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How to calculate attic ventilation with the required net free ventilation area for an entire attic. First you need determine the area of the attic. Measure the width and the length. Multiply the width times the length to get the area. If the attic has several parts that all connect.

Calculate each part of the attic separately. Then add the areas together for the total area. Include any area that is common to the main attic space.

Even if that area is over an unheated part of the house for example if the garage attic doesn’t have a firewall separating it from the main attic then you include the garage attic in your calculation.

## The rule for most ventilation is called the one to 150 rule

You divide the total attic area by one hundred and fifty to get the net free ventilation area required. For example if the total attic area is fifteen hundred square feet you divide fifteen hundred by one hundred and fifty. The answer is ten square feet of net free vent area

Sometimes the one to 150 rule provides more airflow than is necessary to achieve adequate attic ventilation. You can use the one to three hundred rule instead.

If at least three feet of vertical distance separates the high and low vents and 50 percent to 60 percent of the vent area is installed as high vents or you can use the one to three hundred rule.

If either of these one to 300 Rule requirements can be met. Divide three hundred into the total attic area to determine the net free ventilation area you will need to install. Let’s do an example

By holding a tape up from the eve you can quickly see that there is more than a three foot vertical distance between the SOF it will install the low vents in the top part of the roof where we’ll install the roof fence. Also there is room along the roof to place at least half the ventilation area high

Because these conditions exist. We can use the one to three hundred ventilation rule to calculate the total vent area. The attic is measured and the total area is calculated to be fifteen hundred square feet. Divide fifteen hundred square feet by three hundred to get five square feet of net free vent area needed.

### Vents are rated and square inches

There are one hundred and forty four square inches and a square foot. So we multiply five square feet times one hundred and forty four to get seven hundred and twenty square inches.

This is the total net free vent area we will install at least half of this net free area needs to be high vents half of seven hundred and twenty is three hundred and sixty.

The ninety two no printed on these roof vents means each one has ninety two square inches of net free area. For roof vents gives us three hundred and sixty eight square inches of net free area.

Which meets the requirement that at least half the vent Area B high. We locate the roof fence evenly spaced along the roof on the side away from the street so they won’t change the appearance of the house from that side.

Now for the low vents the amount of vent area we need to install is seven hundred and twenty square inches minus the three hundred and sixty eight square inches and high vents or three hundred and fifty two square inches.

These SOF advance each provide 40 square inches of net free vent area nine vents times 40 equals 360 square inches. This gives enough net free area to meet the low venting requirement and still leaves 50 percent of the ventilation in high vents as seen here.

We always add vents evenly distributed on all sides of the house so that air flows throughout the entire attic. Remember you can have combinations of roof vents Gable vents events to achieve the same results. Keep in mind It’s very important to distribute the vents evenly so that all parts of the attic receive airflow.