Attic ventilation

Calculating Attic Ventilation to Prevent Radon

Properly ventilating the attic, as well as the basement can be a helpful way to keep radon levels from climbing, so that you won’t need radon mitigation in the future. Long story short, having too little air circulation in the attic can cause problems, so you need to know how much ventilation would be best for your space. You also need to have the right balance between intake and exhaust. Calculating how much ventilation is needed for your attic space is simple once you know the square footage of the attic.

Step 1: Measuring the Square Footage

To find the square footage of your attic, measure the length and width of the floor, and then multiply the numbers together. So, if your attic is 25 ft. long and 40 ft wide, the square footage would be 1,000 sq. ft. (25×40= 1,000).

Step 2: Finding the Total Net Free Ventilating Area (TNFVA)

The Federal Housing Administration recommends that there be one ventilation unit per every 150- 300 square ft., depending on the age of your home. For the purpose of this equation, we’ll assume the home is newer and needs ventilation for every 150 square ft. Divide your total square footage by 150 to determine the total square footage of venting you need for the attic. 

If we use our previous example, you would divide your 1,000 sq. ft. by 150. (1,000 ÷ 150 = 6.66, rounded up to 7). You’d need about seven ventilation units for 1,000 square footage. 

Step 3: Converting Square Ft. to Square Inches

Ventilation is measured in square inches, so you will need to convert the measurement from square feet to square inches in order to get an accurate quote for your ventilation projects. Multiply the square footage number by 144 to convert it to square inches. (7 x 144 = 1,008).

Step 4: Calculating Intake and Exhaust Requirements

The attic needs a way for air to come in, as well as a way for air to go out. The intake and exhaust ventilation is not a 50/50 ratio, however. The ventilation will be more effective if 60% of it is intake, and 40% is exhaust. Multiply your square inches of total ventilation by .6 to get the needed square inches of intake ventilation. (1,008 x .6 = 604.8, rounded up to 605). Next, multiply the square inches of total ventilation by .4 to get the needed square inches on exhaust ventilation. (1,008 x .4 = 403.2, rounded down to 403). Your intake ventilation will require 605 inches of your 1,008, and your exhaust will require 403.

Once you know how many square inches of ventilation the attic needs for both intake and exhaust, you’re ready to call around and get some estimates for having the work done. These calculations are an example of how calculating your need for ventilation can be done. There are some other factors that affect these numbers. For example, older homes need less ventilation in the attic because they are less efficient throughout. Calculating approximately how much ventilation your attic needs will help you to be prepared when getting quotes for having the work done. Double-check your numbers with a professional before the installation process begins. And remember that the only way to ensure that your home doesn’t have unsafe levels of radon is to get tested!

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