Planning Balanced Attic Ventilation
A Lomanco roof vent plan starts with attic floor area. The floor area drives the net free area requirement. Net free area means the open vent area that air can use. Codes and manufacturers often express the target as a ventilation ratio. Common choices are 1/150 and 1/300. The smaller denominator needs more airflow. The larger denominator may be allowed when intake and exhaust are balanced.
Why Net Free Area Matters
Roof vents do not move air by name or size alone. They move air through listed openings. Each vent has a published NFA value. That value is usually shown in square inches. The calculator uses that value for each selected vent. This keeps the estimate tied to the actual product label. It also prevents guessing from roof length or vent diameter alone.
Balancing Intake and Exhaust
Good attic ventilation needs a path. Air should enter through low intake vents. It should leave through higher roof or ridge vents. A balanced design often places close to half of the required NFA at each side. More exhaust than intake can pull air from living spaces. More intake than exhaust can slow heat removal. This tool lets you adjust the split when a project needs it.
Using the Results
The result shows required NFA, adjusted NFA, intake need, and exhaust need. It also subtracts existing vent area. Then it rounds up the number of additional vents. Rounding up is important. Partial vents are not useful in the field. A safety margin can cover screen blockage, small measurement errors, or future attic changes.
Construction Notes
Use the published Lomanco NFA for the exact vent model. Check local code before cutting the roof. Keep vents clear of insulation, debris, paint, and stored items. Spread intake vents along the eaves when possible. Place exhaust vents high on the roof deck. Do not mix exhaust types without careful review. Final placement should follow the roof design, weather exposure, and installation instructions. Before ordering vents, measure attic length and width carefully. Note vaulted areas, blocked bays, and additions. Treat unusual spaces separately. A simple worksheet helps crews compare options and explain each quantity to owners before materials are purchased for approval.