Mixing ridge, gable and powered attic vents.

Installing and using a powered attic roof ventilator to ventilate an attic space is often regarded as poor practice by some roofing contractors.

Their reasoning is the same as that when they advise to close the gable end vents when installing ridge vents. The powered attic fan will “short circuit” the ventilation system by drawing air from the closest source of air, which would be the ridge vent. They assume that because the ridge vent is closer, it is also the path of least resistance for airflow. And they assume that the force of the attic fans’ volume is great enough to pull not only air, but outside weather elements such as rain, snow and debris into the attic.

First, the ridge vent is not the path of least resistance. It is usually both filtered with some type of mesh or fabric, and capped, to protect from rain, insects and debris infiltration.

Second, the force of the attic fans’ suction is greatly diminished immediately below the fans housing. At the fans’ housing, the airflow may be 1100cfm(cubic feet / minute), but outside the housing that number is reduced by the size of the attic.

Attic fan airspeed 1 to 3 feet away from the fan housing.

Because the fan is drawing air from the whole of the attic space, unrestricted attic space with soffit vents large enough to handle the fans exhaust capacity, the air flow speed is reduced according to the area of the attic.

The important part is to ensure that the source of makeup air is equal or greater to the volume of the air being pulled by the attic vent. If the soffit vents are not large enough, air will be pulled from the living space and any other vents such as the gable or ridge, depending on their resistance to airflow.

The video – Attic Ventilation with Powered Ventilator

The same house model as previous videos on ventilation pages. A simulated power vent hole is cut into the roof just below the ridge vent at the center of the roof. The ridge vent is cut and covered with a thin filter but is not capped. A small household vacuum is used for the source of exhaust.

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