Does a screen attached to a vent affect the vent's Net Free Area?

A screen attached a vent can affect its Net Free Area.

A screen that is attached directly to a vent will be included in the vent's Net Free Area calculations. Typically, the screens that are used have 1/8-inch openings. Such screens will reduce the Net Free Area of the vent by approximately 10%. But that is just an approximation. Each vent manufacturer publishes the vents Net Free Area.