Roof vents

Published on June 13th, 2013 | by Henri

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Why not combine gable vents and ridge vents?

Q.    You advise not to use a gable vent when you have continuous soffit vents and no obstructions blocking the air flow between the soffit and the ridge vent. I don’t understand.

A.    It’s interesting how often this comes up. I’ll try to clarify that for all those who are confused about it.

Air flows from the soffits to the ridge vent by several means. One is wind and another is thermal dynamics. Heat loss from the living space warms the air in the attic and so does the sun when it hits the roof. Warm air rises and exhausts through the ridge vent causing suction through the soffit vents. This creates what is known as an air wash in every rafter bay, removing some moisture and cooling the attic. The pattern looks like this:

Soffit and Ridge Vents

Air flow with soffit and ridge vents

However, air flow, like water, seeks the easiest path. If there are gable vents as well as a ridge vent, the gable vents will feed the need for replacement air caused by the exhaust through the ridge vent and the soffit vents become considerably less effective.

The other effect is the wind. When it blows on a roof with an unbaffled ridge vent, it stops the exhaust through the vent, and therefore the intake through the soffit vents – attic ventilation stops being effective.
This is why I consistently recommend the use of externally baffled ridge vents. By deflecting the wind over the ridge, they create suction in the attic (the Bernoulli Effect) thus increasing the air flow from the soffit vents.

If there are also gable vents, and the wind hits one of them at any angle that allows penetration, there is some turbulence in the attic, but not effective ventilation. I hope this will answer this question to everyone’ satisfaction.

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