Q&A / 

Roof Vent

DEAR TIM: I need to know which roof vent to install on my home as it is time for a new roof. I am considering a continuous ridge vent, but once hot air reaches the peak of the roof it has to go down several inches to escape the attic. This defies physics and doesn't seem logical. What are the roof-venting options I should be considering as many online resources seem to want to sell something? I would also like a green or responsible choice. Leo K., Concord Township, OH

DEAR LEO: You are not the first person to contact me about the mystery of a typical ridge roof vent. Roof-vent installation was a standard practice for all the years I built, and I routinely installed continuous ridge vent on my jobs. Never did I stop to test the manufacturers' claims, thinking they were accurate.

Several years ago, a friend of mine made the same observation you did. He couldn't understand how hot air could go down through a ridge vent to exit a roof. It seems these vents would work well as the carpenters cut 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 the cover that with shingles. The shingles lap down onto the roof to provide the water seal from falling rain.

The continuous ridge vent can be seen on the lower roof. It is under the shingles at the peak that sit above the roof. PHOTO CREDIT: Tim Carter

The continuous ridge vent can be seen on the lower roof. It is under the shingles at the peak that sit above the roof. PHOTO CREDIT: Tim Carter

This means hot air needs to go up to the vent and then someway go down past the shingles before it can escape the attic. Anyone who paid attention in their high-school physics class knows that hot air rises and absolutely doesn't want to go down. Look in the sky on a cool evening and you can see hot air balloons drift as the contained bubble of hot air transports the gondola and passengers up. The balloon comes down when the air cools or the pilot releases hot air from a vent at the top of the balloon.

The same principal, I believe, works with helium balloons. Let one go in a room with a cathedral ceiling, and it will quickly get trapped at the peak of the roof. It takes effort to pull the balloon away from the ceiling.

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 carefully not to set my house on fire. 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. Instead of seeing the smoke get pulled out of the open pathway, it just collected at the peak of the roof. Barely any of the smoke was pulled outdoors.

This experiment of mine was not completely scientific, but it proved to me the roof vent was not working as I thought it would. I am sure some hot air escapes the vent, but my feeling was that there would be a reverse waterfall of hot air readily and continuously moving through the vent. Such was not the case.

You have numerous roof-venting options. There are traditional mushroom or pot vents, powered roof vent fans, a solar-powered roof vent, a turbine vent, etc. There are all sorts of possibilities as well as mixing and matching different types.

On my own home, I have several types. 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 all year. 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.

I prefer the turbine roof vent because it can work day or night with just 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 and environmental choice.

The common mistake many people make is to put in too little roof ventilation. It is impossible to have too much ventilation in your attic. The companies that sell roof turbines have sizing guidelines, but for every 1,000 square feet of attic space I would install two 14-inch-diameter turbine vents.

When you shop for turbine vents, let price be your barometer for quality. The more expensive turbines generally are better made and have better parts. You want the ball bearings to be sealed if possible, and you want the vent to be sturdy.

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 gapping 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.

Column 745

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7 Responses to Roof Vent

  1. Thank you for this very helpful article on roof vents. I will have a new roof installed in the near future and I'm in a quandary as to the type of roof venting I should have. So, just a clarification question: are you saying that ridge vents do not actually work? I have one contractor who wants to install can vents because he says the ridge vents do not work. I have another contractor who says the ridge vent is by far the best method. I'm left in a state of confusion. After reading your article I think the biggest selling point of the ridge vent is probably that it is most pleasing to the eye. Am I correct? Thank you, Tim, in advance for your response.

  2. Just had a ridge vent installed, and the roofer only cut one side of the roof for venting. He did not cut the recomended 1 3/4 inch on either side of the ridge beam. Will this be a problem, and should I have him re-work it to the requirement?

    • David, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  3. Previously my house used two louvered vents one at either end of the main "gable" (for lack of knowing what to call it) (each side of the house under the main part of the roof). These are just openings with screen wire attached to the back of a metal insert with louvers. It's a small house. When I had my house re-roofed, the roofer installed a ridge vent all the way across the top of the roof. I read somewhere that if there is a ridge vent, these other vents should be closed off so the ridge vent will work and pull the hot air out. Now I read where the ridge vent doesn't do the job, so should I leave things as they are...with the side louvered vents and the ridge vent?

  4. Saying that heat will not rise out of a ridge vent because the shingles force air down a bit is like saying the physics of a siphon won't work because some water has to go uphill. Most of the heat is rising as most of the water has to fall.

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