance to heat flow. R = L / k, where L is thickness in meters and k is thermal conductivity in W/(m K). A higher R-value means better insulation. In the US building industry, R-values are often stated in customary units (ft^2 F h/BTU).
What are US customary R-value units vs SI?
US customary R-value is in ft^2 F h/BTU (often just called 'R'). SI R-value is in m^2 K/W. To convert: 1 m^2 K/W = 5.678 ft^2 F h/BTU. This calculator uses SI units but shows the US equivalent.
How do R-values add for layers?
For a wall assembly with multiple layers in series, the total R-value is the sum of individual R-values: R_total = R1 + R2 + R3 + ... This makes it easy to calculate the total insulating performance of a multi-layer wall, ceiling, or floor.
What R-values do common materials have?
Fiberglass batt insulation: R-3.2 to R-3.8 per inch (SI: 0.56 to 0.66 m^2 K/W per 25 mm). Foam board: R-3.8 to R-6.5 per inch. Concrete block (8 in): about R-1.1 SI. Double-pane window: about R-0.35 SI. Values from ASHRAE Fundamentals.
What R-values are required by US building codes?
DOE/ASHRAE 90.1 and the IECC prescribe minimum R-values by climate zone. For example, attic insulation ranges from R-30 (warm climates) to R-60 (cold climates) in customary units. The DOE Building Energy Codes Program publishes these requirements.
Official sources
- DOE Building Energy Codes Program: Energy Codes (DOE).
- ASHRAE: ASHRAE Handbook of Fundamentals (Chapter 26: Heat, Air, and Moisture Control).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.