What is ridge vent net free area, and why can the real number be lower than the label?
What the number measures
A vent's outside size says little about airflow. Two vents the same width can have very different open area depending on their internal baffles and filter. Net free area is the manufacturer's figure for the unobstructed opening, which is what the building code's 1/150 and 1/300 ratios are based on.
The slot sets a ceiling
Air has to pass through the slot in the deck before it reaches the vent. A slot 2 inches wide has 24 square inches of opening per foot of length, since 2 times 12 is 24. Cut it 1 inch wide and the opening drops to 12 square inches per foot, so a vent rated 18 cannot deliver 18. That is why instructions specify a slot width, and why a narrow, timid cut quietly halves the ventilation.
Other things that cut real net free area
- Crushed vent: overdriven nails or heavy foot traffic flatten the internal channels.
- Clogged filter: pollen, dust and debris build up over years. Georgia's spring pine pollen is notorious.
- Felt or debris left in the slot.
- Partial slot length: the vent may run the full ridge but the slot only part of it.
- Paint or sealant smeared across openings.
Net free area is not airflow
It is a size, not a flow rate. Actual air movement also depends on wind, temperature difference and, above all, intake at the soffits. A ridge vent with plenty of net free area over starved soffits still moves little air.