July 1, 2024

How to Calculate the Net Free Area (NFA) for Wildfire Defense Mesh

When discussing ventilation, the term "Net Free Area" or NFA is used to describe the amount of space available in any type of vent that can be used to freely pass airflow.

Wildfire Defense Mesh has a Net Free Area (NFA) of 62% or 62% per square inch.

To calculate the Net Free Area (NFA)of an individual retrofitted vent, this formula can be used:

**Length x Height x 0.62 = Net Free Area of that vent **

To help contractors and homeowners understand the process of calculating the Net Free Area per California Building Code or ICC, we have created a step-by-step process based on the 1/150 rule.

Before we start, let’s be clear on the definitions of the various terms in this post.

** Building Ventilation Area =**The area that is being ventilated. This is typically the floor space of the building on the ground level. This is measured typically by multiplying the length by the width of the building.

*Vent =** *The building component that allows air to move into and out of the Building Ventilation Area.

** Vent Area **=The internal dimensions of the Vent (excluding any mesh, etc).

** Total Vent Area** = The total of all the Vent Areas for the Building.

*Open Area %** = *The percentage area that is open that allows the air to move. Different products have different open areas, a 0% open area will not allow air to pass and a 100% open area has no restrictions.

** Vent Free Area** = The area within the Vent Area that allows air to move. This is impacted based on any baffles, mesh, or screens that may restrict the movement of air.

** California Building Code 1/150 Rule =** A Building guideline that helps builders and architects determine the number of Vents required to provide natural ventilation to avoid moisture buildup.

**Step 1:**

Determine the Building Ventilation Area in Square Feet

Length of the building x Width of the building = Building Ventilation Area

Example: Length (L) x Width (W) = Building Ventilation Area 1500 square feet

**Step 2: **

Multiply Step 1 by 1/150 to determine the Total Vent Area required to meet the California Building code requirements for natural ventilation.

Building Ventilation Area x 1/150 = Total Vent Area

Example: 1500 square feet x 1/150 = 10 square feet

**Step 3**

How many vents does the existing building have?

Count the number of vents in the ventilated area.

Example: 14 Vents

**Step 4**

What are the dimensions of the vents?

The individual vent's Length & Height (Vent Area)

Example: Length – 2 ½ feet (30”) Width – ½ feet (6”)

**Step 5**

What is the Average Vent Area?

Length x Width = Average Vent Area

Example 5: 2 ½” feet x ½ feet = 1 ¼ Square feet

**Step 6**

What is the Total Vent Area?

Number of vents (14) x Vent Area (1 ¼) = Actual Total Vent Area

Example 6: 14 x 1 ¼ = 17.25 square feet

**Step 7**

Determine the Open Area % of Wildfire Defense Mesh

Wildfire Defense Mesh (WDM) is a 1/16” with open area of 62%

Example 7: See Calculations below for Open Area %

**Step 8**

What is the New Vent Free Area using Wildfire Defense Mesh?

Total Vent Area x WDM Open Area = WDM Vent Free Area (square feet)

Example 8: 17.25 x 62% = 10.7 square feet

**Step 9**

Is the WDM Vent Free Area (Answer 8) Greater than the California Building Code requirements (Answer 2) for the 1500-square-foot building?

**If, Yes **

Example 9: 10.7 square feet is greater than 10 square feet: Can remove old barrier (mesh, baffles, etc) and install WDM and meet California Building code requirements for ventilation (1/150 rule)

**If, No **

Example 9: Vent Free Area for 1500 square foot Building has been assessed as less than a total of 10 square feet when using WDM. Additional vent(s) are required to be added to the building to meet the CBC requirements for Ember and Ventilation.

** **

To calculate the NFA of a single retrofitted vent with WDM:

- Vent Area x WDM NFA – Length x Height x 62%
- Noting that if there is no frame and the mesh is attached to the exterior or interior wall (see How to Measure, Cut and prepare WDM for installation), the Length x Height will be the entire open area.

Before installing Wildfire Defense Mesh we suggest removing the existing mesh, as we do not know the distance between the existing and new mesh and what the NFA would be of the existing mesh.

If needed, to calculate the Net Free Area of the existing mesh here is a formula to use:

**EXAMPLE: 12 Mesh and a Wire Diameter of 0.0177"**

To demonstrate the steps with this example, the open area of 12 Mesh, 0.0177" Wire Diameter, can be calculated as follows:

- Multiply the Mesh Count (p/inch) x Wire Diameter
**Example: 12 x 0.0177 = 0.2124**

- Subtract the result above from 1
**Example: 1 - 0.2124 = 0.7876**

- Square the above result (multiply by itself)
**Example: 0.7876 x 0.7876= 0.6203**

- Multiply the result by 100
**Example: 100 x 0.62 = 62**

- The solution should be expressed as %
**Example: 62%**

** **When installing WDM behind retrofitted Louver Vents or other obstructions. The calculations would be:

- Retrofitted Vent with Louver: ((Open Area Length x Height) – (Length x Height of Louver pieces directly touching WDM directly)) x 62%
- Noting the piece of Louver pieces is just the portion directly touching the mesh.

When you have a multilayered venting system, it's important to look not just at the Net Free Area, but also the pressure drop. While spacing in a system might not lower the Net Free Area, it's likely to reduce the pressure drop.

If you need further assistance in calculating the NFA for your project, please don’t hesitate to contact us.

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