If radiant heat is the main reason for body heat loss, then having the reflective layer next to the body makes sense. Less heat is transmitted to the air pad and less is lost by convection. I simply do not know how effective the reflective barriers are. Would a space blanket or an aluminized Mylar sheet provide as much reduction in heat loss? I think that a lot of people would decide not to use a Mylar sheet because they don't like sleeping on a plastic sheet. Reflectix is probably the first choice for a reflective blanket because it is so widely available. If I get around to it, I might try a space emergency blanket under a piece of Tyvek to see how it performs.