Soffit Ventilation Size Requirements: Calculation

Soffit vents are the most popular type of intake vents for attic ventilation. They are installed just below the eves of the roof and they provide an easy, low maintenance path for air to enter an attic.

Soffit vents should have a total net free area of 1 square foot per 250 square feet of attic area. It can be reduced to 1 sq.ft per 500 sq.ft of attic space if the attic has a vapor retarder. This vent area should be divided equally between the soffit areas on both sides of the roof.

Soffit vents installed on a roof

I am pretty sure you are wondering how I calculated this size. Read on to find out;

Soffit vent size calculation

As per the International Residential Code, a ventilated attic needs a net free vent area of 1/150 sq.ft of the total attic area.

Example: If an attic is 1500 sq.ft in size then it needs a net free vent area of 10 sq.ft.

As per the code, this net area should be divided equally between intake vents and exhaust vents. Since soffit vents are intake vents and since most houses use them, let’s assign this 50% to soffit vents.

However, in practice, many experts recommend using 60% of the total net free area for intake ventilation. So in this case the soffit ventilation area would be 60% of the 1/150 sq.ft of the total attic area.

ie, Soffit ventilation = (60/100) x (1/150) x Total attic area = (1/250) x Total attic area.

So if you have an attic of 1500 sq.ft then the soffits will take up 6 sq.ft of net free area.

Calculation of soffit ventilation size

But, the IRC also maintain that the attic ventilation needed is only 1/300th of the attic space if both of the two conditions below are met;

  1. The roof should use a vapor retarder in its warm-in-winter side
  2. 40 to 50% of the vent area is located not less than 3ft from the ridges, and the rest of the vents should be located at the bottom one-third of the attic space.

If the above two conditions are met, then our formula for soffit vent calculation changes to;

Soffit vent area = (60/100) x (1/300) x Total attic floor area = (1/300) x Total attic floor area.

Example: If the total attic space is 1500 sq.ft in size then the total soffit ventilation required is 3 sq.ft.

The total soffit vent area should be equally divided between the soffits on both sides of the roof, and they should be spaced equally.

How many soffit vents do you need

Soffit vents are covered with ‘covers’ to prevent insects and such things from entering the attic. To find the number of soffit vents you need, divide the total soffit vent area of the house by the ‘Net Free Area’ of the soffit vent cover.

So what is this Net Free Area?

The net free area of a vent is the area available for the air to pass. Soffit vents are covered by vent covers which are basically holes/meshes drilled on a cover. These meshes obstruct a portion of the airflow. So if you cut a 3 sq.ft vent on your soffit and use a vent cover over it, the effective area through which the air can flow will be much lesser. This effective area is the net free area, and it changes with the type of vent cover you use.

In many cases, the net free area of a vent cover is not specified. This can be a headache for DIYers. So here is a size guide I saw on the website atticbreeze.netOpens in a new tab. which I find very useful;

Soffit cover typeNet free areaMultiplier
1/4 inch mesh screen100%1.00
1/4 inch mesh screen with rain louvers50%2.00
1/8 inch mesh screen80%1.25
1/8 inch mesh screen with rain louvers40%2.50
1/16 inch mesh screen50%2
1/16 inch mesh screen with rain louvers25%4
Continuous soffit with rain louvers27%3.70
Continuous soffit with expanded metal49%2.04
Continuous soffit with 1/4 inch ventilation holes53%1.89
Continuous soffit with 1/8 inch ventilation holes33%3

To know the total size of the soffit vent to cut, multiply the net free area calculated with the corresponding multiplier of the soffit cover from the above table.

In one of our above calculations, we found that the total net free area of soffit vents for a 1500 sq.ft attic space is 6 sq.ft. If you are using 1/4 inch mesh screens with rain louvers then the vent you cut on the soffit should have an area of 2 x 6 sq.ft = 12 sq.ft.

Can you have too much soffit vents?

Yes, you can have too many soffit vents as long as they are divided equally between both sides of the roof. Having too much intake ventilation is not a problem it is having too many exhaust vents that is the problem.

In a passive ventilation system, the cool air enters the attic through the soffit vents due to a negative pressure created inside the attic by the wind blowing over the exhaust vents. Here the volume of the intaken air is the same as that of the exited air, there is no way the attic will develop a positive pressure due to the excess number of soffit vents. So having too many soffit vents doesn’t have any impact on passive attic ventilation.

In the case of an active ventilation system ie, when you have attic fans, the amount of air intake is regulated by the size of the fans. If you have a very powerful fan, having some extra soffit vents will balance out any negative pressure created in the attic.

You can read more about this in my post here.

Soffit vent installation

Installation of soffit vents is pretty easy especially if you have a broad soffit area. There are 3 types of soffit vents; rectangular, circular, and continuous. You can use any of these three for the roof. It is recommended to add the vents in between the rafters of the roof and away from foam insulation to get unobstructed airflow.

Items needed:

  1. Vent covers
  2. Drill
  3. Reciprocating saw
  4. Screwdriver/staple gun

Step 1: Calculate the soffit vent size using the calculation above. Make sure that you account for the net free area of the soffit cover.

Step 2: Calculate how many soffit vents are needed based on the size of the soffit covers you bought.

Step 3: Cut vents on the soffit based on the vent area needed and the size of the cover. The size of each vent should be smaller than the vent cover.

Step 4: Install the vent cover over the vent.

Here is a video of soffit vent installation;

Can soffit vents be used on a flat roof?

Soffit vents can be used on a flat roof for fresh air intake. Flat and low slope roofs are difficult to ventilate when compared to regular roofs. This is because in a flat roof there is no or little chimney effect to properly exhaust the hot air.

A flat roof is mainly ventilated using exhaust fans. But for the exhaust fans to work, there must be air intake, and soffit vents are the only option here.

Are soffit vents required by code?

Soffit vents are not required by code, any type of intake vents will do. Residential codes require that a ventilated attic should have more than 50% of intake vents but they don’t specifically say it should be soffit vents. Therefore it is safe to say that soffit vents are not required by building code.

Soffit vent alternatives

Soffit vents are the most popular type of intake vents for attics. However, there are other options as well. Here are they;

  1. Eyebrow vents
  2. Gable vents

Eyebrow vents

Eyebrow vents are low profile vents installed on the face of a roof. They are installed on the lower portion of the roof to maximise intake airflow.

Gable vents

Gable vents are installed on the gable side of the roof. Intake gable vents are positioned to the lower side of the attic to maximize intake airflow.

Charles John

A novice DIYer who learns about home ventilation. I am a mechanical engineer and have a basic knowledge of HVAC systems but I learn continuously to make myself the best blogger in that space.

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