Tools
Soffit Intake Calculator
Solar attic fans only work when they have balanced intake ventilation. Use this calculator to size your soffit intake before the install.
Net Free Air required
480 sq in
Pick your soffit vent type
Quantities below cover all 1 fan together.
| Vent type | Unit | NFA per unit | Units needed |
|---|---|---|---|
| Vinyl perforated | per linear foot | 5 sq in | 96 |
| Edge vent | per linear foot | 9 sq in | 53 |
| Hardie perforated | each | 6.25 sq in | 77 |
| 3" continuous soffit | each | 8.5 sq in | 56 |
| 8x16 soffit vent | each | 56 sq in | 8 |
| 4x16 soffit vent | each | 26 sq in | 18 |
Based on the U.S. Federal Housing Administration 1/150 ventilation standard. 480 sq in of Net Free Air per solar attic fan. Round up at the build site. If the attic has existing intake (ridge vents, turbine vents), subtract that NFA from the requirement before sizing soffits.
Why this matters
A solar attic fan pulls hot, moist air out of the attic. Fresh air has to come in from somewhere. Without enough soffit intake, the fan creates negative pressure and works against itself. You lose cooling, you lose moisture reduction, you lose the energy savings the fan is designed to provide.
The 1/150 FHA standard is the floor, not the ceiling. Round up if the attic has any high insulation depth or any restriction to airflow.