Deleting gable vents when adding a ridge vent

A homeowner in Oregon's Willamette Valley asked a building science forum a question worth knowing if you're ever re-roofing: when you add a ridge vent (a vent running along the peak of the roof) to a garage or attic that already has gable vents (vents in the triangular wall sections at the roof ends) and soffit vents (vents under the roof overhang), do you need to block off the old gable vents?

An expert responded that the only real downside of leaving all three vent types open at once is that too many "outlet" vents compared to "inlet" vents can depressurize the attic space, which can pull more air leakage up through the ceiling from the rooms below. That matters in a heated, occupied attic, since it can waste energy and drag conditioned air out of your living space. But in a garage that's rarely heated, as in this case, that concern mostly disappears, so there's little reason to seal up the gable vents.

The practical point for anyone planning similar roof work: adding a ridge vent doesn't automatically mean you have to close existing gable vents. The tradeoff mainly applies to insulated, heated spaces where losing warm air through the ceiling costs you money. For an unheated garage or similar space, it's fine to leave the old vents in place alongside the new ridge vent, keeping installation simpler without a meaningful downside.