The R-value of 1 inch of still air is often misunderstood because air itself is not a good insulator unless it is trapped and kept from moving. A one inch thick sealed air space with still air has an R-value of about R-1.0 to R-1.1 for heat flow across it, but this value varies greatly with the direction of heat flow, temperature difference, and whether the surfaces are reflective. For example, a 1 inch air gap in a wall with heat flowing horizontally is about R-1.0, but the same gap with heat flowing up or down can be slightly higher or lower due to convection loops. If the air space has a reflective surface like a radiant barrier on one side, the effective R-value of that 1 inch air space can increase to R-2 to R-3 for downward heat flow because radiation is blocked. Open air that is free to move and convect, like in a ventilated attic, has essentially zero R-value because wind and convection carry heat away instantly. This is why double-pane windows need a sealed air or argon gap of about 1/2 to 3/4 inch, too small a gap increases conduction, too large increases convection, so the optimum is around R-2 for the gap alone.
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