Requirements for Proper Soffit Ventilation
Proper attic ventilation is important for any home. The 2003
International Building Code (IBC) Section 1203 Ventilation
furnishes a basic guide for determining proper ventilation for any
home. The information provided here may under certain
circumstances not result in enough ventilation. Therefore, the
calculation provided should be used as a guide only.
The IBC guideline requires that any attic or space between top
floor, ceiling and roof must be ventilated. It requires one square
foot of ventilation area for every 150 square feet of attic space.
If a vapor retarder of less than one perm has been installed on
the warm side of the ceiling or if at least 50% of the required
ventilating area has already been provided by gable and vents
or ridge vents, you need add only one-half of the ventilation
area that would otherwise be required. The requirement would
then be one square foot of ventilation area for every 300 sq. ft.
of attic space.
How to Determine Soffit Ventilation
1. Determine local code requirement for total attic
ventilation.
1:150
requires 1 sq. ft. of ventilation for every 150 sq. ft. of
attic space.
1:300
requires 1 sq. ft. of ventilation for every 300 sq. ft. of
attic space, if a vapor barrier having a transmission rate not
exceeding 1 perm is installed on the warm side of the
ceiling.
2. Determine total area of attic (sq. ft.) to be ventilated.
40 ft. x 30 ft. = 1,200 sq. ft.
3. Total free area of ventilation required for attic.
1,200 sq. ft. / 150 = 8.0 sq. ft.
4. Convert square feet into square inches (sq. in.).
8.0 sq. ft. x 144 = 1,152 sq. in.
5. Location of vents.
50% at ridge, roof vent or gable vent = 1,152 x .50 = 576 sq. in.
50% at soffit / eave = 1,152 x .50 =
576 sq. in.
6. Total soffit ventilation area required.
Area of soffit available for ventilation:
80 lineal ft. x 2 ft soffit depth = 160 sq. ft.
Ventilation area required per sq. ft. of soffit = 576 sq. in.
/160 sq. ft. =
3.6 sq. in /sq. ft.
7. Soffit product selection.
Compare soffit ventilation area required to manufacturer’s
soffit product literature.
71
SECTION 7 – Porch Ceilings, Soffit, Fascia
40'- 0"
2'- 0"
2'- 0"
30'- 0"
eave
ridge
Wolverine Fit and Finish Installation Guide
