Ventsure 4 foot strip heat moisture ridge vent.
Roof vent square footage.
Calculate or enter the square footage of the attic or area to be vented.
27 lineal feet required.
If there is no vapor barrier double it to one square foot of nfa for every 150 square feet of attic floor space half for intake half for exhaust.
Pick out your soffit vent size and determine the square footage of the vent.
Ventilation manufacturers assign an nfa value to the non motorized vents they make.
See how simple that was.
Divide this by 144 to calculate how many square feet each vent is.
Attic square footage 2 square inches of exhaust and square inches of intake net free area nfa needed.
1 500 square feet 300 5 square feet.
We know we need 5 square feet total of ventilation area.
There are also gable and roof fans which actively pull the hot air out of the attic.
If your attic floor has a vapor barrier you will need one square foot of nfa per every 300 square feet of attic floor area half of that will be intake half for exhaust.
One square foot equals 144 square inches so the standard 16 inch by 8 inch soffit vent is 0 89 square feet.
For example a 6 by 12 inch soffit vent has 72 square inches which when divided by 144 results in a 0 5 square foot opening.
The calculator cannot account for complex shapes based on a measurement of square footage alone.
Proper attic ventilation consists of a balance between air intake at your eaves soffits or fascias and air exhaust at or near your roof ridge.
Federal housing authority recommends a minimum of at least 1 square foot of attic ventilation evenly split between intake and exhaust for every 300 square feet of attic floor space.
Although more vent is generally better replacing a turbine vent with a standard flat vent is acceptable as long the vent is big enough for the square footage of the attic space.
In cases where a roof has a complex shape such as in the image to the right measuring the dimensions and areas of each part of the roof to calculate total area will result in a more accurate measurement of area.
Thus if your attic is 450 square feet you need roof vents equaling 3 square feet.
Nfa is the unobstructed area through which air can pass through a vent usually measured in square inches.
For example if you use 16 inch by 8 inch soffit vents each vent is 128 square inches.
Flat roof vents have no moving parts to break or squeak but don t vent as much air as wind driven vents.
Based upon just about everyone s recommendations 60 percent or 3 square feet of this should be in the soffit area and the remaining 40 percent 2 square feet should be located at the ridge of the roof.
