Frequently Asked Questions:
Steep Slope Ventilation
General Questions: Why Ventilate and How Vents Work
Q: Is using various exhaust vent types such as roof louvers, wind turbines, gable louvers, and power fans on the same roof with a common attic recommend?
A: We do not recommend mixing two or more types of exhaust vents on the same roof and common attic because it could result in short-circuiting the attic ventilation system. This application inevitably leads to installation of exhaust vents at more than one level on a roof allowing the upper exhaust vent to inadvertently pull air in from lower exhaust vents rather than from the intake vents properly located near or at the eave at the bottom of the roof.
Q: What are the two methods used to ventilate attics?
A: Static and mechanical.
Q: How do static vents work?
A: Static vents rely on convection which is warmer air rising after it enters the bottom of the roof at the eave and exits vents at or near the top of the roof and on outside wind which creates a low pressure or vacuum above the ridge helping to draw warm, moist air out of the attic.
Q: How do mechanical vents work?
A: Mechanical vents use power whether hard wired or solar powered to move air. A power vent with an airflow rate of 1.0 cubic foot per minute per square foot of attic space measured at the attic floor is generally considered to be equivalent to 1 square foot of static venting for every 150 square feet of attic floor space.
Q: What is meant by a balanced ventilation system and why is it so important?
A: A properly designed ventilation system requires balance between the intake and exhaust vents to achieve the desired air flow capacity. In general, the net-free area of intake venting should be equal to or greater than the net-free area of exhaust venting. Net-free area is the total unobstructed area through which air can enter or exit a vent, measured in square inches.
Q: Can there be too much intake ventilation especially if it exceeds the total Net Free Area of exhaust?
A: The exception to the balance rule is at the soffit in passive systems. Since the air that enters at the soffit acts to push out moisture and warm air, having extra soffit ventilation will not create an off-balance system.
Q: What is the pitch (also known as “slope”) of a roof and how does it impact the use of vents?
A: The pitch is the measure of how steep a roof is. If a roof is “4 in 12″, the roof rises four inches for every horizontal run of 12 inches. Vents are designed to operate within a range of minimum to maximum slopes, so make sure that the vents selected are suitable to the slope of your roof.
Q: What are the drawbacks of Turbine Vents aka Whirlybirds?
A: Roof turbines need winds of at least 5 to 6 miles per hour to activate and spin the interior blades, which means that they won’t be effective on days with a slight breeze or no breeze at all.
Q: What are the benefits and drawbacks to gable ventilation?
A: Gable vents provide intake that also functions as an exhaust system. Unlike vertical ventilation, gable vents utilize horizontal cross-ventilation to help keep air moving through the attic space. A gable vent is mostly used with a gable style roof with a vent placed on each side of the home. These vents are less effective on complex roof styles where the cross breeze may be impeded by rafter beams, peaks, valleys, dormers, and other parts of the roof.
Q: What are some of the considerations for use of popular box style or slant back vents for attic exhaust on roofs?
A: Their small size is sometimes a drawback due to exhaust capacity. However, they don’t need to run across the entire peak of the roof, so box vents can be installed strategically in smaller areas that need venting that cannot utilize a ridge vent.
Q: How effective are hardwired (mechanical) vents?
A: They are electric-propelled fans that pull hot air out of an attic space but cause higher electricity costs. Their power is either not powerful enough to make a difference or could actually be pulling cooler air up through the main floors of the home and out of the attic, thereby increasing energy costs by forcing the air conditioning unit to work harder. Vent fans have the potential to reduce measured peak summer attic air temperatures by over 20 degrees. However, the impact over the air conditioning season is modest with well insulated attics. Weaker power vents tend to circulate air rather than expel air. Higher operational costs are a substantial reason why traditional hard-wired systems have transitioned to solar power over the last few years.
Q: Solar powered vents seem like a sensible green alternative. How well do they work?
A: Solar-powered vents remove the electricity costs associated with older hard-wired vents but do not eliminate the downside that come with powered attic vents. The fans are often either too powerful, or not powerful enough. When added on top of a proper vertical ventilation strategy, like a ridge vent exhaust and soffit vent intake, powered vents can have unintended damaging effects. For these reasons, it is best to use more passive, proven methods of exhaust for your roof.