Jerry, radiant barriers don't need an air space. They need a thermal insulator, and preferably an IR transmissive thermal insulator, at the IR reflective surface (the aluminum side). A lack of an IR transmissive thermal insulator is the reason that putting a space blanket against your skin, aluminum side in, does nothing to reduce radiant heat loss. Multilayer radiant barriers are very important to controlling the temperatures of satellite components, and space is a vacuum so there is no air space against those radiant barriers. Multilayer radiant barriers on spacecraft have fabric or scrim between the layers as a thermal insulator, to prevent heating of each layer by solid conduction from other layers ("thermal bridging"). An emergency blanket on the outside of a sleeping bag with the aluminum surface facing in would be in the same situation. It would have lightweight fabric against the reflective surface.
What would a space blanket on the outside of a sleeping bag be doing if the aluminum layer is facing out? If air temperature outside is 30F, what is the temperature of a space blanket on the outside of a sleeping bag? 33F? Freezing of moisture INSIDE the insulation of a sleeping bag is a problem for winter camping, and the outside of a sleeping bag is even colder. It wouldn't be doing a very good job as insulation if it was warm on the outside.
Any heat absorbed by a layer of cold plastic film exposed to cold air will be lost by convective cooling. The cold air will carry it away. The fraction lost to the environment by radiation will be vanishingly small, so you're not accomplishing anything by having the aluminized layer on the outside. But you are putting the mylar layer facing in, where it can absorb radiant energy from the deeper layers of the sleeping bag, which are warm. The space blanket then loses that energy to the outside air by convection.
If the aluminum side is facing in, the radiation from within the sleeping bag is reflected back toward the inner layers of the sleeping bag. Very little gets absorbed by the space blanket, so it can't be lost to the outside air.