Sometimes the hardest part of ordering ventilation products is knowing the difference between the square feet of ventilation needed and how to find the actual net free area (or the amount of air a louver flows) of your vents. Our example here is for when you can use soffit and roof ventilation. If you can not use soffit vents you will need to change your calculations as noted.

First, to calculate the total amount of ventilation your job needs: Multiply the length of the attic by the width in feet to find the attic area, then divide by 300 (Note: use 150 without soffit vents) to find the total square feet of vent space needed. [(length x width of attic in feet) ÷ 300 = total sq. ft. vent area] The volume or height of the attic is not important.

For Example: A 50' x 30' attic would have a total area of 1,500 sq. ft., divided by 300 would mean you need 5 sq. ft. of total ventilation.

Second, you need to know that each type of vent has it's own net free area specification. You will need to know this number, which is usually a percentage of the total area of the louver. For instance, our BL200 extruded aluminum louver has a net free area of 45%. Meaning that a 24"x24" louver would give you 4.0 sq. ft. of total area then multiplied by 0.45 gives you the net free area of 1.8 sq/ft of ventilation, or the amount of air in sq. ft. it will flow.

Industry standards suggests that 60 percent (or 3 square feet) of your total area needed should be in the soffit area and the remaining 40 percent (2 square feet) should be located on the ridge or in the gables.

The vents used will most likely depend on available areas to vent, (gables, ridge, soffit) access and aesthetics. The combinations are many. It might take a little math to figure it all out but in the end you will have a properly ventilated building.