INSULATION · ENGINEERING REFERENCE
Insulation and thermal resistance basics
Understand one-dimensional, steady insulation estimates through resistance, material state and boundary conditions.
Plane-wall model
For one-dimensional steady conduction through a uniform layer with constant conductivity and area, its thermal resistance is R = L/(kA), in K/W. L is thickness (m), k is thermal conductivity (W/(m·K)), and A is the area normal to heat flow (m²). For layers in series with the same area, add the conduction resistances and divide the surface temperature difference by the total.
Th and Tc are wall surface temperatures. Surface convection, radiation, contact gaps and thermal bridges are excluded. Surrounding fluid temperatures require the corresponding additional resistances under the actual conditions; they are not automatically wall surface temperatures.
Worked example
For an area of 10 m², a thickness of 0.05 m, conductivity of 0.04 W/(m·K), and surface temperatures of 20 °C and 0 °C, R = 0.05/(0.04×10) = 0.125 K/W and the heat-transfer rate is 20/0.125 = 160 W. The conductivity is an example input, not a product or recommended value.
Keeping the other inputs and surface temperatures fixed, a thickness of 0.10 m gives R = 0.25 K/W and 80 W. Actual surface temperatures can change when insulation is added, so this does not directly predict real energy savings.
Selecting material properties
Conductivity depends on composition, density, temperature, moisture, ageing and measurement direction. Check the product or data source, state and units, and choose a value representative of the actual conditions. A single value does not describe a material at every temperature and moisture level.
Where k changes substantially with temperature, a constant-k model is an approximation; a segmented or other suitable variable-property method may be needed.
Engineering limits
- Heat-transfer area changes with radius in pipes and cylindrical walls; the plane-wall equation does not replace a cylindrical conduction model.
- Supports, joints, fasteners and discontinuities can create thermal bridges that the one-dimensional model excludes.
- Condensation prevention requires checking local surface temperature, air dew point, vapour transport and actual construction. Conduction resistance alone cannot guarantee a dry surface.
- Operating variation, temperature limits, fire and moisture performance, maintenance and applicable requirements also matter. Qualified review is needed for final selection.