Summary
Highlights
A shiny surface, like aluminum foil, is highly effective at stopping radiant heat by reflecting 95-97% of it back. This prevents heat from soaking into the attic. This material, called a radiant barrier, has low emissivity, meaning it absorbs and radiates very little heat. It's inexpensive, durable, and has no moving parts, offering a permanent solution. The space program uses similar reflective materials to protect spacecraft from solar radiation.
The most crucial aspect of installing a radiant barrier is ensuring it faces an air gap on the shiny side; otherwise, heat conducts straight through. The ideal placement is stapled to the underside of roof rafters, shiny side down, creating an air gap between the foil and the hot roof deck. Perforated foil is recommended to allow moisture vapor to pass through, preventing rot, while still reflecting heat effectively.
The sun heats roof shingles to 160-170°F, turning the roof into a giant radiator. Attics routinely reach 130-150°F, forcing air conditioners to work excessively, leading to high electricity bills. More insulation often doesn't solve this, as it primarily addresses conduction and convection, not the prevalent radiant heat.
Heat travels in three ways: conduction (through touch, like a hot spoon), convection (through moving air or water, like warm air rising), and radiant heat (invisible rays that travel through air until hitting a solid surface, like sunlight warming your face). Traditional insulation is effective against conduction and convection, but almost useless against radiant heat, which is the primary source of attic heat in summer.
Radiant barriers are easy to install in existing homes. For those unable to work on rafters, laying foil over existing attic floor insulation is an easier alternative, though slightly less effective due to potential dust accumulation. The cost is minimal, often less than one month's summer electric bill, and can reduce attic temperatures by 20-30°F, significantly cutting cooling costs.
The widespread focus on 'R-value' insulation, which measures effectiveness against conduction and convection, has led to radiant barriers being overlooked. Radiant barriers have a low R-value, making them seem ineffective by conventional standards. The energy industry profits from selling more insulation and electricity, therefore having little incentive to promote this cheaper, more effective solution for summer heat.
To double the effectiveness of radiant barriers, old barn builders incorporated natural airflow. By leaving openings at the eaves (bottom of the roof) and the ridge (peak), hot air trapped between the foil and the roof deck can escape, pulling in cooler air from below. This creates a constant, natural 'chimney effect' without any power.
The radiant barrier should always be installed with the shiny side facing down into the attic. This single orientation works year-round: in summer, it prevents heat from radiating into the living space, and in winter, it prevents your home's warmth from radiating out through the roof. Its low emissivity works in both directions. Allowing the foil to sag slightly between rafters also deepens the air gap, improving airflow and overall performance.