The shocking truth about ridge vents

There's this theory about attic venting that says since hot air rises that venting should be considered intake when it is near the bottom of the attic space and exhaust when it is near the top. With no wind it actually does happen like that, especially in attic spaces that are very high steep roofs which facilitate vertical flow.  The legal minimum can be met with static vents alone. They allow unwanted attic moisture from collecting in your attic. The purpose of static exhaust vents whether ridge venting, square aluminum mushroom vents or decorative gable wall vents is to meet the legal minimum so the attic wood will not rot. In my personal experience since 1983, having been in excess of 9000 attics during the summer since then; I believe that attics with ridge vents for the exhaust vent are hotter than ones with regular mushroom vents.

Ridge vents were initially invented for houses or rooms that had vaulted ceilings. Without an attic, there is no way to vent every space between the ceiling and the roof where moisture would collect. You would have to put a vent every 16 inches! Ridge vents were a great idea where each cell between the rafters can be vented out by the continuous ridge-line vent. The problem was rain and snow blowing over the top of the roof would infiltrate through into the space and cause leaks and staining along the vaulted ceiling peak.

The solution for that was to make ridge vents with a dense mesh that would stop wind but still allow air to escape. It is a tough challenge especially when you consider the wind whipping over the ridge. The challenge has not been met. While ridge vents allow a little water vapor to leak out in the winter they do not allow enough air out for the purpose of cooling because of their need to restrict wind. People like them because they look better than the "little aluminum boxes" along the ridge. They have become very popular because they look better, not because they work better.

This propensity for ridge vents to stop wind and create restriction to air high speed air flow is why you can actually have both a ridge vent and an attic fan. From a standpoint of cooling there is nothing that can compare to a fan which forcefully moves hot air out while drawing in cooler air from outside. Will air from the ridge go straight to the fan and "short circuit" air-flow? Yes, a little, but that’s the same with any "exhaust" vent. Once the fan comes on any other static vent in the attic "envelope" is now an intake, regardless of whether is high or low in the space. The attic fan's purpose is to cool your attic, not meet legal minimums.

I dream of the day when people will be willing to pay for huge gable wall vents that sense precipitation and close automatically when needed to stop water infiltration. Or, massive ridge vents like a church steeple that actually allow enough hot air out to preclude the need for electric attic fans, but for now attic fans will have to do. They only run when needed, and forcefully maximize air-flow right when you need it, so you don't need massive venting that would change the way houses look as we know them.

One way to deal with short circuit air-flow is to get a bigger attic fan that will create enough suction to absorb the short circuit air and still make enough negative pressure to bring air from the farthest eave vent. As long as there is enough total net free square inches of intake for the fan; it will not draw any air from the rooms of your house. We mount the fan as far as possible from any other "exhaust vents" to create more cross-flow. Sometimes we block mushroom vents that are too close to the fan. Generally, any vent that is more than 6 feet away from the fan is going to get the same amount of suction as any other vent in the attic assuming there is not any restriction to airflow in the attic itself. That is because when a vent is more than 6 feet away, the speed of the airflow is very diminished so Bernoulli's principle of moving fluid being at a lower pressure is negligible. The entire attic is at a negative pressure and every other vent is intake.