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Attic Ventilation Requirements Calculator
Based on the 1/150 or 1/300 Rule
(see Building Codes for more info)
(This calculator is for the exhaust half only. If an intake ventilator is needed, just double the amount of vents. For example, if 2 AV-14 are needed and inlet vents are needed, then an additional 2 PV-14 vents would be required)
To determine how many vents of a given diameter (inches)
are required to properly ventilate an attic space:
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OR
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Application: One vent for exhaust per:
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Vent
Diameter
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Sq. Feet
of Floor
1/150
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Sq. Feet
of Floor
1/300
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Net Free
Vent Area
in Sq. Inches
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Net Free
Vent Area
in Sq. Feet
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4" Diam
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26
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52
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12.5
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.09
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6" Diam
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50
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100
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28
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.19
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8" Diam
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100
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200
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50
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.35
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10" Diam
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163
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327
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78.5
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.55
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12" Diam
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225
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450
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113
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.78
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14" Diam
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300
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600
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144
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1.00
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16" Diam
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418
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837
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200
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1.39
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18" Diam
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525
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1,050
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254
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1.76
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20" Diam
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654
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1,308
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314
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2.18
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24" Diam
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941
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1,883
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452
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3.14
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30" Diam
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1,470
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2,941
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706
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4.90
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36" Diam
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2,118
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4,237
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1,017
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7.06
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42" Diam
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2,883
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5,766
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1,384
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9.61
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48" Diam
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3,766
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7,533
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1,808
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12.56
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54" Diam
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4,769
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9,538
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2,289
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15.89
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60" Diam
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5,886
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11,775
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2,826
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19.62
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72" Diam
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8,475
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16,950
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4,069
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28.25
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Excerpted from IBC-2003 & 2006 Section 1203.2 Attic Ventilation
The net free ventilating area shall not be loss than 1/150 of the area of the space ventilated, with 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents.
Exception: The minimum required net free ventilating area sball be 1/300 of the area of the space ventilated, provided a vapor retarder having a transmission rate not exceeding 1 perm in accordance with ASTM E 96 is installed on the warm side of the attic insulation and provided 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914mm) above eave or cornice vents, with the balance of the required ventilation provided by eave or cornice vents.
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