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Gable louvers and ridge vents

Mike Lamb

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Is it even worth mentioning that the two should not be mixed? . . .

I've never seen the slightest evidence that mixing gable and ridge vents is a problem. The air in Oregon attics doesn't seem to have read the textbook that show it how to behave.

Vents can be described as holes in the roof at a given height. Traditionally, we put some holes low and some holes high. Air is only going to move through these holes when there's a difference in pressure between one side of the vent and the other. Setting aside mechanical ventilation, that difference can come from the buoyancy of hot air or from wind blowing outside.

In terms of buoyancy, hot air in an attic doesn't know the difference between a vent near the ridge and a vent high on a gable wall. The air gets hot, it rises, building up pressure, and the air from the high-pressure zone flows out through whatever openings are there. When this air flow out, the attic develops a low-pressure zone near the bottom of the attic and outdoor air flow in there.

The thing that boogers-up this process is wind. When the wind blows - even a very gentle breeze - the pressure it exerts on the vent openings is far greater than the measly pressure developed by buoyant hot air. If the wind blows toward a vent opening, hot air isn't going to go out; it's just going to sit there. In fact, outdoors air is going to blow in. And when that happens, the overall pressure in the attic goes up and air is going to leave the attic through whatever opening has the lowest pressure - that might be vent openings low on the roof on the lee side of the building.

Different combinations of attic ventilation and wind direction create very complex scenarios that can be very hard to predict. Even Joe L doesn't go there in most of his stuff. It's chaotic and it's the reason that you will find two houses with exactly the same design and venting systems behaving differently when they're oriented in different directions to the prevailing wind.

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