The vapor barrier on under-floor insulation should always be placed close to the floor. The concrete barrier foil in the above post would work. Adding bulk insulation to the underside of the barrier foil would be a great idea.
In practice, I have seen contractors spray silver "aluminizing" paint on the kraft-paper-side of fiberglass batts before installation to help reflect radiant heat. These same guys have nicknamed the "space blanket" type insulation (ecofoil, prodex, reflectex,etc) as "bubble crap". I tend to agree, having lived through the drama of reflect-x having to revise their lofty claims after ASTM testing revealed their product to have an r-value of just about 1, counting the radiant reflectionationing, of course. The novel products' times will come as well.
Last edited by jeff5may; 12-15-13 at 11:15 PM..
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