As a general rule your roof needs 1 square foot of vent area for every 150 square feet of attic space.
Attic vents per square foot.
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.
Add an additional 20 cfm x 1 20 if you have a steep roof and 15 cfm x 1 15 for a dark roof.
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.
Of net free area of exhaust needed at or near the ridge.
6 measure the radius of round vents or take half the diameter of the.
Ridge static attic vents 384 00 minimum sq.
Attic square footage 2 square inches of exhaust and square inches of intake net free area nfa needed.
For the 1 150 attic ventilation requirement simply double the amount of exhaust and intake calculated.
In the article above it says the total vent area for an attic should equal one square foot of vent space for every 150 square feet of attic area.
Nfa is the unobstructed area through which air can pass through a vent usually measured in square inches.
Next multiply the square feet of attic space by 0 7 to get the minimum number of cubic feet of air per minute that the fan should be rated to move.
That number is then divided by 2 so that half 1 300 of the vent area is in the soffit vents and half 1 300 is in the ridge vents which is what is called for in r806 2.
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.
Ventilation manufacturers assign an nfa value to the non motorized vents they make.
There are also gable and roof fans which actively pull the hot air out of the attic.
Attic x 0 7 700 cfm minimum fan rating.
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.
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.
Our roof is 50 feet long so using a standard product we will get 900 square inches of ventilating space.
There are also box or dome shaped vents that go through the roof itself.
Nine hundred square inches equals 6 25 square feet.
So the ridge ventilation has exceeded the minimum requirement.
Enter square footage calculate or enter the square footage of the attic or area to be vented.
