I’m not saying doing math is always fun. And I’m certainly not saying math is always easy. But you have to trust me that there is a way to calculate how many attic vents are needed that you’re going to love. And it’s a piece of cake to use.
Audiences in the seminars I host have told me for years one of the most useful parts of the program is when we walk through the math of how to calculate how many attic intake vents and how many attic exhaust vents are needed for any size attic. We do the math together with the step-by-step calculations displayed on the big screen inside the seminar room. And we go through a sample attic together. Audiences tell me they love it because it gives them confidence how they can do the calculations themselves if needed.
I realize there are handy calculating tools (apps, online spreadsheets) on the market that do the math for you so you don’t have to. These tools are excellent resources. But the nice thing about doing the math calculations yourself is you can confirm the accuracy of the digital tools. Plus, you never know when a client — a homeowner ‑ wants to see the calculations for themselves step-by-step.
I want to focus on the shortcut to those step-by-step calculations because the shortcut leapfrogs straight to the finish line of the calculations. That’s the beauty of it. But first I need to explain two important ground rules before revealing the shortcut to the math:
Ground Rule No. 1
The shortcut to the math applies to non-motorized vents for the attic such as ridge vents, box vents, dormer vents, off-ridge vents, wind turbines, and gable vents. It also applies to any of the necessary intake vents to feed those exhaust vents. The shortcut does not apply to motorized vents. (Sorry.)
Ground Rule No. 2
It matters if you want more or if you want less airflow through the attic. AirVent always recommends optimum airflow through the attic balanced with equal intake and exhaust at a ratio of 1 square foot of Net Free Area for every 150 square feet of attic floor space. I’m going to call the 1 over 150 ratio “optimum attic airflow.” But if you want less airflow through the attic — the allowable minimum amount by the International Residential Code — that ratio is 1 over 300: 1 square foot of Net Free Area for every 300 square feet of attic floor space. The cool thing about the shortcut is it can be used for either of those two airflow ratios.
The Shortcut
The first thing we need to know before any kind of math can be done is, what is the attic’s size? What is the attic’s square footage?
Attic square footage drives the quantity of vents needed. The most accurate measurement of attic square footage is length x width, floor of the attic. In our seminar we offer additional ways to obtain the attic’s square footage, but length x width is the most accurate.
Once we know the attic’s square footage all we need to do next is decide: Do we want optimum attic airflow (expressed in the 1 over 150 ratio) or do we want the bare minimum attic airflow (which is the 1 over 300 ratio)? If it’s the optimum 1 over 150 airflow ratio we want, simply divide attic square footage by 2. That’s the shortcut. Divide by 2.
Attic square footage divided by 2 = the total square inches of Net Free Area airflow needed for intakeAND the total square inches of Net Free Area airflow needed for exhaust.
If it’s the bare minimum 1 over 300 airflow ratio we want, the shortcut to the math is to divide attic square footage by 4.
Attic square footage divided by 4 = the total square inches of Net Free Area airflow needed for intakeAND the total square inches of Net Free Area airflow needed for exhaust.
That’s the shortcut to the math: Divide by 2 or divide by 4.
Let’s say the attic square footage is 2,000 and we want to know how many vents are needed. Let’s apply the shortcut.
2,000 divided by 2 = 1,000 square inches of intake Net Free Area airflow needed AND 1,000 square inches of exhaust Net Free Area airflow needed. That covers the 1 over 150 optimum airflow ratio.
To apply the shortcut to the 1 over 300 minimum airflow ratio we divide by 4.
2,000-square-foot attic divided by 4 equals 500 square inches of intake Net Free Area airflow needed AND 500 square inches of exhaust Net Free Area airflow needed.
Once we apply the “divide by 2” or the “divide by 4” shortcut, the only thing left to do is to select the type of attic intake and exhaust vents you want to use. For exhaust will it be ridge vents, box vents, off-ridge vents, dormer vents, wind turbines, or gable vents? For intake will it be soffit vents, rectangular under-eave vents, vented drip edge, or perhaps a roof-top mounted intake vent?
Next, pick the brand (the manufacturer) you want to use and find the vent’s published Net Free Area value (that info should be easily available on manufacturer’s website). Then, determine how many of those vents you’ll need based on the amount of airflow the vent provides compared to the amount of total airflow the shortcut to the math revealed.
The Shortcut in Action
Let’s do an example from start to finish:
The attic is 2,000 square feet and we want to use the 1 over 150 optimum airflow ratio. We divide by 2. That equals 1,000. And let’s say we want to use ridge vents for exhaust. Most ridge vents on the market provide 18 square inches of Net Free Area per linear foot. Eighteen divides into 1,000 55 times. Thus, we need 55 linear feet of ridge vent.
Or, let’s say we want to use a box vent. A common box vent on the market provides 60 square inches of Net Free Area each. Sixty divides into 1,000 16 times. We need 16 box vents.
Or, let’s say we want to use 12-inch wind turbines. They provide about 95 square inches of Net Free Area each. Ninety-five divides into 1,000 10 times. We need 10 wind turbines.
And you follow the same process for intake ventilation once you know the type of intake vent you want to use, the brand/manufacturer, and the published Net Free Area value of the vent.
Motorized Vents
Although I’m not aware of any math shortcuts for motorized vents, I would like to share with you the formula for sizing power fans so you have it as a reference:
Attic square footage X a factor of .7 = CFM needed or cubic feet of airflow per minute
Every power fan has a CFM rating by the manufacturer. Find one that is close to the desired CFM. By the way, that factor of .7 that we are multiplying is based on achieving 10-12 air exchanges per hour in the attic, which is an industry recommendation. After the power fan for exhaust is determined follow the manufacturer’s Installation Instructions for providing the needed intake ventilation specific to the power fan’s published CFM value.
About the author: Paul Scelsi is marketing communications manager at AirVent and leader of its “Attic Ventilation: Ask the Expert” seminars. He hosts the podcast, “Airing it out with Air Vent,” and he’s the chairperson of the Asphalt Roofing Manufacturers Association Ventilation Task Force. For more information, visit airvent.com and gibraltarbuildingproducts.com.
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