How Much Ventilation Does My Attic Need?
In order for any type of attic ventilation to function properly, the air that is exhausted through the attic vent must be replaced by cooler air from outside. This is especially important when using a powered ventilator because the air turnover rate in your attic will be many times greater than that of passive ventilation methods.
The Home Ventilating Institute (www.hvi.org) recommends that powered attic ventilators provide at minimum 10 attic air volume changes per hour. To calculate the recommend amount of ventilation for your home, simply multiply the total square footage of your attic space by 0.70 as indicated in the table below, or use the Ventilation Requirements Calculator for a more accurate determination of ventilation needs. For particularly dark (black shingles) or steep roofs, HVI recommends multiplying by a factor of 0.80 to calculate your ventilation requirements. See the chart below for examples:
Additionally, HVI recommends a minimum air inlet area of 144 square inches per every 300 CFM of ventilator capacity. For Attic Breeze vents, we choose to exceed this standard and recommend a minimum outside air inlet area of more than twice that of HVI, or about 1600 square inches per a vent unit. We have found that additional air inlet area provides for better performance and reduces the risk of attic depressurization. Our outside air inlet area recommendation translates to approximately 12-13 standard size (16"x8") soffit vents per each solar attic vent.
If you don't have soffits on your home, don't worry. Installing soffits under the eaves of your home is easy and typically inexpensive (see our Installation section for more details). Also, when you have finished determining the amount of ventilation your home needs, don't forget to check out our Estimated Savings calculator to find out how much money Attic Breeze can help you save!