So I was listening to a home improvement show on the radio Saturday when a gentleman called in about a solar powered roof vent suggestion. The host asked some questions and then stated that if you have "whirlybirds" on your roof you should not add any other type of ventilation, especially powered vents. His reasoning was the powered vent would basically draw outside air from the whirlybirds instead of actual attic heated air. He said not to mix vent methods, I currently have 2 of the "birds" and an electric vent (broken) and was going to replace the powered vent with a solar model.
Any roof/ventilation experts here care to chime in?
, in general it’s not recommended to mix different types of exhaust vents because it often creates opposing pressure dynamics inside your attic trapping hot, moist air, thereby defeating the purpose. This is potentially an issue mixing powered with static ridge vents or gable vents with any kind of ridge vents.
Here is some insight on the issue. A natural convection attic ventilation system should be designed to encourage a smooth, balanced, laminar flow of rising air between low intake vents and exhaust vents positioned at the roof peak. Most fall short of the goal for various reasons and result in turbulent or improper airflow that unintentionally hinders or prevents circulation. When that occurs, hot, moist air is trapped in your attic and bad things can happen.
The mixed exhaust vent theory suggests air will unexpectedly enter a windward (upwind) exhaust vent and race along top of the attic until it exits through the nearest leeward downwind exhaust vent. Picture a fast flowing river of air racing along the top of your attic like a raging river. Instead of pulling the lower rising air up through the vent, apparently when the rising air hits the rushing air along the peak it creates a rolling, tumbling disturbance that traps the lower rising air inside the attic.
It’s usually easy to also visualize the gable mixed exhaust vent scenario. Airflow enters the windward side of the static gable vent but instead of exiting at the other gable end the air will exit at the nearest leeward ridge exhaust vent leaving the rest of the air in the attic trapped.
Static gable vents are usually a bad idea and rarely work as intended, especially if you also have ridge vents. Besides the mixed exhaust vent issue, anytime you use powered exhaust vents you should make sure they are not pulling conditioned air into your attic through lights, fans, and vents. Nearly all attics allow conditioned air to leak into the attic.
When possible I prefer to use continuous ridge vents for my own projects with gable roofs. I’ve had good luck using powered vents on some projects with hip roofs but prefer continuous ridge vents if it’s an option.
Keep in mind the biggest failure usually happens because most systems don’t have enough intake vents so always start there when you evaluate your system. Always make sure intakes are not blocked by insulation, bugs, or paint.