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Close off gable vents?

Q:

As built, the attic of our 22-year-old house was ventilated with gable vents and 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, you should close off the gable vents, or 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. Who is right?


No short-circuiting here. Even with the gable vents open, air continues to enter the soffit vents and to move upward due to the stack effect. While some of it exits through the gable vents and some exits through the ridge vent, these outlets are not in competition with each other. Click to enlarge image

No short-circuiting here. Even with the gable vents open, air continues to enter the soffit vents and to move upward due to the stack effect. While some of it exits through the gable vents and some exits through the ridge vent, these outlets are not in competition with each other.





A: Bruce Harley, technical director of Conservation Services Group in Westborough, Mass., responds: Everyone has an opinion about this stuff, and most folks overthink it. Here’s what you need to know.

The newspaper guy is wrong; your roofer is right. There is no need to block the gable vents unless they are prone to collecting windblown rain and snow. (This would not be related to adding the ridge vent; that would have already been true before the roof work.)

Although unvented roof assemblies work if they are built properly, attic venting is a good thing for most houses, and having too much venting will not harm the structure. Airflow promotes drying, whether the water comes from inside (condensation in the winter) or outside (roofing or flashing leaks).

For any attic venting to work properly and for the house to work properly, you must thoroughly air-seal and insulate the attic floor. If there are air leaks between the attic and the living space, attic vents become outlets for conditioned air driven by the stack effect. If you don’t seal the leaks in the attic floor, the heat loss can melt snow (causing ice dams) and create condensation and moisture problems.

It can even increase the risk of combustion backdrafting, especially for a fireplace with an exterior chimney. Venting is still better than no venting, but for it to work optimally, you must seal the leaks first and insulate well.

Once leaks are sealed and the attic is insulated, the attic is an isolated space. The main force that now drives venting is solar radiation on the roof, heating attic air. Adding exhaust vents up high on a roof can help to accentuate the inflow of air through the soffit vents. It doesn’t matter if these exhaust vents are ridge vents, gable vents, roof vents, or some combination. They don’t fight with each other, and the added vents just raise the neutral pressure plane in the attic to accentuate the lower vents’ function as inlets.

If windblown rain or snow is frequently entering the gable vents, close them. Don’t worry about an airtight seal; just nail a board over the opening from the inside.
To summarize: Seal air leaks in your attic floor so that roof venting doesn’t suck conditioned air into the attic and cause moisture problems. Adding a ridge vent is a fine idea, and don’t worry about it competing with the gable vents. If the gable vents aren’t letting in rain or snow, leave them alone.


From Fine Homebuilding 225, pp. 78-80 January 19, 2012