Select hard-wired over photo voltaic attic fan
Q: I covered my two-story stucco house from the 1930s in San Mateo. It currently has two layers – composition shingles over cedar shingles – that are to be removed. Half an inch sheet of plywood and new composite clapboard are installed. The roof has a lot of reveal openings and I add eyebrow openings. I am thinking of installing a roof fan. I’d love to hear your thoughts on a hard-wired roof fan versus a solar-powered roof fan, or am I going overboard in testing a roof fan? The electrical connection is no problem for the hardwired device.
A: The simple answer is to go with the hardwired version. While we got some good reviews about the solar powered models, they have drawbacks.
Solar powered roof fans require a small solar panel (usually 10 or 20 watts) to power a DC motor. The fans are equipped with inlet openings to ensure powerful ventilation without electrical operating costs. Most vents are mounted high on the roof near the ridge and are combined with soffit openings on the roof overhang or gable openings on the building walls near the top of the roof to ensure a balanced flow of inlet and outlet air.
The plus is that they work for free, take advantage of the sun’s rays, vent hot air and don’t need a separate power source. The negative is that they only work when sunlight hits the solar panel. When a cloud passes and blocks the sun, the fan will stop. Worse, when the sun moves through the sky, contact with the solar panel is eventually lost and the fan stops. At best, you get part-time cooling.
The hard-wired version is thermostatically controlled to activate when the attic reaches a certain temperature. You use electricity from the grid, but the consumption is minimal. The bottom line is that regardless of the whims of the sun and cloud movement, you can deflate hot air when you have to.
The perfect time to improve roof ventilation is to replace the roofing. The base is exposed so that more or different ventilation slots can be installed. One of the basic laws of thermodynamics is that hot air rises through the convection process. Attics are ventilated when heated air in the attic exits the ventilation slots at the gable ends or the roof edge and is replaced by the relatively cooler outside air that enters through the reveal openings. The roof air temperature tries to equalize with the outside temperature.
They write that you have adequate soffit openings and that you are adding “eyebrow” vents near the ridge line. We would like to suggest an alternative. Forego the eyebrows and have the roofer install ridge vent along the entire length of the ridge line.
Ridge vents are installed by cutting back the roofing membrane a few inches on each side, nailing a long plastic vent over the opening, and covering the vent with composite roofing material. The result is a continuous vent that sucks hot attic air and is replaced with cooler outside air drawn in through the reveal openings.
If you do this, you can forego installing a roof fan of any kind. Let convection do the work. No need to pump out hot air. And if you’re a “belt and suspender” thing, you can install a hard-wired loft fan for the occasional super hot day in San Mateo.