Well dead air space becomes live airspace if large enough. For example: If you take a sheet of foam insulation and put it on a wall it has only the value of the insulation, lets use R4 just for giggles. BUT if you put a 1/8 to 1/4 air space between the insulation and the wall, you effectively double to R8, same amount of insulation just the dead air space. BUT if you open that up to greater than a half inch, you are right back to the R4 because air now can move much easier. Same principal, except we are now horizontal. The space above the rack is the raising the R2 of the wood of the rack to R4 because of the bee space that is there. (those are not factual numbers just examples)
Air is an excellent insulator, in fact most insulations provide dead air space in one form or another and that is the insulative power. So it is not the rack really, it is what the rack creates. Also the rack turbulates the air as it moves up from the bottom of the hive mixing colder and warmer, better maintaining the bee's natural temps.
This works both ways, I had no bearding at all this summer even though my hives are painted dark colors, in the sun, and even on 100 degree day no issues presented. Maybe I am talking out my buns, because it was my first year, so take it an aspririn.