It's one of the most common questions we get: "A solar attic fan runs on sunlight, so what happens when there isn't any?" The sun sets every night, clouds roll in, and winter days are short. Here's an honest look at what a solar attic fan does in each case.
How a solar attic fan gets its power
A solar attic fan has a small solar panel wired straight to the fan motor. When sunlight hits the panel, the fan spins, and more sun means more speed. There's no plug and no wiring to your electrical panel. The fan follows the sun: it runs hardest at midday, when your roof soaks up the most heat, and slows as the light fades.
Do solar attic fans work at night?
No, a solar attic fan doesn't spin at night. With no sunlight on the panel, there's no power to the motor.
But "not spinning" doesn't mean "not working." The fan sits in an open vent opening near the top of your roof. Our product pages for the Solar Mega RoofBlaster and the Solar RoofBlaster say each one boosts "the effectiveness of what would otherwise be a passive vent while also acting as a passive vent anytime the sun is not out."
At night, your attic goes back to the way passive vents have always worked: cooler air enters low at the soffits, and warm air rises and leaves high. The fan just stops adding its push until morning.
Do solar attic fans work on cloudy days?
They work, but more slowly. Clouds cut the amount of sunlight reaching the panel. The U.S. Department of Energy explains in its Solar Radiation Basics page that "Atmospheric conditions can reduce direct beam solar radiation by 10% on clear, dry days and by 100% during thick, cloudy days."
Some light still gets through as scattered, or diffuse, light. So on a thin overcast day, the fan may keep turning at a lower speed. Under thick, dark clouds, it may slow way down or stop and act as a passive vent until the sky clears.
Do solar attic fans run in the winter?
Yes, whenever the sun is shining on the panel. Winter just gives them less sun to work with. The same DOE page notes that "The sun's rays are far more slanted during the shorter days of the winter months," and that Denver "receives nearly three times more solar energy in June than they do in December."
So expect shorter run times and lower speeds in December than in June. Luckily, the fan's winter job is different from its summer job.
Why winter ventilation still matters
In summer, attic ventilation is mostly about heat. In winter, it's mostly about moisture. Warm, humid air from your living space leaks up into the attic through ceiling gaps, light fixtures and the attic hatch. Then it meets the coldest surface up there: the underside of the roof deck.
Building scientist Joseph Lstiburek puts it this way in BSD-102: Understanding Attic Ventilation: "In cold climates, the primary purpose of attic or roof ventilation is to maintain a cold roof temperature to control ice dams created by melting snow, and to vent moisture that moves from the conditioned space to the attic."
Shingle roofs get colder than you might think on clear nights. The same digest notes that "Membrane roofs and shingle roofs can experience night sky cooling that can depress roof deck temperatures significantly below ambient air temperatures." That cold deck is where condensation forms, the same physics we covered in Container Rain. Left alone, that dampness is what mold and wood rot need, which we wrote about in Black Mold in the Attic.
The roof consultants at IIBEC sum up both seasons in their Attic Ventilation 101 article: "Attic ventilation serves two main functions: to lower attic temperatures and to remove excess moisture."
On sunny winter days, a solar fan helps move that moist air out.
Should you close attic vents in winter?
It's a common search, and the usual answer is no. Building codes require attic ventilation year-round. IIBEC quotes IRC R806.1, which says enclosed attics "shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow." IRC R806.2 then sets the minimum vent area at "1/150 of the area of the vented space."
Covering vents can trap the moisture your attic needs to get rid of. If drafts are the worry, seal ceiling air leaks and add insulation instead.
Balance still comes first
Day or night, summer or winter, a solar fan only works as well as the intake feeding it. Keep these in mind:
- Intake low, exhaust high. Soffit or eave vents bring air in, and the fan and roof vents let it out.
- Bigger is not better. An oversized fan with too little intake will pull air from your ridge vent, a gable or the house below.
- Size the fan to the space. With proper intake, the Mega RoofBlaster does about one air exchange every 10.4 minutes in a 1,000 cubic foot space, and the standard RoofBlaster about one every 28 minutes. Our post What Size Solar Attic Fan Do I Need? walks through the math.
Being honest about the limits
- No sun, no spin. A solar fan doesn't run at night, and it slows or stops under heavy clouds or when snow covers the panel.
- Winter output is lower. Short days and a low sun mean less power. Don't expect June performance in December.
- A fan doesn't stop air leaks. If warm, moist air is pouring into your attic through ceiling gaps, ventilation helps, but sealing those leaks and fixing insulation matter too.
- It doesn't fix existing damage. Wet insulation, moldy sheathing or rotted wood need to be repaired by a pro. Ventilation helps keep things drier going forward.
- Every roof is different. If you're not sure your intake is adequate, have a roofer check it before adding exhaust.
The bottom line
Do solar attic fans work at night, on cloudy days and in winter? They spin when the sun gives them power and rest as passive vents when it doesn't. Paired with balanced intake, they help your attic deal with summer heat and winter moisture alike.
As Daniel Rheaume of Solar Blaster puts it, "Our products move air, which is a natural cleanser. Clothesline fresh."
Questions about your attic? Call us at (480) 747-7097 or email sales@solarblasterfans.com.
