Radiant Barrier Belongs on Rafters

QUESTION: I would like to install a foil radiant barrier in my attic. There are batts of insulation between the ceiling joists with another two inches of blown-in insulation on top of that. I need the foil barrier more to keep the attic cool than for heat, and I would like to lay it on top of the insulation. Can I install it this way?

ANSWER: For readers not familiar with them, a foil-faced radiant barrier reflects radiant heat from the sun away from the attic, reducing the amount of air conditioning necessary to cool the rooms below.

Don’t install the barrier on the existing floor insulation because in most cases it acts as a vapor barrier and causes condensation problems. Instead, the barrier should be attached to the underside of the roof rafters. The joints should not be sealed so moisture accumulating in the cavity between the roof deck and the radiant foil barrier can escape. If only one side of the foil barrier is shiny, install that side face down. This prevents dust from settling on the surface and lowering the barrier’s reflective capability. According to tests, dust accumulation can reduce the barrier’s performance by 50%.

To allow for air circulation behind the barrier, leave a gap at the bottom and top of the rafters. The industry trade group that oversees radiant barriers specifies a three-inch gap at the eaves and a three-inch gap on each side at the ridge (six inches total gap at the ridge). Finally, install a radiant barrier according to the manufacturer’s instructions.