Does the internal weather filter affect Net Free Area?

The internal weather filter does not affect the Net Free Area of the vent.

The internal weather filter used for ShingleVent II (and other Air Vent ridge vents) does not reduce the Net Free Area of the vent because it is not placed near any of the ridge vent's restrictions. For example, if the filter were placed behind the louvers the Net Free Area would be reduced because a restriction is added at the louvers. If anything the filter reduces the Net Free Area of the slot because that is the only restriction it is in contact with. The internal weather filter would be approximately 97% open.