Attic Ventilation Calculator
A 30 by 50 foot attic at the 1/150 rate needs 10.00 square feet of net free area, 12 intake vents and 12 exhaust vents at 60 square inches each.
Last updated . The one hundred fifty path is selected. Upper share opens at fifty percent.
An attic ventilation calculator for the net free area of a roof. It follows the 2021 IRC rule for the vented space. It does not size heating or cooling equipment.
Squares on the roof: roofing calculator. Eave length and leaders: gutter calculator. Batts under the ceiling: insulation calculator.
Net free area and vent count
Sixty square inches is a stand-in so the vent count can be checked. It is not a listed vent. The 1/300 choice is the 2021 IRC R806.2 exception, and it applies only when the vapor-retarder condition and the 40 to 50 percent upper-vent condition are both true. This form cannot confirm the climate zone or the retarder.
Results
1/150, and the narrower exception
A 30 by 50 foot plan is 1,500.00 square feet, 139.35 square meters. Divided by 150, the minimum net free area is 10.00 square feet, 0.93 square meters, 1,440.00 square inches. Half of that, as a planning split, is 720.00 square inches low and 720.00 square inches high. At an example 60 square inches per vent, each side is 12 vents. The code section does not require that half-and-half split on the 1/150 path.
Divided by 300, the same attic is 5.00 square feet, 720.00 square inches. That smaller area is available only when both exception tests in 2021 IRC R806.2 are met: the Class I or II vapor retarder on the warm-in-winter side in Climate Zones 6, 7, and 8, and an upper share from 40 percent through 50 percent, with those upper vents not more than 3 feet below the ridge and the balance in the bottom third. Fifty percent is the top of that band. Thirty percent is outside it. The sentence is printed at up.codes/s/minimum-vent-area. Vent counts round each side up separately, from a net free area you type. Sixty square inches is only an example.
Worked attic
| Ratio | Upper share | Net free area | Each side | Vents at 60 in² |
|---|---|---|---|---|
| 1/150 | 50% | 1,440.00 in² | 720.00 in² | 12 and 12 |
| 1/300 | 50% | 720.00 in² | 360.00 in² | 6 and 6 |
| 1/300 | 30% | 720.00 in² | 504.00 and 216.00 in² | 9 and 4 |
Attic vent questions
What does the opening attic require?
Leave length 30 ft, width 50 ft, the 1/150 choice, upper share 50, and net free area per vent 60. Attic area reads 1,500.00 ft² · 139.35 m². Net free area reads 10.00 ft² · 0.93 m² · 1,440.00 in². Intake reads 5.00 ft² · 720.00 in². Exhaust reads 5.00 ft² · 720.00 in². Intake vents reads 12. Exhaust vents reads 12. The note reads This is the 1/150 minimum. The upper share is a planning split, not a requirement of that minimum.
What changes on the 1/300 choice?
Switch the choice to 1/300 and leave the other boxes. Net free area reads 5.00 ft² · 0.46 m² · 720.00 in². Intake reads 2.50 ft² · 360.00 in². Exhaust reads 2.50 ft² · 360.00 in². Intake vents reads 6. Exhaust vents reads 6. The note reads Upper share is inside the 40 to 50 percent band. The 1/300 exception still needs the vapor retarder in Climate Zones 6, 7, and 8. This form cannot check that.
Why does 1/300 need two conditions?
The 2021 IRC R806.2 exception, as printed on UpCodes, allows 1/300 only when both of these are true. In Climate Zones 6, 7, and 8, a Class I or II vapor retarder is on the warm-in-winter side of the ceiling. And between 40 and 50 percent of the vent area is in the upper portion of the attic, not more than 3 feet below the ridge, with the balance in the bottom third. The form can test the percent. It cannot test the zone or the retarder. A code that says “one or more” of those conditions is a local amendment, not the sentence used here.
What if each vent is 50 square inches?
Switch back to 1/150 and set net free area per vent to 50, with the upper share still 50. Intake vents reads 15. Exhaust vents reads 15. Each side has 720 square inches, and 720 divided by 50 rounds up to 15. The area lines stay on the 1/150 figures.
What if the upper share is 30 percent on 1/300?
Choose 1/300, set the upper share to 30, and set net free area per vent back to 60. Intake reads 3.50 ft² · 504.00 in². Exhaust reads 1.50 ft² · 216.00 in². Intake vents reads 9. Exhaust vents reads 4. The note reads Upper share is outside the 40 to 50 percent band, so this split does not meet exception condition 2 of 2021 IRC R806.2.
What if the vent size is cleared?
Clear net free area per vent, return the upper share to 50, and choose 1/150 again. Net free area still reads 10.00 ft² · 0.93 m² · 1,440.00 in². Intake still reads 5.00 ft² · 720.00 in². Exhaust still reads 5.00 ft² · 720.00 in². Intake vents reads Enter a net free area. Exhaust vents reads Enter a net free area.
Is this a heating or cooling load?
No. The result is vent area for the roof space under R806.2. It is not a furnace, an air conditioner, or a fan wattage.