[Snail]

I must disagree with berniebenz. Any material that is a good reflector of thermal radiation is automatically a very poor emitter of it as well. A barrier that has the reflective side facing away from the heat source will, of course, heat up but it will emit little from the other side, hence acting as a barrier just as well as if it was mounted the other way around.

Regardless of the theory however, I have to agree with creepy that such barriers are almost never worth doing. Dust and condensation can ruin both the reflective and emissive effects and in an attic you can never guarantee to avoid them. Same goes for most places where people place such barriers.

If you must fit a barrier, be very careful to ensure that you do not trap water, whether it be condensation or possible driven rain, where it can cause structural or toxic mould problems.