Attic Ventilation

Attic ventilation is an important part of an overall ventilation strategy to make homes more durable and comfortable. Without proper attic ventilation, the home's HVAC equipment must work harder to maintain the indoor comfort level. Utilizing a properly sized and installed powered attic ventilator (PAV) along with accommodations for adequate make-up air, one can ensure that the hot and humid attic air is exhausted and replaced with the fresh outdoor air. The resulting cooler, drier attic will be less likely to suffer mold and moisture damage.

Advantages of Using Powered Attic Ventilators

• Prevent costly roof, structural, or remedial repairs. One of the greatest advantages of ventilating your attic is to preserve the structural integrity of your home. Excess heat and moisture in the attic can lead to a shorter life for roof shingles, and can cause rot damage to roof sheathing and structural members. Mold and mildew can thrive in this hot, humid attic environment. Using PAVs helps mitigate the harmful effects of an excessively hot and humid attic space by actively moving the air through the attic space.
• Lower air-conditioning costs. Keeping your house cool can be a real challenge when your attic is too hot. As the morning sun rises high overhead, the roof takes the full force of the sun's energy. The roof shingles can reach temperatures as high as 150°F! As this heat is transferred through the shingles, it is absorbed into the roof deck and structural members of the home's attic. Heat from the hot ceiling drywall will radiate down into the living space. The effects are even more pronounced if proper insulation and air sealing has not been completed. Lowering the attic temperature with ventilation using PAVs can have a direct correlation to your cooling expenses.
• Prevent the formation of ice dams. Ice dams are destructive to your home, but they can be avoided. Understanding how they are formed is the first step. It starts where heat escapes into the attic, slightly warming an area of the roofing materials. An accumulation of snow can mask these relatively warmer spots. As that under-layer of snow melts, the water from the melted snow then travels under the snow to the eave. The roofing is colder at the eave, since it is not above the living space. At the eave, the colder temperatures freeze the water and the ice continues to accumulate. This ice accumulation creates a dam and the melted snow can no longer escape. The backed-up water can then work its way underneath the shingles and damage the roof structure, insulation, and other building materials. Proper attic ventilation will help even out the roofing temperatures to avoid the damage from this thawing and freezing cycle.

Sizing a Powered Attic Ventilator

In selecting an HVI-Certified Powered Attic Ventilator for your home, choose one with an airflow rating that will exhaust at least 10 times the volume of attic air per hour. For most homes, a rule-of-thumb is to just multiply the total square footage of the attic by 0.7 to find the minimum ventilation rate in cubic feet per minute (CFM). For homes that have dark shingles and/or steep roofs, the multiplier is increased by 15%, to 0.805.

Importance of Adequate Make-up Air

It is important that the air being exhausted from the attic is properly replaced with outside air drawn up through ample soffit vents that have been properly installed with even spacing under the eaves. In an exhaust-based ventilation system, the air that is drawn into a space is called make-up air. A minimum of one square foot of inlet area for every 300 CFM of fan capacity is required for proper fan operation. To calculate the minimum soffit vent inlet area in square inches, divide the airflow rating of the PAV(s) by 300 and multiply the result by 144 (one square foot). This works out to a recommended inlet area in square inches that is roughly half of the fan's rated airflow in CFM.

For the current list of HVI-Certified Powered Attic Ventilators, please check the Certified Products Directory (CPD). Products must appear in the current publication to be considered HVI-Certified. Not listed = Not Certified.