Is it necessary to close off gable vents when roof ridge vent added?
suzypq
| Posted in General Discussion on
When it was built, the attic of our 22 year old house was ventilated with gable vents and continuous soffit vents. We replaced the roof recently and added a ridge vent. According to the guy who writes the home improvement column in the local paper, if you add a ridge vent, it’s important to close off the gable vents or else the air pathway through the attic is short-circuited, flowing only from the gable vents to the ridge vent and not ventilating the lower part of the attic. The roofer said he thought this wasn’t necessary and the more ventilation the better. Opinions?
Replies
venting
I would have to agree with the newspaper guy. Left in, the gable vents all but eliminate the purpose of the soffitt vents.
Every attic is different
The newspaper guy is generalizing and I wouldn't jump up and follow his advice. My guess is that the gable vents have been doing a good job of venting your attic, and that closing them is a bigger risk than leaving them open. Is your attic humid? What is the dewpoint? Is there condensation on anything? I would monitor conditions and watch for excess humidity. If there is excess humidity, determine the source and solve it.
Close those gable vents. You spent the money to get good ventilation in your attic by installing soffit and ridge vents. If you leave the gable vents open they short circuit the soffit to ridge system and you've thrown your money away. It is just like putting your A/C return right next to your supply, lots of air flow but your house won't get cooled. I'd put foam board over the inside of the gable vents and seal them well.
The newspaper guy has somewhat of a point about short circuiting the flow, but he has the logistics wrong. Air may indeed enter the ridge and exit a gable vent or vice versa. But this will not impede air from entering the sofit vents and rising up to the top.
If you have continuous sofit vent inlets and a continuous ridge vent outlet, this is the ideal system. The air will flow up each rafter bay and uniformly bathe the underside of the roof deck, which will keep it cool to help prevent ice dams. This even distribution is also ideal for ventilating any vapor that has gotten past the vapor barrier, but is not essential for that vapor removal function. Vapor will find its way to the airflow no matter where that airflow is.
If you have a ridge vent, sofit vents, and gable vents, the gable vents may short circuit the ridge vent outlets, by offering an additional outlet, but they will not short circuit the sofit vent inleets. Air will still enter the sofit vents and be drawn upward by the chimney effect regardless of whether the outlet is gable vents or a ridge vent.
But if there happens to be wind blowing crosswise to the gable vents, it might create a vacuum on them that will pull out some of the air that was heading for the ridge vent. So it could eliminate a portion of that uniform airflow that you want bathing the roof deck. But generally, I don’t think the trading of air outlet function between the ridge vent and the gable vents would make a significant enough difference in the roof temperature to be a concern. Moreover, there may be times when the ridge vent is plugged with snow, and then the gable vents could take over as a backup.
Perhaps a better reason to block off the gable vents is the rule of thumb that there should be a larger total vent area for the intake than for the outlet. If it is the other way around, the venting can pull a vacuum on the attic space and draw air in through leaks from the conditioned space below.
So calculate your total cross-sectional area of the openings for your sofit vents, ridge vents, and gable vents. If the sum total of the area of ridge and gable vents is vastly more (say twice as much, for example) than the area of the sofit vents, then you might want to consider blocking the gable vents just to reduce the total outlet size in relation to the inlet size.
Roofer's wrong
Gable Vents
I have an attic fan and that's why I have gable vents. I also have a roof vent but I agree that the more ventilation you have the better because of condensation. Attics can get pretty warm.
You should also have vents in the eaves. The problem with gable vents is that they can "short-circuit" airflow, causing air to flow through the gable vents rather than from the eave vents. This leaves the edges of the attic poorly ventillated.