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Attic Ventilation Strategies - Are Attic Fans a Good Idea ?

Every homeowner has been told at some point or another about how important ventilation is to your attic, especially by roofing companies and those in the business of selling attic fans. A well ventilated attic helps to avoid problems with moisture and heat buildup. Be it winter or summer, the goal is to keep the ventilated attic as close as possible to the outside temperature.

In the summer time a well ventilated attic helps to keep the attic cooler. On the other hand, a poorly ventilated attic can get extremely hot and impact the comfort of adjacent spaces and result in higher energy bills. Temperatures can soar to over 130 degrees and lessen the life of the roofing above it.

Ventilation can be even more important in the colder winter months. Warm moist air escaping the home through the attic floor can condense on cold surfaces soaking the attic space and rotting the roof deck. Improperly vented bath and kitchen exhaust fans can pump huge amounts of warm moist air in attic spaces. Proper ventilation helps this moisture to escape and dry out any wet surfaces. Ice damming and mold are common problems found in attics with inadequate ventilation.  Ice dams form when the warm air from the house enters the attic and warms the air above. Ineffective air sealing and inadequate insulation of the attic floow allows warm air to enter the attic. As this warm air builds the snow and ice in contact with the roofing begins to melt and run down. As that water hits the colder soffits and gutters it refreezes forming ice dams along the edges of the roof causing damage to gutters and soffits. Good ventilation, in addition to effective air sealing and insulation, can prevent this buildup of heat from creating these kinds of problems.

So what is the best strategy for attic ventilation ? Most experts agree that passive ventilation using high and low vents is the most effective method of venting an attic. You need at least 1 square foot of ventilation for every 300 square feet of attic floor divided evenly between high and low venting.  A continuous ridge vent in conjunction with soffit vents works great (make sure the soffit vents remain clear of insulation). If soffits are not present, gable vents are the next best option. Regardless of the type of vent used, a balanced hi-lo configuration is essential to ensuring proper air flow throughout the space.

And what about those attic fans ? Although attic fans can move a lot of air, we don't recommend installing them for various reasons. Most homes, even those that have lots of insulation have not had their attic floors professionally air sealed to prevent indoor air from reaching the attic. Because of this, attic fans can draw that expensive conditioned air right out of the home through the attic floor. Those equipped with humidistats will fire up in the winter as well and draw warm moist air out of the home through the attic floor resulting in even more attic moisture, not less. One other potentially dangerous aspect of powerful attic fans is that they can create a negative pressure situation around the combustion appliances resulting in dangerous spillage of carbon monoxide and other combustion gases into the home. We have witnessed this phenomenon of attic fan induced combustion air spillage on many occassions.

If you are experiencing problems such as ice dams, hot bedrooms, attic condensation or mold growth, contact us for an evaluation today.



The standard code formula requires 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the soffit, and half along the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area.

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