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I am finishing out an above-garage room that is currently attic space. The roof has radiant barrier sheathing (ie foil-lined OSB). I have a few walls that are currently just studs open into the attic. I plan to add OSB on the back of these walls and insulate them along with the outside walls. I went to price the OSB and found I can get the same foil-lined stuff used on the roof, or regular unlined OSB sheathing.

Looking at the specifications section on the Lowes website, there is an X (as opposed to a checkmark) by the "For use with walls". Why is this? Is there any real reason not to use this for the attic-facing walls? If it is OK to use, should the lining face into the room?

This is in southern US. The floor and garage are already insulated, and I plan to do blown fiberglass or cellulose insulation on the walls and ceiling.

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Yes, if you don't have a way for the heat to escape rapidly. Radiant Barriers aren't insulation. So, if the reflected heat can't get away quickly, like in a wall, the Radiant Barrier won't do anything of benefit in your location since you would want it facing the exterior to repel heat.

It really would only be useful if it were installed as your exterior sheathing, like the roof, to reflect heat back out through your siding or finished surface. Up North it's used on insulation, not sheathing, to reflect escaping heat back into the living space during the heating season.

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