A ridge vent runs along the very top of the roof, right where the hottest attic air collects. So it's natural to ask: can you have a ridge vent and an attic fan on the same roof? The short answer is that a big powered fan and a ridge vent usually fight each other. Here's why, what the vent makers say, and where a small solar fan fits in.
What a ridge vent is built to do
A ridge vent is passive. Cool air comes in through intake vents at the eaves, warms up, rises and leaves at the ridge. Wind blowing across the top of the roof helps pull it out. That's the same "cool air in low, hot air out high" idea we covered in What a Hot Air Balloon Teaches Us About Attic Ventilation.
The code sets the minimum. IRC R806.2 reads, "The minimum net free ventilating area shall be 1 / 150 of the area of the vented space." It allows 1/300 when the vents are balanced, with part of the vent area up high and the rest down low.
Why a ridge vent and a power fan fight
Attic vent maker Air Vent answers this exact question in Can I install a power fan if I have a ridge vent already on my roof? Its answer:
- "Mixing a power vent with a ridge vent can short-circuit the attic ventilation system just as a gable vent can as discussed above."
- "This happens because air follows the path of least resistance."
- "When the power vent turns on, it can pull air from the ridge vent, which could lead to weather infiltration and unbalanced airflow along the underside of the roof deck."
- And when the fan shuts off, it becomes just another hole in the roof: "the ridge vent pulls its intake air from the power fan."
In plain English: instead of pulling fresh air from the eaves and sweeping it across the whole roof deck, the fan grabs the closest, easiest air, which is often coming in through the ridge vent a few feet away. The air takes a shortcut, and the lower part of the attic gets skipped.
It isn't just fans: don't mix exhaust types
The same problem shows up with any mix of exhaust vents. In Is mixing exhaust vents of any combination OK?, Air Vent puts it bluntly: "It is never OK to mix two different types of attic exhaust vents."
It explains, "There are generally five groups of exhaust vents: ridge vents, wind turbines, roof louvers, gable louvers, and power vents. Each group uses the wind differently." And the result of mixing them: "Short-circuiting the ventilation system will result in areas of the attic being bypassed (skipped)."
Power fan makers say it too. The TopShield Power Vent 2000 instructions list these rules:
- "Install ONLY ONE TYPE of Exhaust Ventilation within a common attic area."
- "Exhaust Vents pull air from the easiest intake source."
- "Installation of exhaust vents at more than one level on a roof allows the upper exhaust vent to pull air in from lower exhaust vents rather than from the intake vents."
Intake is still the real limit
Even with one type of exhaust, the attic can only push out as much air as it pulls in. GAF's Cobra plastic ridge vent installation instructions say, "In no case should the amount of exhaust ventilation exceed the amount of intake ventilation."
This is the heart of "bigger is not better." A high-CFM fan in an attic that's short on intake will pull air from wherever it can: the ridge vent, gables, chimneys or the house below. We explain it in Why Solar Blaster Bets on Balance, Not Brute Force. If you're not sure your eaves are pulling their weight, How Many Soffit Vents Do I Need? walks through the math.
Solar Blaster's approach: assist the ridge, don't compete with it
A ridge vent's weakness is that it's passive. On a hot, still afternoon, it depends on rising warm air alone. Instead of adding a big fan somewhere else on the roof, Solar Blaster puts a small solar fan right at the ridge:
- The Solar RidgeBlaster is made for shingled or metal-roofed homes, garages and sheds that have ridge venting. It has three fans that turn on one at a time as the sun gets stronger, and it still acts as a passive vent when the sun isn't out.
- The Solar Metal RidgeBlaster is a smaller version with a 4.5W solar panel and one fan, a good fit for sheds and garages with metal roofs. It mounts right inside the ridge vent opening, under the ridge vent cover.
The exhaust stays at the ridge, at the highest point, instead of adding a second exhaust at a lower spot on the roof. They run hardest when the sun is strongest and assist the passive system, intake low and exhaust high, instead of overpowering it.
Being honest about the limits
- Read your ridge vent's instructions. Some makers rule out any fan. GAF's Cobra instructions say, "Do not use with gable-end or roof louvers, wind turbines, solar or powered vents, or other exhaust vents." If your roof warranty depends on following them, talk to your roofer before adding anything.
- No intake, no airflow. If your eaves are blocked or short on vents, fix intake first. No fan can make up for it.
- Already mixed? If your roof has a ridge vent plus box vents, turbines or gable vents, ask a roofer which one should stay.
- A fan doesn't fix a roof. Leaks, damaged shingles and worn flashing are a job for a roofer.
- Air leaks come first. If house air leaks into the attic around lights, pipes or the hatch, seal those gaps.
- Your building official has the final say on how much vent area your roof needs.
The bottom line
A ridge vent and a big powered attic fan usually work against each other, and the vent makers agree. Keep one type of exhaust, make sure intake down low matches exhaust up high, and if you want help on still, hot days, assist the ridge vent you already have instead of competing with it.
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.
