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How Much Attic Ventilation do I Need?

An equalized roofing system is paramount to the stability of the structures of your residential and commercial buildings, but interpreting the solutions to how much attic ventilation do I need can sometimes be counterintuitive because you may have already planned to insulate your house to hinder the entrance of air, keeping in view all the contaminants that come along with it, while simultaneously wishing to ensure your homes with maximum cross-ventilation to avoid humidity build-up.

Understanding the effective contributory factors to how much attic ventilation do I need and the way they function altogether facilitates the regulation of homeostasis and prevents the roofs and walls from deterioration. Constructing your homes from scratch or renovating them requires appropriate planning and timely improvisation. If you’re unaware of how much ventilation do I need in my attic, your roofing contractors or friends experienced in roofing will surely let you know that the temperature of the attic regulates the overall atmosphere of your house.

Attractive Basics to Attic Ventilation

Attic ventilation works on the principle that hot air naturally rises towards the ceiling. Before insulating your home, the two main types of vents that ventilate an attic are:

Attractive Basics to Attic Ventilation

Exhaust vents:

are located at the peak of the roof and allow rapid escape of hot air from your home.

Intake vents:

allow cool air to enter the attic. These are located at the lowest part of the roof, which could be either under the eaves, high on the sides of the ceilings within a gable, or as shingled intake vents installed in a low part of the roof.

The types mentioned above are the key foundations for how to improve attic ventilation.

Attic Ventilation Before Insulation

It is a good idea to insulate your homes as it prevents the accumulation of hot air in the attics while simultaneously reducing your burden to save you on utility bills. However, no matter how hot or humid the environment gets, we all need fresh air to circulate the house.

Owing to this and many other important reasons, it is crucial first to certify maximum ventilation to avoid the probability of unnecessary sensations of suffocation and synchronize the calefaction of your residential properties.

How Much Attic Ventilation Do I Need? Understanding Proper Airflow

Regardless of the availability of calculating software and applications for attic ventilation, homeowners must have the basic knowledge on how much attic ventilation do I need so they can make accurate estimates regarding the required number of intake and exhaust vents, guaranteeing a balanced roofing system with durable longevity. Some terms and steps that you need to consider while making these calculations are as follows:

Net Free Area

Net free area, abbreviated as NFA, is the open area through which airflow can pass through the vent - the unit in which NFA is measured in square inches. An NFA value is assigned to non-motorized vents by ventilation manufacturers.

Calculate NFA

The first step to how much attic ventilation do I need is to calculate the net free area. Its measurement is further categorized into two parts, depending on the presence or absence of a moisture barrier.

  • 1:150 – No Moisture Barrier

This method is taken into consideration when moisture barriers are not installed on the roofs. The minimum attic ventilation required is one square foot of total net free area per 150 square feet of the attic when no moisture barrier is used. Soffit ventilation is needed in an amount equal to or greater than the amount of ridge ventilation.

  • 1:300 – Moisture Barrier

This ratio is used when moisture barriers are installed on the roofs. The minimum attic ventilation required is one square foot of total net free area per 300 square feet of the attic when a moisture barrier(s) is used. Similar to the 1:150 ratio of NFA, soffit ventilation up to or more than the required amount of ridge ventilation is needed.

Most of the roofing contractors recommend the utilization of 1:300 (as it is a ratio, it can also be denoted as 1/300) for minimum residential attic ventilation. It implies that 1 square root of ventilation is required for every 300 square feet of enclosed attic space, with half at the upper portion as exhaust vents and half at the lower portion as intake vents. Traditionally, this formula is best suitable for static roof vents, which are rated for the net free area in terms of square inches.

An Example to Calculate Attic Ventilation

An Example to Calculate Attic Ventilation

Suppose the attic floor space with the installation of moisture barriers of a home is 2000. The first step will be to divide 2000 by 300, and the answer you’ll get will give an estimate of overall attic ventilation.

  • Step 1: 2000 ÷ 300 = 6.66

The answer infers that you need 6.66 square feet of attic ventilation.

The next step is to divide 6.66 by two because certifying a balanced roofing system requires the installation of both intake and exhaust vents.

  • Step 2: 6.66 ÷ 2 = 3.33

It means that you need 3.33 square feet of attic ventilation for intake and 3.33 square feet of attic ventilation for exhaust vents.

As vents are rated in square inches, you need to convert square feet into square inches, which is accomplished by a general conversion

1 foot = 12 inches; therefore, 1 square foot = 144 square inches

whereas; (12)2 = 144

  • Step 3: 3.33 × 144 = 480 square inches

Therefore, you need 480 square inches of attic ventilation each for intake vents and exhaust vents.

The next step is to divide the NFA required by the NFA rating of the vent; regarding this example, which attic floor space is 2000 square feet. For instance, if your roofing contractors use 750 vents (each of 50 square inches NFA) for exhaust vents and 36 square inches NFA for intake vents, divide 480 by 50, as shown below.

  • Step 4: Exhaust Vents Calculation

480 ÷ 50 = 9.6

Rounding up the answer, you would need ten exhaust vents. A general rule of thumb is to increase the number of intakes NFA from exhaust NFA; the amount of exhaust provided is divided by the NFA rating of 36 square inches NFA for intake vents.

(10 × 50) ÷ 36 = 13.9 vents

Rounding up the answer, you would need 14 intake vents.

Click here to read about who to call for attic ventilation.


If your head is spinning because of these calculations on how much attic ventilation do I need, you don’t need to worry because the skills of Only Roofing professionals are not limited to the investigation of roof damages or their replacement. We ensure that your home has a proper ventilation system so you can save the stress and hassle of emergency roof repairs.