Radiant barrier: worth it on the sunny side of the Bay

What a radiant barrier does

On a summer afternoon the sun heats your roof shingles, and the roof deck under them gets hot too. A hot surface sends heat out as radiation, the same way you can feel a campfire on your face from a few feet away. The underside of the roof deck radiates heat straight down onto whatever is below it: the insulation on the attic floor, and any ducts running through the attic.

A radiant barrier is a reflective, low-emissivity surface, usually aluminum foil on a backing, fixed under the roof deck. A shiny foil surface gives off very little radiant heat, so much less of the roof's heat reaches the attic floor and the ducts. Two details matter:

• It needs an air space. A radiant barrier works because it faces open air. Pressed directly against something, it simply conducts heat like any thin sheet of metal.
• It isn't insulation. It doesn't have a useful R-value of its own and it doesn't slow heat moving through the attic floor in winter. It's an add-on to a properly insulated attic, not a replacement.

Fog side or sun side?

The Bay Area has four climate zones in the cities we serve, and they behave very differently on a July afternoon:

• Climate zone 3, the fog side. San Francisco, Daly City, Oakland, Berkeley, Alameda, San Mateo, Redwood City, Fremont, Hayward and the rest of the cities along the Bay. Marine air keeps many summer afternoons mild, and the roof never gets as hot as it does inland. A radiant barrier here is a low priority.
• Climate zone 4, the Santa Clara Valley. San Jose, Santa Clara, Sunnyvale, Mountain View, Palo Alto, Cupertino and Milpitas get far more sun and heat. Upstairs rooms under the roof feel it.
• Climate zone 12, inland East Bay. Concord, Walnut Creek, Dublin, Pleasanton and Livermore get some of the hottest afternoons in the region. This is radiant barrier country.
• Climate zone 2, the North Bay. Napa and Santa Rosa have hot summer days, and parts of Marin and Sonoma sit in different zones depending on the address.

Within any zone, a few things make a radiant barrier more worthwhile: ducts and an air handler in the attic, a dark roof, a large west-facing roof slope, and rooms with sloped ceilings tucked right under the roof. If your ducts carry cold air through an attic that's far hotter than the outdoors all afternoon, keeping that attic cooler matters more than it would in a house with its ducts under the floor.

Signs a radiant barrier could help

• Ducts or the furnace live in the attic. The air in them picks up heat on the way to your rooms, and a cooler attic is the fix that reaches them.
• A dark roof on a big west- or south-facing slope, with little shade from trees.
• The attic floor is already well insulated and the house still runs hot in summer. That's when a radiant barrier stops being an extra and starts being the next step.

If the question is simply why the bedrooms are hot, start with our page on a too-hot upstairs; the attic floor usually comes first.

Radiant barrier or more insulation first?

Insulation first, in almost every Bay Area attic. A radiant barrier only deals with heat radiating down from the roof deck on sunny afternoons. Insulation on the attic floor slows heat in both directions, summer and winter, and on the fog side, where the heating season is long, the winter half matters most. So the order is: seal the attic floor, bring the insulation up to the code target (see blown-in insulation), then ask whether a radiant barrier still earns its place. In the hot inland zones, with ducts in the attic, it often does. On the fog side, rarely.

Reroofing? Read this first

If you're replacing more than half of a steep-sloped roof in climate zone 4 or 12, the 2025 Energy Code brings in cool-roof rules, and an attic radiant barrier that meets the code's specification is one listed exception. The other exceptions, and when to do the attic relative to the roof, are in our guide to attic insulation and a roof replacement.

Foil or spray-on?

There are two main products:

• Foil radiant barrier, stapled to the underside of the rafters with the reflective side facing down into the attic. Perforated foil lets moisture pass. It's the more common choice and usually the less expensive one.
• Spray-on radiant barrier, a low-emissivity coating sprayed onto the underside of the roof deck. It's quicker to apply around crowded framing and nails, and it costs more per square foot.

Each product's data sheet lists its emissivity, which is the number that says how well it works.

How we do it

1. Check the attic floor first. If the insulation is thin or the attic floor is leaky, those come first. A radiant barrier over an R-11 attic floor is the wrong order.
2. Check the ventilation. Soffit vents must be clear and the attic needs a path for hot air to leave. If it doesn't have one, see attic ventilation.
3. Install. Foil goes up between or across the rafters, seams overlapped, stopping short of ridge and roof vents so they keep working, with an air space below it.
4. Keep the hatch path clear. The barrier goes up without blocking access to anything that needs service, like the furnace or the duct connections. If those ducts need work, see ductwork.