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How Much Attic Ventilation Do I Need?

September 13th, 2023 | 6 min. read

How Much Attic Ventilation Do I Need?

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Your attic’s ventilation system is crucial to your home and roof’s health. Unfortunately, most attics aren’t properly ventilated. 

Not only does this mean your AC unit is running more, but your roof won’t get anywhere near its maximum lifespan. The last thing you want is to spend money because your roof is missing a vent or two. 

For over 30 years, the team at Bill Ragan Roofing has helped homeowners understand the nitty-gritty of the roofing industry. That’s why I’ll help you determine how much ventilation your attic needs. 

This article answers the following questions:

  • How much attic ventilation do you need?
  • How many vents do you need to properly ventilate your attic?

How much attic ventilation do you need?

According to the International Residential Code (IRC), attics require a minimum of 1/150 of net free area (open area for air to pass through) for each square foot. This means that for each 150 square feet section in the attic, 1 square foot of net free area is required for ventilation.

attic space being inspected for proper attic ventilation

Let’s say you have 1,500 square feet in your attic. All you have to do is divide 1,500 by 150 to determine that you need 10 square feet of attic ventilation. 

Just know that vents are rated by inches, so you’ll have to convert the square footage to square inches to determine the number of vents needed for the space. There are 144 square inches in a square foot, so you’ll multiply the square feet of attic ventilation needed by 144 to get the square inches. 

In our example above, that would be 10 times 144 to get 1440 square inches of net free area needed. With this number, you can determine how many of the chosen exhaust vents (and accompanying intake vents) are needed to properly ventilate your attic.

How many vents do you need to properly ventilate your attic?

A properly ventilated attic requires a certain number of roof vents based on the attic’s size and the type of roof vent you choose. Just know that the information below is about exhaust vents, the ones you actually see on your roof. 

While intake vents are important, the amount you’ll need depends on the exhaust vent. Let’s break down how many exhaust vents you would need for the attic example we used above. 

The information below is provided by Lamanco’s vent calculator and based on their vents. 

Passive ventilation roof vents

Passive ventilation works by moving air in the attic around by natural sources like wind or convection (the cool air coming in pushes the warm air out). It’s called passive because nothing is actively moving the air around continuously or on a cycle. 

Because of this, this ventilation system doesn’t rely on the vents to let trapped air escape. 

Static roof vents

Static roof vents (aka box or turtle vents) look like little boxes all over your roof. This is the most common type of passive vent, but it also covers the least amount of square footage. 

box vent on an architectural asphalt shingle roof

A standard box vent has 50 square inches of net free area. For a 1,500 square foot attic, you’ll need 8 box vents for proper ventilation. 

Ridge vent without a baffle

A ridge vent is cut into the ridge of your roof and runs the entire length of the ridge. This passive roof vent is popular for its low profile when looking at the roof. 

ridge vent on an architectural asphalt shingle roof

Unlike most other roof vents, the amount of ridge vent is calculated per linear foot. The standard ridge vent without a baffle is 4 feet long and covers 72 square inches of net free area.

With that in mind, you’ll need a total of 20’ of ridge vent to ventilate a 1,500 square foot attic properly. 

Active ventilation roof vents

Active ventilation works by pulling in fresh air from the outside through the intake vents and pushing the air in your attic out through the exhaust vents. Unlike passive ventilation, this system and its vents actively work to let air flow in and out of your attic correctly.

Turbine roof vents

Turbine roof vents (also known as whirly birds) work by using a drawing effect through convection to move the air in your attic around, even when there is no wind. As long as it’s installed correctly, it’ll move the air in your attic around 10-12 times per hour.

turbine vent on an architectural asphalt shingle roof

For a 1,500 square foot attic, you’ll need 2 standard 12” turbine roof vents for proper ventilation. There’s a common misconception that turbine vents let rain, snow, and insects enter your home through the slats, but they’re specifically designed to avoid this.

Power roof vents

Power roof vents are circular-shaped vents with very low profiles installed near a roof’s ridge. This active roof vent uses electricity to pull the hot air from the attic. 

power roof vent on an architectural asphalt shingle roof

For a 1,500 square foot attic, you’ll need 2 power roof vents for proper ventilation. During the winter, you’ll want to run your power vents with a humidistat to avoid condensation in your attic space. 

Just know that you may have to get an electrician to hook the power vents up once installed.

Ridge vent with a baffle 

While I already went over the passive version of a ridge vent, there’s also an active ridge vent. The only thing that makes it active is a baffle that provides a channel for air to flow.

ridge vent

Just like a ridge vent without a baffle, you’ll need a total of 20’ of ridge vent to properly ventilate a 1,500 square foot attic. While both ridge vents are good options, I always recommend the active version. 

This is because ridge vents without a baffle can allow debris, rain, snow, insects, etc., to enter your attic.

Solar-powered roof vent

Solar-powered roof vents are like power vents, but they use the sun as the power source instead of electricity. For a 1,500 square foot attic, you’ll need 2 solar-powered roof vents. 

solar powered roof vent on a 3 tab asphalt shingle roof

Unfortunately, the battery won’t hold a charge long enough to run the vent(s) all day because it has to turn off while charging. This means you may notice the air conditioner running more while the battery charges.

Is your attic properly ventilated?

Now you know how much attic ventilation you need and how many vents are needed. I recommend active ventilation for the functionality and health of your roof. 

But no matter what system or vent you choose, you won’t have problems as long as it’s done right. But as I said earlier, most attic spaces aren't ventilated correctly. 

Even worse, most homeowners don’t know until it’s too late, or the rooms upstairs are unlivable in the summer. This comes with costly problems that could even lead to a full roof replacement. 

But how are you supposed to know if your attic is properly ventilated or not? It’s as simple as noticing the signs. 

Check out 6 Signs of Poor Attic Ventilation to identify if your attic is improperly ventilated.

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