How to Use Thermal Resistance Calculator
The Thermal Resistance Calculator turns simple geometry and material data into the R-value used by HVAC, building, and electronics engineers. Choose a mode for a single layer, a multi-layer stack, or a heat-flow estimate from temperature difference, then enter the data the calculator asks for.
- Pick a mode - Single layer when you have one material slab; multi-layer for stacked insulation, drywall, sheathing, and siding; or the ΔT and R mode when the R-value is already known.
- Enter geometry and conductivity - The Thermal Resistance Calculator needs thickness d, area A, and the thermal conductivity k in W/(m·K) for each layer. Common values: mineral wool 0.04, EPS foam 0.036, concrete 1.7, brick 0.7, copper 400 W/(m·K).
- Read the result - The panel highlights the total R in K/W, and where applicable the area-specific R” = R · A in (m²·K)/W (the construction-industry R-value) and the steady heat flow Q̇.
- Add surface convection coefficients - For a complete wall U-value, add the interior (h_i ≈ 8 W/(m²·K)) and exterior (h_o ≈ 25 W/(m²·K)) film resistances as thin extra layers with k = h·d.
Formula & Theory - Thermal Resistance Calculator
The Thermal Resistance Calculator uses the steady-state conduction definition of thermal resistance:
R = d / (k · A) (single layer, K/W)
R_total = Σ R_i (series, K/W)
Q̇ = ΔT / R_total (steady heat flow, W)| Symbol | Meaning |
|---|---|
| d | Layer thickness (m) |
| k | Material thermal conductivity (W/m·K) |
| A | Cross-section area perpendicular to heat flow (m²) |
| ΔT | Temperature difference across the stack (K) |
| Q̇ | Steady-state heat power (W) |
In construction the area-specific R-value R” = d / k in (m²·K)/W (also called the RSI value or thermal resistivity) is commonly quoted. The Thermal Resistance Calculator keeps the area factor so you can compute heat flow directly.
Typical Material k Values
| Material | k (W/m·K) |
|---|---|
| Mineral wool batt | 0.035–0.045 |
| EPS polystyrene | 0.030–0.040 |
| PIR foam board | 0.022–0.028 |
| Brick | 0.60–0.80 |
| Concrete block | 0.51–1.7 |
| Timber (softwood) | 0.12–0.14 |
Assumptions and Limits
The calculator assumes purely one-dimensional, steady-state conduction with no contact resistance. For real walls, also include interior and exterior surface film coefficients; for refrigeration or transient scenarios, a U-value plus thermal mass model is more appropriate.
Use Cases for Thermal Resistance Calculator
The Thermal Resistance Calculator is useful when you need a quick, transparent calculation for heat-flow-limited problems. Common uses include:
- Building envelope - Stack insulation, sheathing, cladding, and drywall layers and compare the total R-value against local energy codes (e.g., ASHRAE, Building Regs Part L).
- Electronics cooling - Compute the conduction resistance of a thermal interface material (TIM) + heat-spreader + heatsink sandwich to check if junction temperature stays within limits.
- Refrigerated cabinets - Estimate cold-loss through insulated panels to size the compressor and verify daily energy consumption per cubic meter.
- Pipe lagging - Approximate heat loss per meter of insulated pipe for HVAC hot-water distribution or process engineering.
- Passive house design - Compare different insulation build-ups side by side to find the minimum cost combination that meets the target U-value (e.g., U ≤ 0.15 W/m²K).
- Fire barrier assessment - Estimate time to reach a critical temperature on the cold side of a fire-rated partition, as a preliminary check before full finite-element thermal modeling.
Real performance also depends on air leaks, thermal bridges, and convective films, so add a safety margin and always validate critical designs against measured data. The Thermal Resistance Calculator provides the solid, transparent conduction baseline for any multilayer thermal analysis.