Overall U from Total Resistance

Two trades describe the same wall with reciprocal numbers. Heat-transfer analysis likes absolute resistance in K/W because it adds; equipment datasheets like U in W/(m²·K) because it multiplies straight into Q̇ = UAΔT. This converts between them: U = 1/(R_tot A). A 12 m² assembly totalling 0.005 K/W has U = 1/(0.005 × 12) = 16.7 W/(m²·K) — a bare metal partition. Building envelopes live three orders of magnitude lower, at U = 0.15–0.35.

The trap is the area you divide by, and it has cost real money. Shell-and-tube exchangers are almost always rated on outside tube area, but the inside film resistance is physically attached to the inside area, which on a 19 mm tube with a 2 mm wall is 21% smaller. Mix the two and your U is off by a fifth, which shows up as an exchanger that will not make its approach on the hottest day of the year. Always write the area basis next to the U — "850 W/(m²·K), outside" — and the ambiguity disappears.