How this calculator works
Steady heat crosses each layer and film at the same rate, so their temperature drops add. Dividing each thickness by conductivity gives resistance per unit area; dividing again by common area gives K/W. The overall U value is the inverse of the summed resistance per area. Material tokens use approximate OpenStax Table 1.5 values, including stainless steel and the lower listed wood value; use custom data at the actual temperature. Contact resistance, thermal bridges and cylindrical geometry are excluded.
Formula
Rtotal = sum[Li/(ki A)] + 1/(hi A) + 1/(ho A); U = 1/(A Rtotal); conductance = UA.
Worked example
Layers 0.1 m at 0.5 W/(m K) and 0.05 m at 0.025 W/(m K) give 2.2 m² K/W. For A = 2 m², U = 0.454545 W/(m² K) and R = 1.1 K/W. Adding films hi = 10 and ho = 20 gives U = 0.425532 W/(m² K).
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Frequently asked questions
Does area change U?
For uniform layers with the same area, U stays unchanged while total resistance varies inversely with area.
Can I enter more than one material?
Yes. Each row represents a layer, up to eleven layers, and its contribution appears in the table.
Do the material tokens cover every building product?
No. Products, moisture, direction and temperature affect conductivity. Enter manufacturer or measured data for unlisted materials.