radiant barrier

i was listening to a home improvement talk show recently and the narrator kept recommending a 'radiant barrier' in the attic. since i didn't get to listen to the complete show, i was never quite sure what was meant by a 'radiant barrier'.
i did hear enough to determine that one side has foil, which must face downward.
is a 'radiant barrier' just a piece of foam insulation with foil on one side?? or, is it something special that you must purchase?
what is meant by 'radiant barrier'??
i also believe that i heard him say that 'radiant barriers' were really not effective in cold weather; mostly effective in hot weather.
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stevie wrote:

Generally it is any substance that has a shiny metallic service. The shiny side goes toward the heat.
I would not get all excited about it. In practice they tend not to do as great a expected for a number of reasons.
First it can only reduce radiant heat. Second if it gets dusty it quickly loosed efficiency. Third it has to have somewhere to reflect the heat. In other words the heat has to go somewhere. Generally there are not many good places to use it in home construction. You really don't want to reflect a lot of heat up towards your roof, after all heat damages most roofing materials and during the winter you want to keep the roof cold to prevent ice dam damage.
--
Joseph Meehan

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The information I have says it matters little whether the shiny part faces up or down, out or in. It also matters little if the surface gets dirty. Research is still being done, but recent studies are more favorable than early studies. It is hard to keep up with all the information on them. Even installed wrong they still work, just not as well as when installed properly.
Stretch
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Probably the best attic radiant barrier is light-colored roofing, to reduce IR transmission where it matters most.
With adequate insulation & sealing below, and ventilation of attic space. Ventilation to dissipate summer heating, and prevent snow-melt and ice-dams in winter.
HTH, J
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