What's the Best Way to Vent the Cabin Attic?
Prevent the formation of ice dams and mold with proper venting
Published: August 5, 2014
|Q: What’s the best and most effective way to vent the attic of my cabin in the Upper Peninsula? I originally had a vent in each gable end, but when I added an addition with a vaulted ceiling, the vent was removed.|
– Lonnie, Mich.
A: Adequate attic ventilation is a very important part of maintaining a healthy cabin, but it’s often overlooked. Proper ventilation allows cool outside air into the attic while permitting warm air to exit. Letting hot attic gases escape prevents melting of snow and subsequent ice-dam formation on roofs in northern climes; during hot summer months, inadequate ventilation can cause internal moisture buildup, decay, and mold.
SOFFIT – In a soffit-to-ridge ventilation system, cool air enters through vented soffit panels under the eaves, working its way up to the attic by way of a baffle system.
RIDGE CAP – Warm air in the attic naturally travels to the peak of the roof, where a ridge cap allows heat to escape. The vent is covered with shingle material to prevent moisture from entering the roof.
TURBINE – This type of vent uses wind currents to help draw warm air out of the attic. However, they are typically uninsulated, so unwanted melting around the vent may occur.
Gable vents, which are installed on the gabled side of the cabin, are immune to snow blockages, which is likely the reason that your cabin was originally constructed with them. Now that gable vents are no longer an option, there are a number of alternatives for you to consider.
The soffit-to-ridge vent is one the most effective attic ventilation systems available. Cool air enters through vented soffit panels under the eaves, working its way up to the attic by way of a baffle system. Warm air in the attic naturally travels to the peak of the roof, where a mesh-like material – typically extending most of the length of the roofline – allows heat to escape. The vent mesh is covered with shingle material to prevent moisture from entering the roof.
Typically, the combination of winds at the peak and the flow of escaping warm air keeps ridge vents snow-free. In heavy snowbelt regions like the U.P., however, blockages can still occur, and the insulating layer of snow can quickly cause melting at the shingle-snow interface. You can avoid this by regularly removing the excess snow from your roof with specially designed rakes or shovels. If this is not possible, there are still some other options to consider.
Turbine, or whirlybird, vents use wind currents to help draw warm air out of the attic. However, they usually extend about 1–2 feet above the roof and are subject to the vagaries of wind strength. They are also typically uninsulated, so unwanted melting around the vent may occur.
Power venting systems use a fan, often thermostatically controlled, to force hot air out of the attic. These systems are typically coupled with an inlet system to bring cold air in as hot air is being exhausted. Power venting may sound like a logical solution, but actual results can be spotty, and the associated maintenance and energy costs make them more of a last-resort option.
Finally, consider creating an insulated false chimney near the roof’s peak, a passive system that would extend the vent opening several feet above the roofline. A general rule of thumb is to create 1 square foot of vent per 150–300 square feet of attic space, so you would need to size the false chimney accordingly. Install screening to keep animals out, and add a chimney cap to prevent rain and snow from entering the attic. Enlist the help of an experienced builder or architect if you pursue this route.