Combine layers of insulation into a total thermal resistance and U-value.
Thermal resistances in series add, exactly like electrical resistors. The U-value is the reciprocal of the total, which is why adding insulation gives diminishing returns as R grows. A single thermal bridge can dominate the effective U-value of an otherwise well-insulated assembly, because it provides a parallel low-resistance path.
Thermal Resistance
R_total = Σ (thickness ÷ conductivity) + surface resistances; U = 1 ÷ R_total
R_total = Σ (thickness ÷ conductivity) + surface resistances; U = 1 ÷ R_total Thermal resistances in series add, exactly like electrical resistors. The U-value is the reciprocal of the total, which is why adding insulation gives diminishing returns as R grows.
A single thermal bridge can dominate the effective U-value of an otherwise well-insulated assembly, because it provides a parallel low-resistance path.
This calculator takes 5 inputs: Layer 1 thickness, Layer 1 conductivity, Layer 2 thickness, Layer 2 conductivity, Combined surface resistances. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.