Net Free Area Tables
1:150 Ratio-no Moisture Barrier
1:150 ratio: Net Free Area Table-Use when no Moisture Barrier is installed.
The MINIMUM attic ventilation required by the Building Codes is 1 square foot of total net free area per 150 square feet of attic when no moisture barrier is used.
A Balanced System requires soffit ventilation in an amount equal to or greater than the amount of ridge ventilation. See soffit vent manufacturer’s information on the Net Free Area provided by soffit vent product.
| Attic Area (SF) | Minimum NFA (SF) | Exhaust NFA (SF) | Length of Roof Saver (FT) | Intake NFA (SF) |
|---|---|---|---|---|
| 1000 | 6.67 | 3.33 | 31.6 | 3.33 |
| 1100 | 7.33 | 3.67 | 34.7 | 3.67 |
| 1200 | 8.00 | 4.00 | 37.9 | 4.00 |
| 1300 | 8.67 | 4.33 | 41.1 | 4.33 |
| 1400 | 9.33 | 4.67 | 44.2 | 4.67 |
| 1500 | 10.00 | 5.00 | 47.4 | 5.00 |
| 1600 | 10.67 | 5.33 | 50.0 | 5.33 |
| 1700 | 11.33 | 5.67 | 53.7 | 5.67 |
| 1800 | 12.00 | 6.00 | 56.8 | 6.00 |
| 1900 | 12.67 | 6.33 | 60.0 | 6.33 |
| 2000 | 13.33 | 6.67 | 63.2 | 6.67 |
| 2100 | 14.00 | 7.00 | 66.3 | 7.00 |
| 2200 | 14.67 | 7.33 | 69.5 | 7.33 |
| 2300 | 15.33 | 7.67 | 72.6 | 7.67 |
| 2400 | 16.00 | 8.00 | 75.8 | 8.00 |
| 2500 | 16.67 | 8.33 | 39.5 | 8.33 |
| 2600 | 17.33 | 8.67 | 41.1 | 8.67 |
| 2700 | 18.00 | 9.00 | 42.6 | 9.00 |
| 2800 | 18.67 | 9.33 | 44.2 | 9.33 |
| 2900 | 19.33 | 9.67 | 45.8 | 9.67 |
| 3000 | 20.00 | 10.00 | 47.4 | 10.00 |
| See soffit vent manufacturers instructions for NFA calculation. | ||||
1:300 Ratio-w/ Moisture Barrier
1:300 ratio: Net Free Area Table-Use when Moisture Barrier is installed
The MINIMUM attic ventilation required by the Building Codes is 1 square foot of total net free area per 300 square feet of attic when a moisture barrier is installed.
A Balanced System requires soffit ventilation in an amount equal to or greater than the amount of ridge ventilation. See soffit vent manufacturer’s information on the Net Free Area provided by soffit vent product.
| Attic Area (SF) | Minimum NFA (SF) | Exhaust NFA (SF) | Length of Roof Saver (FT) | Intake NFA (SF) |
|---|---|---|---|---|
| 1000 | 3.33 | 1.67 | 15.8 | 1.67 |
| 1100 | 3.67 | 1.83 | 17.4 | 1.83 |
| 1200 | 4.00 | 2.00 | 18.9 | 2.00 |
| 1300 | 4.33 | 2.17 | 20.5 | 2.17 |
| 1400 | 4.67 | 2.33 | 22.1 | 2.33 |
| 1500 | 5.00 | 2.50 | 23.7 | 2.50 |
| 1600 | 5.33 | 2.67 | 25.3 | 2.67 |
| 1700 | 5.67 | 2.83 | 26.8 | 2.83 |
| 1800 | 6.00 | 3.00 | 28.4 | 3.00 |
| 1900 | 6.33 | 3.17 | 30.0 | 3.17 |
| 2000 | 6.67 | 3.33 | 31.6 | 3.33 |
| 2100 | 700 | 3.50 | 33.2 | 3.50 |
| 2200 | 7.33 | 3.67 | 34.7 | 3.67 |
| 2300 | 7.67 | 3.83 | 36.3 | 3.83 |
| 2400 | 8.00 | 4.00 | 37.9 | 4.00 |
| 2500 | 8.33 | 4.17 | 39.5 | 4.17 |
| 2600 | 8.67 | 4.33 | 41.1 | 4.33 |
| 2700 | 9.00 | 4.50 | 42.6 | 4.50 |
| 2800 | 9.33 | 4.67 | 44.2 | 4.67 |
| 2900 | 9.67 | 4.83 | 45.8 | 4.83 |
| 3000 | 10.00 | 5.00 | 47.4 | 5.00 |
| See soffit vent manufacturers instructions for NFA calculation. | ||||