How can you tell if you need more vents in your attic? There are numerous things to look for: Your ceiling feels warm to the touch on a hot day. The eaves of your roof don’t have any vents. You constantly get ice dams. Your attic feels damp. Most homes do not have enough vents in their attic. The good news is there is a formula to follow to know exactly how many vents you should have.
The formula to know how many vents you need is called the 1/150th rule or the 1/300th rule. The only difference is that you’ll use the 1/150th rule if you don’t have any kind of intake vent. The 1/300th rule is the is the standard we can follow. The purpose of this formula is to create a Net Free Ventilation Area (NFVA). Benefits include lower cooling bills and longer shingle life.
If math isn’t your thing, you might want to just call us. Lol. Here is the formula broken down step by step:
Step 1: Find the Square Footage of Your Attic.
The square footage of your attic will include the overhangs of your house and the garage if it’s attached. Formula: Attic floor space / 300 = Square ft. of NFVA. Example: 1,500 sq. ft. / 300 = 5 sq. ft. NFVA.
Step 2: Convert the Square Footage to Inches.
Ventilation accessories are calculated in square inches instead of square feet. Formula: Sq. ft. of NFVA x 144 sq. in. = sq. in. of NFVA. Example: 5 sq. ft. x 144 sq. in. = 720 sq. in.
Step 3: Figure Square Inches Needed for Intake and Exhaust.
Formula: Sq. in. of NFVA / 2 = amount needed for both intake and exhaust. Example: 720 sq. in. / 2 = 360 sq. in. of intake and 360 sq. in. of exhaust.
Step 4: Pick the Type of Ventilation to Install.
Divide the square inches of exhaust and intake separately by the number of square inches that the vent provides. Example: 380 sq. in / 50 sq. in (1 roof vent) = 7.6 Roof Vents.
If your house meets the 1/300th rule, you can expect lower cooling costs in the summer and longer shingle life. If you have any issue with a step or want a company that uses this formula every day, get in touch with Green Country Roofing at (918) 607-7409.