How it Works
What is Attic Ventilation Calculator (NFA 1:300)?
Step-by-Step Guide
- Measure Attic – Input floor width and length.
- Select Rule – 1:300 (Standard) or 1:150 (No Vapor Barrier).
- Calculate NFA – Find total vent area needed.
- Balance System – Ensure 50% Intake (low) and 50% Exhaust (high).
Example
Input: 1500 sq ft Attic
Result: 5 sq ft NFA (2.5 In / 2.5 Out)
FAQ
What happens if I have too much exhaust?
It can create negative pressure, pulling conditioned air from your home into the attic (energy loss).
What is NFA?
Net Free Area - the actual open space of a vent, accounting for screens and louvers.
1:150 vs 1:300 rule?
Use 1:300 if you have a balanced high/low system and vapor barrier. Use 1:150 otherwise.
Can I use gable vents with ridge vents?
No. Gable vents can disrupt the airflow of ridge vents, turning them into intake vents and pulling in rain/snow.
Do I need vents in winter?
Yes. Ventilation removes moisture to prevent mold and keeps the roof cold to prevent ice dams.
Conclusion
Balanced ventilation uses thermal buoyancy to pull cool air in low and push hot air out high. Never mix exhaust types (e.g. Ridge vents + Wind turbines) as it can short-circuit airflow. Ensure insulation baffles keep soffit vents clear.