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How Much Attic Ventilation Do I Need?

Local building codes may differ but this is a good rule of thumb method of calculating how much attic ventilation that is needed. Basically, you need 50% of the ventilation to come from the lower soffit or eave vents. The other 50% should come from the upper ventilation, such as roof jacks, ridge vents or gable vents. 

The method of determining how much attic ventilation that is required is fairly simple. For new construction you need one square foot of net free area (NFA) ventilation for every 300 square feet of attic floor. 

Example:�

A 900 square foot attic would be calculated as follows:

900 Divided by 300 = 3 sf net free area needed. Next convert it to inches:

3 X 144 = 432 inches. Finally, divide it by 2 to determine how much you need:

432" Divided by 2 = 216 square inches.

So, you can see for a 900sf attic you would need 216 square inches for the lower ventilation and 216 square inches for the upper ventilation. Remember, this is "net free area". The screens that cover the vents are considered an obstruction and don't calculate into the equation. 

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Why convert to inches?  Be cause most upper and lower vents are sold based on the number of net free inches that they produce. 

See also:

How To Install Roof Vents

How To Install Gable Vents

How To Install Soffit Vents

How To Install Attic Ventilation Baffles (or Rafter Vents)