Model "C" Soffit
Requirements Calculator
(Soffit "A" Calculator, Soffit "B" Calculator)

Based on the 1/150 or 1/300 Rule
(see Building Codes for more info)

To determine how many vents
of a given length (inches)
are required to properly
ventilate an attic space:

Enter the length of
the building in feet
Enter the width of
the building in feet
Choose the desired vent




vents required - 1/300 rule



vents required - 1/150 rule

Enter the total area of attic
in square feet



Choose the desired vent




vents required - 1/300 rule



vents required - 1/150 rule

OR

Application: One vent for intake per:

Model
Sq. Feet
of Floor
1/150
Sq. Feet
of Floor
1/300
Net Free
Vent Area
in Sq. Inches
Net Free
Vent Area
in Sq. Feet
SV-C-22
45
90
22
0.15
SV-C-45
93
186
45
0.31
SV-C-68
141
282
68
0.47
SV-C-91
189
378
91
0.63
SV-C-114
237
474
114
0.79
SV-C-136
282
564
136
0.94
SV-C-159
330
660
159
1.10
SV-C-182
378
756
182
1.26
SV-C-206
426
852
205
1.42
SV-C-227
474
948
227
1.58
SV-C-250
522
1,044
250
1.74
SV-C-273
570
1,140
273
1.90

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.