Passive attic venting (no fans) functions based upon the difference in height between the low vents and the high vents.
As for how many vents the calculation is based upon the NFA (net free area) of all vents. In general when a vent is covered with a screen to keep the bugs out its NFA is about half the actual area.
As for the calculations they want 1 ft² of NFA for every 150 sq ft of attic floor when the ceiling has not been carefully air sealed. That number drops to 1 ft² per 300 sq ft of attic floor when well sealed.
If you don't know the NFA rating of your vents search for something similar and use their listed NFA,
Whatever you calculate for total vent area is then divided helf high and half low, but don't panic if you can't meet that it originated as a wild guess 70 years ago.
Can I ask what your concerns are?
Bud
As for how many vents the calculation is based upon the NFA (net free area) of all vents. In general when a vent is covered with a screen to keep the bugs out its NFA is about half the actual area.
As for the calculations they want 1 ft² of NFA for every 150 sq ft of attic floor when the ceiling has not been carefully air sealed. That number drops to 1 ft² per 300 sq ft of attic floor when well sealed.
If you don't know the NFA rating of your vents search for something similar and use their listed NFA,
Whatever you calculate for total vent area is then divided helf high and half low, but don't panic if you can't meet that it originated as a wild guess 70 years ago.
Can I ask what your concerns are?
Bud
