Thermal Resistance Calculator
The thermal resistance calculator determines how well a material or wall assembly resists heat flow. Using Fourier's law of heat conduction, it computes the R-value of a single layer, the total R-value of composite walls with multiple layers in series, the heat flux (rate of heat flow per unit area), and the temperature at each layer interface. This tool is used by building engineers designing insulated wall assemblies, HVAC engineers sizing heating and cooling systems, electronics engineers managing heat dissipation in circuit boards, and process engineers designing insulated pipe systems. Enter the material thickness, thermal conductivity, area, and boundary temperatures to get all heat transfer quantities.
Thermal resistance formula
R = L / (k * A) [K/W] or R_area = L / k [m^2*K/W]
U = 1 / R_area [W/(m^2*K)]
Heat flux: q = (T_hot - T_cold) / R_area [W/m^2]
Total heat loss: Q = q * A [W]
Thermal conductivity reference values (W/m*K)
- Expanded polystyrene (EPS): 0.033 to 0.040
- Mineral wool / fiberglass: 0.035 to 0.045
- Concrete: 1.0 to 1.7
- Brick: 0.6 to 0.8
- Timber (softwood): 0.12 to 0.14
- Steel: 45 to 58
Thermal resistance: frequently asked questions
What is thermal resistance?
Thermal resistance (R) is the measure of a material's resistance to heat flow. For a flat layer: R = thickness / (thermal conductivity * area) = L / (k * A). Higher R means better insulation. In building science, the R-value is quoted per unit area: R = L / k, with units of m^2*K/W (SI) or hr*ft^2*F/BTU (US).
How is heat flow through a wall calculated?
Heat flow Q = (T_hot - T_cold) / R_total, where R_total is the total thermal resistance of all layers (insulation, drywall, brick, etc.) added in series. This is directly analogous to Ohm's law for electrical circuits, with temperature difference as voltage and heat flow as current.
What is thermal conductivity?
Thermal conductivity (k) is a material property measuring how readily it conducts heat. Low k means good insulation. Air: 0.025 W/(m*K). Fiberglass insulation: 0.04 W/(m*K). Wood: 0.15 W/(m*K). Concrete: 1.7 W/(m*K). Steel: 50 W/(m*K). Copper: 400 W/(m*K).
How do I combine thermal resistances in series?
For layers in series (like a composite wall), total thermal resistance R_total = R1 + R2 + R3 + ... This is the same as adding electrical resistances in series. For parallel paths (like a stud wall with insulation between studs), use 1/R_total = 1/R1 + 1/R2.
What is the relationship between R-value and U-value?
U-value (thermal transmittance) is the reciprocal of R-value per unit area: U = 1 / R. Lower U-value means better insulation. Building codes often specify minimum R-values (or maximum U-values) for walls, roofs, and floors. For example, a US R-30 roof insulation has U = 1/30 BTU/(hr*ft^2*F).
Official sources
- US DOE: US Department of Energy - Insulation.
- ASHRAE: ASHRAE Handbook of Fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.