Attic ventilation and radiant barrier for Phoenix heat

With summer highs above 110 F and roof surfaces reaching 150 F to 170 F, the space under the deck in an Arizona home can easily top 140 F to 150 F on a July afternoon. Heat soaks into the rafters, the sheathing, the underlayment and any ductwork running through the attic. Ducts in that space are a big deal here, because cold air from the AC picks up heat on its way to the rooms before it ever reaches a vent.

Trapped heat also shortens the life of the roof. Felt and synthetic underlayments break down faster when they're cooked from above by the sun and from below by a stagnant attic. On tile roofs, where the tile itself can outlast the underlayment by decades, a cooler attic helps that underlayment reach the full 20 to 25 years instead of failing early. We look at ventilation on every roof inspection for that reason, not just when a homeowner asks about comfort.

Good attic ventilation is a loop. Cooler air enters low, through soffit vents or low-profile intake vents near the eaves, and hot air leaves high, through ridge vents, off-ridge vents or roof-mounted exhaust. The two sides need to be roughly balanced. Too much exhaust with too little intake and the attic starts pulling air from the house itself, including conditioned air through ceiling gaps and light fixtures. That's a common result when someone adds a powered fan to an attic with blocked soffits.

We start by measuring the attic floor area and counting existing net free vent area at both ends. Then we look for blockages: insulation packed against the soffits, eave vents painted over, or vent screens choked with dust after years of haboobs. Often the fix is opening up intake and adding baffles so insulation stays clear, rather than cutting more holes in the roof. Where exhaust is short, we add vents matched to the roof type, with tile vents that blend into concrete or clay tile and low-profile metal vents on shingles.

Most of the heat a Phoenix attic picks up arrives as radiant energy from the hot underside of the roof deck. Radiant barrier is a reflective foil or foil-faced material that turns much of that energy back instead of letting it reach the insulation and ductwork. It works best with an air gap on at least one side and the shiny face toward open air. It doesn't replace insulation. It cuts the heat insulation has to stop.

There are two common ways to add it. In an existing attic, foil sheets are stapled across the underside of the rafters, leaving the vent path open from soffit to ridge. During a full roof replacement, radiant barrier sheathing, with foil already laminated to the deck panels, can go down in place of standard plywood or OSB. We avoid laying foil directly on top of attic insulation, where dust collects quickly in the Valley and cuts down how well it reflects.

The most common one is the power attic fan installed to solve a hot attic without any added intake. It pulls conditioned air out of the house, runs up the electric bill and can even draw dust in through gaps. A close second is mixing exhaust types on one roof, so a turbine and a ridge vent pull against each other. We also see bathroom and dryer vents ending in the attic, dumping humid air under the deck, when they should run all the way outside.

On tile roofs, we sometimes find vents that were never flashed into the underlayment correctly, which causes leaks every monsoon season. When a vent is leaking, that's a flashing repair first and a ventilation question second. For more on how heat reaches the roof itself, our article on cool roof coatings in Phoenix covers the outside half of the problem.