Radiant barriers can be anywhere in any position as long as one side of them is not contacting any surface as in it has to be in free air on one side. Hence radiant if it's touching both sides all the heat is conducted through conduction and radiant heat isn't in the equation if they are touching.
You can attach it to the underside of the roof rafters and it will work fine. In fact it's best if the shiny side is facing down as the build up of dirt on the radiant material will reduce it's effectiveness.
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