Q. DEAR TIM: It's time for a new roof, and I need to know which roof vent to install. 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? I would also like it to be a green choice. -- Leo K., Concord Township, Ohio
A. DEAR LEO: In all the years I built houses, installing roof vents was standard practice, and I routinely installed continuous ridge vents. Never did I stop to test the manufacturers' claims; I 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 three- or four-inch opening at the highest point. Then the roofers install the continuous ridge vent and cover that with shingles. The shingles lap down onto the roof to seal against rain.
This means hot air needs to go up to the vent and then somehow go down past the shingles before it can escape the attic. Anyone who paid attention in physics class knows that hot air rises. Think of a hot-air balloon in the cool evening air: The 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 principle at my home with a stick of incense. On a blistering hot day with no wind, I went into my attic and lit the incense. With a flashlight illuminating the wafting smoke, I set the 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 that the roof vent was not working the way I thought it would. I am sure some hot air escapes through the vent, but I had imagined that it would continuously move like a reverse waterfall. 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 home, I have several types of vents. The static pot vent covers a 12-inch diameter hole in the roof. Hot air 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. There's also 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. It can help to keep your attic cooler if tremendous amounts of air are moving. The movement will exhaust humid air that otherwise might condense on cool or cold surfaces in your attic.
I prefer the turbine roof vent because it can work day or night with the slightest amount of wind. You can't say that about a solar-powered roof vent. Plus, a turbine vent uses no electricity. It's a really green choice.