Steady-state heat flow through a flat wall or plate (Fourier's law, Q = k·A·ΔT/L), with the layer's R-value and U-value. For insulation, enclosures and thermal sizing.
Q = k · A · ΔT / L. R-value = L/k (m²·K/W), U-value = 1/R. This is one solid layer at steady state — for a real wall, add the R-values of every layer (plus surface films) in series, then U = 1/ΣR.