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.
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.
|Attic area in Square Feet||Minimum Recommended Airflow||+15% for dark/steep roofs|
|1,000 square feet||700 CFM||805 CFM|
|2,000 square feet||1,400 CFM||1,610 CFM|
|3,000 square feet||2,100 CFM||2,415 CFM|
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.
|PAV Airflow Rating in CFM||Soffit Vent inlet area in Net Square Inches|
|805 CFM||386 net square inches|
|1,610 CFM||773 net square inches|
|2,415 CFM||1,160 net square inches|
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.