Radiant Barrier

Regular insulation slows heat that moves by conduction, but a different path needs a different fix: the radiant heat that pours off the underside of a sun-baked roof deck and beams down onto everything below it. A radiant barrier, a shiny, low-emissivity sheet (most often aluminum foil bonded to kraft paper, plastic film, or roof sheathing), goes in the attic to reflect that heat instead of soaking it up.

In summer the sun heats the roof, and that hot deck radiates heat into the attic. A radiant barrier facing an open air space bounces most of that radiant heat back, so less of it reaches the attic floor, the ceiling, and any ducts running through the attic. The U.S. Department of Energy reports that radiant barriers can cut cooling costs 5% to 10% in a warm, sunny climate. They work best exactly where Arizona homes struggle: hot attics with ductwork in them.

For example, a Scottsdale home with the air handler and ducts in a vented attic loses cooling all July as attic air pushes past 140 degrees. Adding a radiant barrier under the roof rafters drops the radiant load on those ducts, so the air conditioner does not fight a superheated attic as hard. Some homes can even get by with a slightly smaller AC unit.

A radiant barrier is not insulation and has no useful R-value on its own. It does not replace attic insulation; it adds to it. The reflective face has to point at an air gap, and dust on the foil cuts its reflectivity over time, so placement matters. For a tight, efficient roof, many Arizona builders pair a radiant barrier or reflective roof deck with a strong attic R-value rather than choosing one over the other.