Best Roof Vents for Michigan Attics, Compared

A continuous ridge vent exhausts along the peak, where the warmest, wettest air collects, and draws evenly from the whole length of the attic. Air Vent ranks ridge vents as the best attic exhaust, ahead of power vents, turbines and louvers. The full-width baffled models from GAF, Air Vent, CertainTeed and Lomanco are each rated at 18 square inches of net free area per linear foot.

For Michigan, look for two features. An external baffle deflects wind so it pulls air out of the attic instead of blowing in; Air Vent warns that unbaffled ridge vents can let weather in and let strong wind pass straight through. And a snow filter: GAF's Cobra SnowCountry models carry a Snow Guard filter, Air Vent says ShingleVent II's internal filter helps trap fine snow, and CertainTeed's filtered ridge vent has an internal weather filter against windblown snow.

Rolled ridge vents are a lower-profile option with less airflow per foot: 12 square inches for CertainTeed's Rolled Ridge Vent and Air Vent's Peak Performer, 14.1 to 16.9 for GAF's original Cobra roll depending on the version.

Intake isn't an alternative to a ridge vent. It's the other half of it. GAF calls intake arguably the most important part of attic ventilation, and Air Vent says you cannot have too much of it: extra intake on the downwind side simply switches to exhaust. Too much exhaust is the problem, because the vents then pull make-up air from wherever they can.

Continuous soffit vents are rated at 9 square inches per foot in Air Vent's literature, and CertainTeed says its Triple 3-1/3 in. InvisiVent vinyl soffit gives more than 10 square inches of net free intake area. Houses with little or no overhang can use a roof-edge intake instead: Air Vent's The Edge Vent and CertainTeed's Intake Vent are both rated at 9 square inches per foot, and CertainTeed's needs at least a 3:12 pitch.

Intake only works if it's open. Insulation pushed over the soffits blocks it, which is what rafter baffles are for.

Box vents are fixed, passive, and simple. The catch is that each one vents a single spot, so they need to be spread and sized correctly, and Air Vent puts them in its "good" tier with turbines and gable louvers because they depend on wind direction and can leave hot spots.

Sizes vary. GAF's Master Flow IR65 is rated at 65 square inches of net free area, which GAF says is 30% more than a typical 50-square-inch box vent. Air Vent recommends keeping box vents on the same side of the roof over a common attic; otherwise, depending on the wind, one side can act as intake for the other and draw in weather.

Gable louvers sit in the end walls, so they add no holes in the roof. Air Vent rates them "good" alongside box vents and turbines, because louvers depend on wind direction and vent only one area of the attic.

The rule that matters in a replacement: if the new roof gets a ridge vent, the gable vents should be closed off. Air Vent says left open, they become intake for the ridge vent, which risks weather getting in and poor airflow through the rest of the attic.

Turbines move a lot of air per opening when the wind blows. Air Vent rates its 12-inch turbine at 95 square inches of net free area and its 14-inch at 132, with an adjustable base for 3:12 to 12:12 pitches.

They share the weaknesses of box vents: each one is a single spot, performance depends on the wind, and Air Vent keeps them in its "good" tier. Like box vents, they belong on one side of the roof over a common attic, and never on a roof that also has a ridge vent.

Air Vent ranks power vents above turbines and louvers. We rank them last for Michigan, because the U.S. Department of Energy's Building America Solution Center is blunt about them: attic fans create negative pressure that draws conditioned air out of the house through gaps in the ceiling, can make combustion appliances draft poorly or backdraft, and in winter pull warm, moist air into the attic. DOE says studies show they often cause more overall energy loss than they save and can increase moisture problems in winter, and notes they are not recommended by ENERGY STAR.

If one is used, it needs intake to match. Air Vent sizes a power vent at the attic's square footage times 0.7 for the minimum CFM, then divides the CFM by 300 for the square feet of intake; a 1,500-square-foot attic needs a 1,050 CFM fan and about 504 square inches of intake. GAF's Master Flow roof-mount fans include an adjustable thermostat with an optional humidistat. A power fan should never share an attic with a ridge vent.