What Homeowners Should Know About Attic Ventilation.by Parameters Construction on 09/12/12
Inadequate or improper ventilation will cause moisture damage.
Aside from leaks or not enough insulation, a more common cause of moisture damage is inadequate ventilation. So how do you know if you have enough ventilation? To determine this, you must first know the square footage (area) of your attic.
Say your house (excluding garage if attached) measures 30 feet by 40 feet. 30 feet x 40 feet = 1,200 square feet. Most building codes require one square foot of ventilation for each 150 square feet of attic floor space—assuming there is a vapor barrier or the proper ratio of exhaust to intake vents (50% upper exhaust vents / 50% intake vents).
Using our example above with an average attic and average pitch, you simply
divide 1,200 by 150. 1,200 square feet divided by 150 = 8 square feet. This
means you need a total ventilation area of 8 square feet. Remember though, the
ideal ventilation ratio is 50% exhaust and 50% intake, so divide 8 by 2 to
achieve the ratio of exhaust to intake. 8 square feet divided by 2 = 4 square
feet. (Note: If your roof pitch is 7/12 to 10/12, add about 20% to your
calculation; add 30% for pitches greater than 11/12).
We now know you need 4 square feet of attic ventilation for intake, and 4 square feet of attic ventilation for exhaust. But before you assume you’re ventilation is fine as is, or you run out and install an extra roof vent, it is important to determine how your current set up works. Remember, the goal is to keep your attic as consistently cool as possible—out with the hot air, in with the cool. The wrong placement, combination or amount of vents will short-circuit the ideal airflow. Also, a guarantee on shingles will likely not be honored by the manufacturer without sufficient ventilation. Most experts agree that a good configuration is 50% intake in the soffit area and 50% exhaust at the ridge of the roof.
The last calculation requires us to convert square feet to square inches. For this, we simply multiply the required 4 square feet by 144. 4 x 144 = 576 square inches, so we need 576 square inches of intake and 576 square inches of exhaust.
Exhaust: Nothing beats a ridge vent in our opinion. An average ridge vent produces 18 square inches of ventilating area per linear foot. Since our example roof is 40 feet long, we would get 720 square inches of exhaust space. (18 x 40 = 720). Divide 720 by 144 and we see that 720 square inches equals 5 square feet. So our ridge ventilation has exceeded the minimum requirement of 576, which is perfect!
Intake: An average soffit vent measures 16 inches by 4 inches creating an intake an area of 64 square inches. We know we want 576 square inches of intake, so we can divide 576 by 64. 576 divided by 64 tells us that 9 soffit vents are required; but let’s round up to 10.
There you have it. Of course the figures provided are for example only, and you should always consult with an expert, but now you have a good idea whether you roof and attic are ready for winter!