Proper attic ventilation is calculated by dividing the total square footage of the attic by the Total Net Free Ventilating Area value. The Federal Housing Administration recommends a TFNVA value from 150 to 300.
The use of the TFNVA value of 150 provides the maximum ventilation of 1 square foot of ventilation to 150 square feet of attic space. Stricter building codes result in more airtight construction that requires greater ventilation capacity. Older, less energy-efficient buildings can use a 1:300 ventilation ratio.
The result of a ventilation ratio calculation provides the recommended ventilation in square feet. Divide the result by 12 to convert the result to square inches to simplify vent product evaluation and selection.
The ratio of intake to exhaust vents is important to providing sufficient ventilation. Intake vents in the soffit, fascia and eaves should make up 50 to 60 percent of the installed ventilation area to ensure adequate air exchange. Exhaust vents near the roof ridge are used for the remainder of the necessary venting.
Proper attic ventilation removes heat in summer and reduces moisture in winter. Careful consideration and planning regarding attic ventilation reduces energy consumption and prevents structural damage such as mold growth, roof leaks and gutter damage.