The whole reason for using Reflectix instead of a simple foil barrier is that it has a built-in air space. If you're hanging a reflective barrier in your attic, Reflectix is a waste of money since you could hang aluminized or foil-laminated material and have an air space on both sides. Confused?
Reflective barriers work because they are not only good reflectors or infrared (duh!), but, by definition, they are poor emitters as well. Likewise, good heat absorbers, such as flat black surfaces, also make very good infrared emitters. Aluminum works very well as a reflector of both visible light
and most of the infrared band.
If a single layer of foil is used as a barrier and both sides are glued to some other material, heat is transferred right through the aluminum (which is a very good conductor of heat) into the adjacent bonded layer. An air space on the warmer side of the foil will allow the warmer material to emit its heat, a little of which will be absorbed by the air, but most of it will reflect off of the foil surface, where it will be reabsorbed by the warmer surface. This is pretty intuitive. The second, non-obvious way the barrier works when the foil is on the OUTSIDE of the warmer surface. Remember, shiny aluminum foil makes a poor heat emitter as well. What happens is that the warmer but foil-covered surface will emit very little of its heat into the air space. Again, if the foil is glued to something (like your cool new fake ostrich leather headliner) without that air space, the heat will pass right on through.
Reflectix has two layers of foil with what is basically bubble wrap in between. It was made so that it can be glued or sandwiched on one or both sides and still function as a reflective barrier because (this question will be on your semester exam) it has a built-in air gap
. New Scamp
trailers (and probably other ones as well) use two layers of Reflectix.
Oak Ridge National Laboratory has a nice little tutorial on reflective barriers as used in buildings at Radiant Barrier Fact Sheet
The take-away here is that you'll do best with radiant barrier plus thermal insulation. Thick insulation in a little egg camper
takes up a lot of space, so two layers of Reflectix is a nice compromise. I plan to put low-E film on my windows
to augment the Reflectix when I reline my Scamp