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The shiny side must face an air space. Otherwise it doesn't do anything. Theoretically, it doesn't matter if the shiny side is facing up or down, as long as it is facing an air space. Practically, dust tends to collect on it if it faces up, which degrades its performance. So down is probably preferred, as long as it faces an air space. It is usually recommended that the air space be at least 0.75 inches deep to get the full performance specified for radiant barriers.
Search "radiant barrier" to confirm this and for more info.
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