Association of Home Appliance Manufacturers
Calculating Requirements for a Fixed Vent System
The math involved in calculating ventilation requirements is simple. A pad and pencil are all you need. NOTE: The following five-step process is used to calculate requirements for non-powered ventilation system.

1. Determine the square footage of attic area to be ventilated. To do that, just multiply the length of the attic (in feet) by its width. Example: For this and the following calculations, we'll assume a project involving a home has a 40' by 25' attic area.
Calculation: 40'x 25' = 1, 000 squarefeet of attic area

2. Establish a goal. The specific rate of air flow required to both reduce summer temperatures and remove winter moisture. Tests conducted at the University of Illinois significantly simplify this step. Those tests established that when air flow is 1.5 cubic feet per minute per square foot (CFM/sq. ft.) of attic floor area, temperatures are reduced nearly 24'. What's even more interesting about this finding is that it's true whatever the ambient air temperature. The tests were conducted in summer conditions, with attic temperatures ranging from 85 degrees to 110 degrees. In every case, attic temperatures fell about 24 degrees.

3. Determine the total net-free area required to establish the desired air flow. To calculate the total net-free area required (in square inches), multiply the square footage of the attic by the appropriate net-free area. Example: Because we want the most efficient and cost-effective system, we choose a ridge vent and undereave vent system. (The net-free area specifications for under-eave vents are the same as those for soffit vents.)

Calculation:
1, 000 sq.ft. x 1.5 sq. in.Isq.ft. = - 1,500 square inches of total net-ftee area required

4. Determine the amount of intake and exhaust (low and high) net-free area required. This is a simple calculation: just divide the answer from Step 3 by 2.

Calculation:
1,500 -- 2 = 750 square inches of venting for each area

5. Determine the number of units of exhaust and intake venting you'll require. To make these calculations, first refer to the Net-Free Area Table on the following page. The table lists the approximate net-free area, in square inches, for all common intake and exhaust ventilation units. To perform the calculations, divide the net-free area requirement from Step 4 by the appropriate figure from the Net-Free Area Table (below) For our example, we will use the figures for Multi-Pitch FilterVent and unscreened undereave vents.

Calculation:
(for 8-foot length of ridge vent) 750 sq. in. + 144 = 5.2 pieces of vent (or five eight-foot leng-ths of ridge vent)
(for 16" x 8" undereave vent) 750 sq. in. + 56 = 13.4pieces (or 14 vents - 16" x 8)

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Net-Free Area Table
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Type of Vent Net-Free Vent Area
in Square inches (approximate)
High Vents-Exhaust
FilterVent (8' length) 144
ShingleVentII(4') 72
Roof Vent 50
Turbine Vent 112
Rectangular Gable Vents
12" x 12" 56
12" x 18" 82
14" x 24" 145
18" x 24" 150
24" x 30" 324
Low Vents-Intake
16" x 8" 56
16" x 6" 42
16" x 4" 28

Strip vent & drip edge vent
8' length 72

Perforated aluminum soffit
One square foot 14

Lanced aluminum soffit
One square foot 4 - 7

Note: be sure to check specifications for individual products to determine actual net-free vent area.

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Meeting minimum code requirements
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According to building codes, you need one square foot of ventilation for every 150 square feet of attic floor space. The minimum is one square foot for every 300 square feet of attic floor space, if there is a vapor barrier or the number is balanced between the ridge and intake vents.

To determine how many feet of NET-FREE AREA you need for a balanced ridge vent system, use this formula:

Sq. ft. of attic floor space divided by 300 (150 for homes without a vapor barrier or are not balanced) = Sq. ft. of net-free area needed.

To determine how many feet of AIR VENT RIDGE VENT you need, use this formula:

1/2 net-free area x 144 divided by 18 = feet of ridge vent needed.

To determine how many feet of AIR VENT CONTINUOUS SOFFIT VENTS you need, use this formula:

1 /2 net-free area x 144 divided by 9 = feet of soffit vent needed.

If your minimum ridge vent requirement is less than the full length of the roof ridge, Air Vent recommends adding the extra feet needed to run the vent from end to end for a more attractive appearance.


Source: Air Vent Inc. A Certainteed Company
Reprinted with permission

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