Mixed-exhaust short-circuiting — why combining ridge with gable, powered, or turbine breaks airflow

When an attic has more than one exhaust type running simultaneously, the exhaust openings tend to compete with each other rather than work together. The airflow path the system was designed for — cool air entering at the soffits, warming as it traverses the attic, exiting at the ridge — is replaced by a shorter path that bypasses most of the attic interior. Three common configurations show this pattern. Ridge vent plus gable vent: outside air enters at the nearest gable and exits at the nearest ridge slot, leaving the attic interior with stagnant air. This is the most common Texas re-roof issue because older homes typically had gable vents that are not always closed when a new ridge vent is added. Ridge vent plus powered ventilator: the powered ventilator pulls strongly enough to draw outside air in through the ridge vent (reverse flow), making the ridge vent functionally a second intake. The intended soffit-to-ridge path collapses, and the powered fan can pull conditioned air through the ceiling assembly when soffit intake is inadequate. Ridge vent plus turbine: similar to ridge plus gable — the turbine and ridge become competing exhausts on the same roof plane, short-circuiting airflow. Across all three configurations the underlying mechanic is the same: airflow takes the path of least resistance, and the path of least resistance is the shortest one. Building-science consensus and major manufacturer install specifications (GAF, Owens Corning, CertainTeed) all direct contractors to close or seal non-ridge exhausts when installing a ridge vent. The accurate framing is: International Residential Code Section R806 sets the NFVA requirement; the "no mixing exhausts" rule comes from manufacturer install specifications and building-science consensus rather than directly from code. Homeowners who notice their re-roof installed a ridge vent without addressing existing gable, turbine, or powered vents typically ask a qualified Texas roofing professional to assess whether those openings should be closed. Mixed-exhaust configurations on older homes were common and often need remediation when re-roofing. [Source: IRC Section R806 attic ventilation; GAF Cobra ridge vent installation guide; Owens Corning VentSure documentation; CertainTeed ventilation install specifications; Air Vent Inc. balanced ventilation documentation]