Well, the foil faces the living space. I have the the roof decking that has the foil on the underside of it. Called Techshield.
LP Building Products | LP TechShield Radiant Barrier
I can vouch for using just plain old Standard weight Aluminum foil as a way to keep the heat in the house.
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Read more here starting at post #14.................
http://ecorenovator.org/forum/applia....html#post9662
If I were going to retrofit foil in my attic I would use this something way less expensive than the product at Lowes or Home Depot...........
FOIL ROLL 18"X1000' ROLL STANDARD WEIGHT FOIL - Dean Supply