There should also be a 50 50 balance of attic venting and roof venting.
Attic ventilation formula.
Remember the minimum area requirement when using a continuous system is a ratio of 1 300.
Exception 1 allows you to only ventilate 1 300 th of the attic area if 50 to 80 of the ventilation occurs at least 3 above eave or cornice vents.
Calculation formula there is a generally agreed upon formula that you can apply for balanced attic ventilation known as the 1 150 rule.
Calculate or enter the square footage of the attic or area to be vented.
The example below uses this exception and the calculation is based on a 100 x120 building with a hip roof and 2 overhangs on all sides.
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.
Ok so now we know the square footage of the attic area to be ventilated.
Attic area including eave soffit 104 x 124.
Now let s figure out how much ventilation space we need.
Attic ventilation calculations made easy 84 24 3 7 2 212 768 56 x 84 56 x 39 3 2 2 120 0 3 8 5 3 3.
Nfa is the unobstructed area through which air can pass through a vent usually measured in square inches.
Tips for assessing your needs.
If your roof is 1200 sq ft soffit floor space you will need 4 sq feet of roof ventilation.
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.
According to the national roofing contractors association nrca a power vent with an airflow rate of 1 0 cubic foot per minute per square foot of attic space measured at the attic floor is generally considered to be equivalent to a 1 150 ventilation ratio.
To achieve a balanced system half that amount is intake and the other half is exhaust so each should be 3 3 square feet or 475 square inches.
According to their requirements you should have 1 square foot of attic ventilation per 300 sq ft of attic floor space.
Of attic space you need 1 sq.
It states that for every 150 sq.
Assembly ventilation coalition ravc that all roofing professionals have a basic understanding of the attic ventilation calculations necessary to help the system fight heat in the summer moisture in the winter and ice dams in cold climates.
So for an attic that s 1 000 square feet you would divide 1 000 by 150 to calculate that 6 6 square feet nfa of ventilation would be required.
Ventilation manufacturers assign an nfa value to the non motorized vents they make.
