Attic Ventilation Calculator
*Note: nfa means "net free area". That's the way to describe the real and true opening of an attic vent after all blockage & obstruction, such as screening & weather protection, has been deducted.
The main ways to provide attic ventilation are:
1. Under eve vents (soffits)** 15 to 50 square inches nfa* per vent, if clean. Perhaps 0 if covered with dust or insulation.
2. Roof caps (or roof jacks). 50 to 75 square inches nfa* per vent (8" to 10" diameter).
3. Turbine (10", 12", 14") 75, 100, 150 cubic inches nfa* respectively.
4. Ridge vents**. About 1 square foot nfa* of opening for every 12-15 linear feet of ridge vent.
5. Gable vents (louvres), After calculating the size of a gable vent, divide by 1/2 for the nfa". Gable vents are usually round, square, rectangular or triangles. Example of nfa calculation: Formula for the area of a circular vent is: radius x radius x 3. ie, 10" circle = 5 x 5 x3 = 75 square inches. Half of this is the nfa* of a circular gable vent.
6. Roof mounted power ventilator (not shown): these provide the amount of venting equal to the size of the hole. The motor system does NOT increase the amount of air that can get through the hole itself. See # 5 above.
*nfa= net free area. This is how a ventilation engineer describes the real opening of a vent. The net area is like net income, what is left after you take away all the deductions. After the hole is cut for a vent, some system of weather and insect blockage is added, like screening, for example. This blockage reduces the amount of air that can get through the hole, causing some restriction. In a gable vent, it can reduce the original hole by over 50%!
**For all existing vents, especially #1, under eave vent, be sure that the vent has not been blocked with dirt or insulation and that light can be seen coming into the attic where the vents are located. For #2, roofcap, check to see if the hole cut into the roof sheathing is as large as the circle of the vent hole. If not calculate the smaller hole size only.
