What is a radiant barrier?

Three installations show up in American houses. Retrofits are stapled to the underside of the roof rafters, shiny face pointed down into the open attic. New construction usually arrives as foil laminated to the roof sheathing, so the decking reads silver from below. The third version, foil laid face up over the attic floor, is the weakest of the three, because settled dust raises its emissivity and kills most of the effect within a few years. Check yours from the attic hatch with a flashlight on a sunny afternoon: rafters that look wrapped in heavy kitchen foil mean you have one.

Reflection is the entire job, and that is what separates it from insulation. Fiberglass and cellulose carry an R-value and slow conduction; foil has almost no R-value and slows radiation. In a hot, sunny climate the barrier knocks roughly 20 to 30 degrees off peak attic temperature, and the U.S. Department of Energy puts the resulting cooling savings at 5 to 10 percent. Move the same product to Minnesota and the payback disappears, since the benefit only shows during hard cooling weather. Two installation mistakes cause real harm. Foil draped over the eaves blocks the intake vents that keep an attic dry, and solid, unperforated foil stapled to rafters in a cold climate can hold moisture against the sheathing. Perforated products let vapor pass, which is why they are the default.

Nothing here needs service, so budget 5 minutes during the annual attic look. Confirm the staples still hold and no sheet has sagged across the soffit openings, verify that blown insulation has not buried a floor-laid barrier, and scan the roof sheathing for dark staining, which points at a ventilation fault rather than a foil fault. Fold the check into your yearly attic insulation inspection. If you find frost on the nails or damp decking in January, chase that first: a frosty or wet attic is a moisture problem, and foil will not fix it.