Q: I am having my two-story, 1930s stucco house in San Mateo, Calif., re-roofed. It currently has two layers — composition shingles over cedar shingles — that are going to be removed. One-half-inch plywood sheeting and new composition shingles will be installed. The roof has plenty of soffit vents and I am adding eyebrow vents. I am considering installing an attic fan.

I would like to know your thoughts on a hard-wired attic fan versus a solar-powered attic fan, or am I going overboard in considering one? The electrical connection is not an issue for the hard-wired unit.

A: The straightforward answer is to go with the hard-wired version. Although we’ve gotten some good reports on the solar-powered models, they have drawbacks.

Q: I am having my two-story, 1930s stucco house in San Mateo, Calif., re-roofed. It currently has two layers — composition shingles over cedar shingles — that are going to be removed. One-half-inch plywood sheeting and new composition shingles will be installed. The roof has plenty of soffit vents and I am adding eyebrow vents. I am considering installing an attic fan.

I would like to know your thoughts on a hard-wired attic fan vs. a solar-powered attic fan, or am I going overboard in considering one? The electrical connection is not an issue for the hard-wired unit.

A: The straightforward answer is to go with the hard-wired version. Although we’ve gotten some good reports on the solar-powered models, they have drawbacks.

Solar-powered attic fans rely on a small solar panel (typically 10- or 20-watt) to power a DC (direct current) motor. The fans are installed with intake vents to provide high-capacity-powered ventilation without electric operating costs.

Most vents are mounted high on the roof near the ridge and are combined with soffit vents located at the roof overhang or gable vents located on the building walls near the roof peak for balanced intake and exhaust airstreams.

The pluses are that they work for free, using the sun’s rays; they exhaust hot air; and they don’t require a separate power source. The negative is they work only when sunlight hits the solar panel. If a cloud drifts by, blocking the sun, the fan stops. Worse, as the sun moves through the sky, eventually contact with the solar panel is lost and the fan stops. At best you’ll get part-time cooling.

The hard-wired version is thermostatically controlled to kick on when the attic reaches a certain temperature. True, it does use electricity from the grid, but usage is minimal. The bottom line is that you’ll exhaust hot air when you need to, regardless of the vagaries of sun and cloud movement.

The perfect time to upgrade attic ventilation is when replacing the roof covering. Underlayment is exposed, allowing for installation of more or different vents. One of the basic laws of thermodynamics is that hot air rises by the process of convection.

Attics are ventilated when heated air inside the attic is exhausted from vents on the gable ends or at the roof’s ridgeline and replaced by the relatively cooler outside air entering through the soffit vents. Attic air temperature will seek to balance with the outside temperature.

You write that you have adequate soffit vents and that you are adding "eyebrow" vents near the ridgeline. We’d like to suggest an alternative. Forgo the eyebrows and have the roofer install a ridge vent along the entire length of the ridgeline.

Ridge vents are installed by cutting the roof sheeting back a few inches on each side, nailing a long plastic vent over the opening, and covering the vent with composition roofing material. The result is a continuous vent exhausting hot attic air that is replaced with cooler outside air drawn in through the soffit vents.

Do this and you may forgo installing an attic fan of any kind. Let convection do the work. No need to pump hot air out. And, if you’re a "belt and suspenders" person, you can install a hard-wired attic fan for the occasional super-hot day in San Mateo.

***

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