Total R-Value of an Assembly

Because every layer of a wall passes the same heat through the same square metre, their R-values add: $R_{\mathrm{tot}} = R_1 + R_2 + R_3$. Set an unused layer to zero. A conventional 2×6 wall totals R-19 in the cavity, R-5 for an inch of exterior polyiso and about R-1.13 for the inside air film plus gypsum, giving R-25.13 — and note that the two "small" layers together are 24% of the answer, which is why the air films and finishes are in the ASHRAE tables at all rather than being waved away. In metric the same sum runs RSI 0.12 for the inside film, 3.32 of cavity and 0.08 for drywall to make RSI 3.52, the Canadian code number that everyone quotes as R-20.

Two things do not belong in this sum. Airspaces are not insulation in proportion to their thickness: a sealed 20 mm cavity is worth about RSI 0.17 (R-1) and a 100 mm one is worth barely more, because convection takes over as soon as the gap is wide enough to circulate — the exception is a low-emissivity foil facing the gap, which doubles it. And the exterior air film depends on wind, dropping from RSI 0.12 in still air to 0.03 in a 25 km/h breeze, so any R-value quoted with a generous outdoor film is quoting a calm day. The rule to hold on to is that only series layers add. Studs, joists and fasteners run alongside the insulation rather than behind it, and that is a parallel path, handled on the effective R-value page.