Thermal Conductivity of Air

Still air is one of the best cheap insulators there is, and essentially every building insulation product is a scheme for holding air still. Fibreglass batt at 0.040 W/(m·K) is worse than the air it contains, because the glass fibres and the radiation across the cavity add paths that pure still air does not have. Only foams blown with heavy gases, aerogels, or evacuated panels beat still air, and blown foams do it by trapping a gas with lower conductivity than air — which is why their R-value drifts down over years as the blowing agent diffuses out and air diffuses in.

The trap is that air in a real cavity is never still. Above about 20 mm of unobstructed gap, natural convection sets in and the effective conductance stops improving with width; a 100 mm empty wall cavity is nowhere near four times the resistance of a 25 mm one. Air conductivity also rises with temperature, roughly 0.030 W/(m·K) at 100 °C, and is nearly independent of pressure until you approach vacuum — the principle a Dewar flask exploits.