Balance is the key to having an effective attic ventilation system. A properly designed ventilation system will assure that a continuous supply of air moves through the attic space, reducing heat and moisture. The lack of proper ventilation most often results in severe damage to asphalt shingles due to heat blistering. All roof systems that are poorly ventilated result in increased energy costs, and even mold growth in the attic caused by high moisture levels. Patriot Roofing makes it their business to insure that your new roof is installed with a proper ventilation configuration for optimum performance.
Surprisingly, many contractors, even builders, do not adhere to the requirements for proper attic ventilation. It is not rocket science but it is science.
Here is an example of the formula for calculating ventilation for one of the most common types of roof vents. These are commonly called slant backs or box style vents: Lomanco 750s.
The first step in determining how many vents are needed is to calculate the Net Free Area (NFA) required. Most codes use the 1/300 rule for minimum residential attic ventilation recommendations. This means that for every 300 square feet of enclosed attic space, 1 square foot of ventilation is required – with half at the upper portion (exhaust vents) and half in the lower portion (intake vents). This formula is traditionally used for static roof vents which are rated for Net Free Area in terms of square inches.
For a home with 2000 square feet of attic floor space, you’ll first divide 2000 by 300 (2000 / 300 = 6.66). You need 6.66 square feet of attic ventilation. Since you want a balanced system, you divide by 2 so that half of the ventilation is intake and half is exhaust. Thus, 6.66 divided by 2 = 3.33 square feet of attic ventilation for intake and 3.33 square feet of attic ventilation for exhaust. Because vents are rated in square inches, you need to convert the square feet required to square inches. This is accomplished by taking the square feet recommended and multiplying by 144. Thus, 3.33 X 144 = 480 square inches of attic ventilation is required for intake and 480 square inches for exhaust.
The next step is to divide the NFA required by the NFA rating of the vent. In our 2000 square feet example, we determined we needed 480 square inches for intake & 480 square inches for exhaust. For this example, let’s use the Lomanco 750 Slant Back Vent (50 square inches NFA) for the exhaust vents & the Deck-Air DA-4 (36 square inches NFA) for the intake vents. To calculate the number of 750 vents needed, divide 480 by 50 to get 9.6 vents. Rounding up, you would need ten (10) 750 Vents. Because you always want your intake NFA to meet or exceed the amount of exhaust NFA, we will take the amount of exhaust provided and divide by the NFA rating of the Deck-Air. To calculate the number of Deck-Air Vents needed, divide 500 (10 X 50) by 36 to get 13.9 vents. Thus, you would need 14 Deck-Air Vents.
Look for future posts with additional ventilation information utilizing ridge vents and solar powered electric vents.