It's actually a little more complicated than that. In real life, a
BOUNDARY LAYER tends to form in the fluids (air, here) on both sides
of the metal. On the 'pure-air' side, the boundary layer is actually
just some air molecules which have already been heated by the exchange
surface.Those molecules get in the way of subsequent heat transfer, keeping the cooler air molecules away from the hot surface. This effectively represents an INSULATION layer which reduces heat exchange.