Use this worksheet to calculate how much actual ventilation your attic has. Click on the magnifying glass symbol for a larger view, then print out the sheet and start measuring.
*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.
Calculator Instructions: Go outside and identify the type and amount of vents in you house. Go into the attic to determine the actual size of the vent holes, the length (Amt) of the ridge vents, etc. If you have soffit vents, determine if there is any light coming through them. If you cannot see any light during the daytime, then the soffit vents are probably covered with dust or insulation.
If you want your whole house fan to work properly you must open enough windows so that the fan can "breathe" in as much air as it needs. Usually this means opening 6 to 9 square feet of window, about one or two normal windows.
This is simple to do.
After the air is pulled into the house, the fan blows it into the attic. If you don't have ENOUGH opening out of the attic, then not all the air will be able to get back outdoors. The attic needs enough opening to the outdoors to equal about 6 or 9 square feet. Since there are usually no windows in the attic, the venting has to be provided through the roof, end gables or the overhangs.
Without enough venting, the result is the same as trying to get off a bus or an airplane: there is congestion and backup. Some of the air gets out but the rest just pushes back against the fan. The fan works, but not as well as it could. Since the point of the fan is to move enough air through the house quickly, it is a false economy not to provide sufficient attic ventilation.
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. Measure the area of the vent and multiply by 1/2 to get the net free area.
2. Roof caps (or roof jacks). 50 to 75 square inches nfa* per vent (8" to 10" diameter). Measure the radius of the circle of the hole from inside the attic. Multiply the radius by itself (radius squared) then multiply by Pi (3.14) for the net free area.
3. Turbine (10", 12", 14") 75, 100, 150 cubic inches nfa* respectively. Measure the radius of the circle of the hole from inside the attic. Multiply the radius by itself (radius squared) then multiply by Pi (3.14) for the net free area. We do not recommend adding spinning turbine vents since the air blowing through them will cause premature wear & tear.
4. Ridge vents**. About 1 square foot nfa* of opening for every 12-15 linear feet of ridge vent. Measure the length of the ridge then multiply by .075 for the nfa. For every 12 to 15 feet of ridge vent you get about 1 square foot nfa.
5. Gable vents (louvres), After calculating the size of a gable vent, then divide by 1/2 for the nfa. The finished gable vent loses 1/2 or more of the original cut hole after the vent is installed. Gable vents are usually round, square, rectangular or triangular.
6. Roof mounted power ventilator (not shown): these provide the amount of venting equal to the size of the hole. They provide the same amount of ventilation as a roof vent without the motor and blade. 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 No. 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 No. 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.