Calculate insulation surface temperature, heat flux, thickness, or thermal conductivity by selecting the unknown and entering the other four known values in °C, °F, K, and common units.
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Insulation Surface Temperature Formula
The calculator uses the one-dimensional steady-state conduction relationship for heat flow through a flat insulation layer. It converts all inputs to base units before solving: temperature in °C, heat flux in W/m², thickness in m, and thermal conductivity in W/m·K.
- Ts = insulation surface temperature
- Ta = opposite-face material temperature, not ambient air temperature
- q'' = heat flux through the insulation per unit area
- d = insulation thickness
- k = thermal conductivity of the insulation material
To calculate insulation surface temperature, the calculator adds the temperature rise across the insulation, q''d/k, to the known-side temperature. Select a different quantity to rearrange the same conduction equation to solve for known-side temperature, heat flux, insulation thickness, or thermal conductivity.
The sign of q'' matters. With the formula shown, a positive heat flux makes the surface temperature higher than the known-side temperature. A negative heat flux would represent heat flow in the opposite direction.
Common Insulation Thermal Conductivity Values
Thermal conductivity depends on material, density, temperature, moisture, and manufacturer data. The values below are typical reference ranges for quick checks.
| Insulation material | Typical k value, W/m·K | Notes |
|---|---|---|
| Fiberglass batt | 0.035 to 0.045 | Common building insulation |
| Mineral wool | 0.035 to 0.045 | Often used for thermal and fire resistance |
| Expanded polystyrene, EPS | 0.032 to 0.040 | Rigid foam board |
| Extruded polystyrene, XPS | 0.029 to 0.036 | Rigid foam with relatively low conductivity |
| Polyisocyanurate foam | 0.020 to 0.028 | High insulating performance |
| Calcium silicate | 0.050 to 0.080 | Often used for higher-temperature pipe and equipment insulation |
Unit Conversions Used by the Calculator
| Quantity | Input unit | Conversion to base unit |
|---|---|---|
| Temperature | °F | °C = (°F - 32) × 5/9 |
| Temperature | K | °C = K - 273.15 |
| Heat flux | Btu IT/(h·ft²) | W/m² ≈ value × 3.154590745 |
| Heat flux | kcal th/(h·m²) | W/m² = value × (4184/3600) |
| Thickness | in | m = value × 0.0254 |
| Thermal conductivity | Btu IT/(h·ft·°F) | W/m·K ≈ value × 1.730734666 |
Example Problems
Example 1: Calculate insulation surface temperature
You know the opposite-face temperature is 25 °C, the heat flux is 40 W/m², the insulation thickness is 0.10 m, and the thermal conductivity is 0.04 W/m·K.
The insulation surface temperature is 125 °C.
Example 2: Calculate required insulation thickness
You know the surface temperature is 80 °C, the known-side temperature is 20 °C, the heat flux is 24 W/m², and the thermal conductivity is 0.04 W/m·K.
The required insulation thickness is 0.10 m, or 10 cm.
FAQ
What does the known-side temperature mean?
The known-side temperature is the material temperature at the opposite face of the flat insulation layer. Ambient air is not generally at that face temperature; convection and radiation require separate boundary conditions. Positive heat flux in this formula flows from the Tₛ face toward the Tₐ face.
Why does the calculator use heat flux instead of total heat transfer rate?
Heat flux is heat transfer per unit area, usually in W/m². The insulation conduction equation used here is based on heat flow through a unit area, so area cancels out. If you have total heat transfer rate instead, divide it by the heat-transfer area first to get heat flux.
Does this include convection and radiation at the outside surface?
No. This calculation only models conduction through the insulation layer. Real surface temperature can also depend on outside air movement, convection coefficients, radiation exchange, contact resistance, gaps, moisture, and curved geometry such as pipes. For a more complete surface-temperature estimate, those effects need to be included separately.
