Tips for choosing the right roof vent

on October 01, 2008 at 3:23 PM, updated October 01, 2008 at 6:11 PM
The continuous ridge vent can be seen on the lower roof. It is under the shingles at the peak that sits above the roof.

Q. It's time for a new roof, and I need to know which roof vent to install on my home. I am considering a continuous ridge vent, but it seems to defy physics and logic. Once hot air reaches the peak of the roof it has to go down several inches to escape the attic. What are the options for roof vents? Many online resources I've checked seem to want to sell something. I would also like a green or responsible choice.

A. You are not the first person to ask me about the mystery of the ridge roof vent. In all the years I built, installing roof vents was a standard practice, and I routinely installed continuous ridge vent on my jobs. Never did I stop to test the manufacturers' claims; I just assumed they were accurate.

Several years ago, a friend made the same observation you did. He couldn't understand how hot air could go down through a ridge vent to exit a roof. Here's how these vents are installed: A carpenter cuts away the top roof sheathing, leaving a 3- or 4-inch opening at the highest point in a roof. But then the roofers install the continuous ridge vent and cover that with shingles. The shingles lap down onto the roof to provide the water seal from falling rain.

This means hot air needs to go up to the vent and then some way go down past the shingles before it can escape the attic. Anyone who paid attention in physics class knows that hot air rises and absolutely doesn't want to go down. Think of a hot-air balloon in the cool evening air: the contained bubble of hot air transports the gondola and passengers upward. The balloon comes down when the air cools or the pilot releases hot air from a vent at the top of the balloon.


This got me to thinking, so I tested the principal at my own home with a stick of incense. On a blistering hot day with no wind outdoors I went into my attic and lit the stick of incense. With a flashlight illuminating the wafting smoke, I set the incense stick a few inches below the peak of the roof where the continuous ridge vent was. The smoke was not pulled out through the open pathway; it just collected at the peak of the roof. Barely any of the smoke seeped outdoors.

This experiment was not completely scientific, but it suggested to me the roof vent was not working as I thought it would. I am sure some hot air escapes the vent, but had imagined hot air would readily and continuously move through the vent like a reverse waterfall of hot air. Such was not the case.

You have numerous roof vent options. There are traditional mushroom or pot vents, powered roof vent fans, solar-powered roof vents, turbine vents, and so forth. There are all sorts of possibilities, which can be mixed and matched.

On my own home, I have several types of vents. The static pot vent just covers a 12-inch-diameter hole in the roof. Hot air just floats out of these vents. I also have two solar-powered roof vents. When the sun shines on the solar collectors, a low-voltage fan spins, pulling out hot air. I then have a sleek turbine vent that has excellent ball bearings. The slightest breeze sets the turbine in motion, sucking hot air from the attic.

Keep in mind that a roof vent is just as important in the winter. In fact, roof ventilation is critical year-round. You need a continuous, or nearly continuous flow of air moving through your attic space. This air movement can help to keep your attic cooler if tremendous amounts of air are moving, and the air movement will exhaust humid air that otherwise might condense on cool or cold surfaces in your attic.




If you live in an area that may be struck by a hurricane, you will probably have to remove the turbine vent if a monster storm is forecast. Sustained winds can rip a turbine vent from a roof, leaving a gaping hole in your roof. Talk with professional roofers who have weathered a hurricane or two. They possibly can tell you the best way to install a temporary patch in the roof that will not be blown off during the storm.