Remember how hot your attic was when you went up there to check the antenna wire last summer? Remember that ice dam on the roof? Proper attic ventilation can help with both those problems, but how do you know how much you need — and just as importantly, how much do you already have?
Determining what you need is simple — all you need is the size of your house and a calculator. Attic ventilation should equal approximately 1 square foot of vent area for every 300 square feet of attic, so figure out roughly how many square feet the footprint of your attic is, and then divide by 300. To ensure effective air movement throughout the attic, the total vent area should be split approximately evenly between high and low vents, so now divide that number by two to get a rough idea of how much low ventilation and how much high ventilation your home needs.
Finally, since vents are measured and sold based on square inches, you’ll want to convert from square feet. To do that, take the total amount of ventilation you need in square feet and multiply by 144 to convert it to square inches.
DETERMINING WHAT’S EXISTING
In order to determine how much ventilation you currently have, you need to measure the sizes of the existing vents, and then make a few adjustments in order to figure out exactly how much air is able to actually get through them.
Let’s say you have a 12-inch-by-18-inch gable-end vent. That equals an area of 216 square inches (12 x 18), and that’s how much ventilation area you would have if you left the hole wide open. However, to prevent animals, insects and rain from getting into your attic, you would need to install a gable-end vent, which has a screen and louvers on it. You have now reduced the amount of area that the air can pass though by the amount of area taken up by the screen, the louvers and the framework of the vent. The remaining open area that the air can actually pass through is called the net free area (NFA), and that is how vents are rated.
If you are purchasing new vents, the NFA should be printed right on the vent itself. If it isn’t, or if you are trying to figure out how much vent area you currently have with your existing vents, here are some common vents and their approximate net-free area:
7-inch round roof vent
30 square inches
8-inch round roof vent
40 square inches
9-inch round roof vent
50 square inches
12-inch x 18-inch gable vent
96 square inches
3.5-inch x 22.5-inch soffit vent
40 square inches
5.5-inch x 22.5-inch soffit vent
72 square inches
Continuous ridge vent
11 to 16 square inches per linear foot
For other types of vents, you can calculate the NFA using the following formula: Gross vent area / area factor = NFA. The area factor is how much of an adjustment you need to make for the screen and other obstructions, based on the following approximations:
area factor = 1.0
1/4-inch screen with louvers
area factor = 2.0
area factor = 1.25
1/8-inch screen with louvers
area factor = 2.25
Louvers, no screen
area factor = 2.0
So, using that formula, let’s say you have a big 12-inch-by-24-inch gable-end vent with 1/8-inch screen and louvers. The gross size of the vent is 288 square inches (12 x 24), and the area factor for 1/8-inch screen with louvers is 2.25. Divide 288 by 2.25, and you can determine that your vent has approximately 128 square inches of net free area.
ADD MORE VENTS AS NEEDED
If, after all this math, you determine that your attic does not have enough ventilation, you need to give some serious thought to adding more.
Separate what you need into high and low, and decide how many of each type of vent you need. Remember that the half and half ratio of high to low is only an approximation — if you have almost enough low vents and are short on high vents, you can add a little more high-vent area than you need to make up the difference.
Hardware stores, home centers and lumberyards all carry a wide variety of vents for different applications. You can install them yourself, or contact a licensed roofing contractor to have it done for you.
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