Thermal Resistances in Series

Because every layer of a composite wall passes the identical heat flow, their resistances add just like series resistors: $R_{\mathrm{tot}} = R_1 + R_2 + R_3$. Set an unused layer to zero. The sum immediately shows where the money goes: an inside film of 0.02, a 6 mm steel skin of 0.006 and 50 mm of mineral wool at 0.04 K/W total 0.066 K/W, of which the steel is 9%. Adding a second steel skin changes almost nothing; adding a second inch of wool changes a great deal.

The same arithmetic exposes the most expensive mistake in insulation work, the thermal bridge. Series resistances add, but parallel paths do not: a steel stud, a through-bolt or a pipe hanger sits alongside the insulation rather than behind it, and a bridge occupying 2% of the area with 300 times the conductivity can carry a third of the heat. That is why fastener manufacturers sell thermal-break washers, and why an infrared camera pointed at a well-insulated wall on a cold morning still draws you a perfect picture of the studs.