CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationHard
A contractor is installing a powered attic ventilator (PAV) rated at 1,200 CFM. Compared to a passive, wind-driven ventilation system, why must the net free intake area be carefully verified when using a powered fan?
- APowered fans automatically balance intake and exhaust regardless of NFA
- BInsufficient intake area can cause the fan to draw conditioned air from the living space through ceiling penetrations, increasing energy loss
- CIntake area only matters for gable vents, not powered systems
- DPowered fans require no intake air since they pressurize the attic
Show answer & explanationAnswer & explanation
Correct answer: B. Insufficient intake area can cause the fan to draw conditioned air from the living space through ceiling penetrations, increasing energy loss
A powered attic ventilator actively exhausts air, creating negative pressure in the attic. If soffit or other intake NFA is inadequate, the fan will pull makeup air from the conditioned living space through ceiling gaps, light fixtures, and other penetrations — wasting cooled/heated air and increasing utility costs.
Why the other options are wrong
- A. Powered fans do not self-balance; NFA sizing must be verified manually.
- C. Intake sizing matters for all ventilation types, including powered systems.
- D. PAVs exhaust air, creating negative pressure, not positive pressurization.
Powered Attic Ventilator Intake Sizing
Because powered attic ventilators actively exhaust air, adequate soffit/intake net free area must be provided to prevent the fan from drawing conditioned air from the living space below.
- PAVs create negative attic pressure during operation
- Undersized intake NFA leads to energy loss from conditioned space
- Manufacturer specs typically require intake NFA to meet or exceed fan CFM demand
Memory trick: A hungry fan without enough soffit air eats your AC instead.