The need for proper attic ventilation cannot be overstated enough. Bad attic ventilation can raise energy costs, cause roof warping due to moisture buildup and lead to the formation of ice dams in the winter. These problems can lead to roof leaks, gutter damage and increase your annual maintenance costs considerably. A well-vented roof, with proper intake and exhaust venting not only provides numerous benefits to the structural integrity of your roof and attic, but will end up saving you money in energy bills by removing heat in warm seasons and moisture in cold. Calculating the proper amount of ventilation for your home is quite easy, all you need is the measurements of your attic and a calculator.

Step 1) Find the square footage of your attic. If you have a garage, include this as well since proper garage ventilation confers similar benefits. For example, if your attic space is 50 feet x 40 feet, then you multiple the numbers together and get 2000 feet. 50 × 40 = 2000 square feet.

Step 2) Divide the final number by 150. This number is called the Total Net Free Ventilating Area (TFNVA) and refers to the fact that there should be proper ventilation for every 150 square feet of attic space. The number is a recommendation by the Federal Housing Administration, which recommends between a 1/150 to a 1/300 ratio depending on the age of the home. If you have an older home then a 1/300 ratio is suitable, since the home is probably less energy efficient and thus less airtight. The building codes of new homes over the past 20 years are much stricter, making them more airtight and therefore requiring additional ventilation.

Let’s use the maximum recommended TNFVA of 1/150 to get maximum ventilation: 2000 ÷ 150 = 13.33. This is the total square footage of attic venting needed in your home.

Step 3) Convert square feet into square inches: 13.33 × 144 (the number of square inches in a square foot or 12 × 12) = 1919.52 (you can round this up to 1920). This is the total square inches of venting you need. Since vents are measured in inches it is easier to do the final vent calculation if you use inches.

Step 4) Calculate the intake and exhaust venting percentage based on 60% intake vents (soffits and eaves) to 40% exhaust vents (near the roof ridge). You could make simpler calculation of 50/50, but most experts recommend a 60/40 split. So 1920 × .60 = 1152 square inches for intake vents and 1920 × .4 = 768 square inches for exhaust vents.

Step 5) Choose and install the right number and types of vents needed for proper attic ventilation. Different types of vents provide various rates of venting efficiency and also a different visual aesthetic to your home, so choose carefully.

Additional Attic Ventilation Resources:

• 7 Common Problems Caused By Poor Attic Ventilation
• Homeowner Guide: Inspecting Your Attic Before Winter
• A Guide to Installing Attic Ventilation