The Federal Housing Administration and most state and local building codes recommend a minimum of 1 square foot (144 sq. in.) of ventilation per 300 square feet of attic floor space. Always check your local building code regulations, as you may need to provide 1 square foot of ventilation per 150 sq. feet of attic space in some instances.
Bear in mind that this is a minimum recommendation. A number of factors specific to your home—geographical location, roof style and pitch, orientation to prevailing winds and weather extremes, and so on—as well as local building codes may require even more attic ventilation.
Here's an example of calculating the minimum attic ventilation required for a home that has a 50 ft. by 32 ft. attic area:
1. Multiply length by width
| 50 ft. x 32 ft. = 1,600 sq. ft. of attic area |
2. Divide the total square footage of the attic area by 300 sq. ft. (or 150 if that is required in your code)
| 1,600 sq. ft. / 300 = 5.3 sq. ft. |
This home requires 5.3 sq. ft. of attic ventilation.
The "Balancing" Act: Use the 50-50 Rule
A well-ventilated attic breathes-in (intake ventilation) and breathes-out (exhaust ventilation) through vents located at the lowest and highest parts of your attic. Properly placed, the correct "balance" of intake and exhaust vents will create a constant flow of air through your attic, cooling it in the summer and preventing moisture build-up in the winter.
As a general rule, attic temperatures should be within 15°F of the outside air. A smaller temperature difference is preferable. Temperature differences in the attic cause more work for air conditioning systems and can also cause extra humidity in the attic.
The most efficient attic ventilation system—and the one recommended by roofing engineers—is a balanced system of 50% intake vents and 50% exhaust vents, known as the 50-50 Rule, which allows the most efficient and constant airflow through the attic. The outside air enters the attic through vents placed in the soffits or undereaves, and exits the attic (by convection) through vents placed at or near the ridgeline of the roof.
The ventilation needed for proper attic ventilation in the example above is 764 square inches. To "balance" the vents for this attic, 50% (382 sq. in.) of the vents must be allocated for intake ventilation and 50% (382 sq. in.) for exhaust ventilation.
When choosing vents for the attic in this example, be sure to "balance" the 764 square inches of NFA by placing 382 sq. in. of intake ventilation at the soffits or undereaves, and 382 sq. in. of exhaust ventilation at or near the roof ridgeline.
Is Over-Ventilation Possible?
Over-ventilation is possible if you exceed the 50/50 rule of having half intake and half exhaust. There are minimum amounts of ventilation required by city codes, usually the 1 sq. ft. of ventilation per 150/300 of attic sq. ft. rule we covered in the sections above. Depending on your situation, exceeding those requirements may only cost more and not actually provide a benefit. However, some houses may need more ventilation than normal, so be sure to consult a General Contractor when deciding on attic ventilation.
