How do you calculate overall U-factor for a multi-layer wall?

• A. Multiply all R-values to get U-factor.
• B. U-factor is equal to the largest R-value among layers.
• C. U-factor is unrelated to R-values.
• D. Sum the thermal resistance of all layers (R-values) including interior and exterior boundaries; U = 1/R_total for layers in series; for complex assemblies use area-weighted calculations.

Answer: (D)

Heat flows through a wall in layers that add up in series, so you’re looking at the total thermal resistance. The overall U-factor is the reciprocal of that total resistance. Add up the resistances of every layer, including the interior and exterior boundary resistances (the surface films). Once you have R_total, compute U as 1 divided by R_total. For more complex assemblies with different paths, you’d use area-weighted calculations, but the core idea stays: U = 1 / R_total for layers in series.

This shows why multiplying R-values or picking only the largest resistance won’t give the correct overall heat transfer. And U is indeed linked to R-values, not unrelated to them. For a concrete sense, if R_total = 4.5 (in ft^2·°F·h/BTU), then U = 1 / 4.5 ≈ 0.22 BTU/(h·ft^2·°F).