09/26/2026
Roofers call it the short circuit. Most homeowners have never heard the term.
Air takes the easiest path it can find, so a ridge vent sitting a few feet above an open gable louver will pull from that louver instead of from intake vents twenty or thirty feet down the roof plane. The top ribbon of the attic gets a pleasant breeze while everything below it sits dead still. That stagnant lower zone is where the damage collects: dark staining on the underside of the sheathing, rusted nail points, and eave insulation gone flat and damp. None of it is visible from a ladder at the gutter line, which is why the problem runs for years before anyone opens the hatch.
The manufacturers say this in plain language on their installation sheets. GAF's Cobra instructions and Owens Corning's VentSure literature both tell installers never to combine a ridge vent with another exhaust vent, and that list covers gable louvers, wind turbines, box vents, and powered attic fans. Mixing exhaust types can also undercut the ventilation clause that some shingle warranties depend on. Pick one exhaust type for the whole roof and stay with it.
Sizing is a separate rule, and it is the rule that tricks people into adding vents they never needed. IRC Section R806.2 asks for one square foot of net free vent area for every 150 square feet of attic floor, and that requirement drops to one square foot per 300 when intake and exhaust are balanced. Balanced means 40 to 50 percent of the total sits up at the ridge with the remainder down low at the eaves. A house with a full-length ridge vent and clear eave intake usually clears the number without one extra hole cut anywhere in the roof.
Closing a pair of gable louvers costs about twenty dollars and one hour in the attic. Cut rigid foam board or scrap plywood to match each opening, screw it to the framing from the inside, then seal the perimeter with canned foam so air cannot sneak around the edges. Leave the exterior louver on the wall so the house still looks right from the street. You get a second benefit at no cost, because gable louvers take in wind-driven rain and blowing snow far more easily than a baffled ridge vent does, and that water lands on insulation nobody ever inspects. [410NE]
