Why Ventilate? It's Only an Attic, after all... Just as houses have to breathe to create a healthy environment for the people living in them, attics also need to "breathe" in order to keep the roofing components "healthy," well-functioning, and structurally sound.
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Why Ventilate? It’s Only an Attic, after all…
Just as houses have to breathe to create a healthy environment for the people living in them, attics also need to “breathe” in order to keep the roofing components “healthy,” well-functioning, and structurally sound.
Seasonal Issues Affect the Structural Integrity of Your Home’s Roofing Components
Anyone who has ever been in the attic during the summer knows that the attic gets hot—very hot. All that heat is first absorbed by the wood sheathing, the framing lumber, and the ceiling joists, and then those roofing components begin to radiate heat. Eventually all that heat will begin to affect the structural integrity of roofing components, and it is for this reason that many manufacturers of roof shingles will consider the shingle warranty voided if an attic is unventilated.
During the winter months the major concern with an attic that is not well ventilated is moisture build up. Heated air from the living space below as well as moisture (from cooking, bathing, and washing clothes) leaks into the attic through the ceiling (even with the use of vapor barriers) through light fixtures and fans, access panels, and fold-down attic stairways. Condensation then begins to form on the framing lumber and the inside of the roof decking, leading to mold, mildew, and eventually wood rot. (Take a look at the ends of roofing nails sticking through the roof deck. If the ends of the nails are rusty, you have a moisture problem!) A second problem created by heat and moisture leaking into an un-ventilated attic is the creation of relative warm and cool spots on the roof deck, which may lead to “ice dams” when the roof is snow covered (and the inevitable roof leaks caused by water behind melting ice).
An Attic with Proper Ventilation Translates into Energy-Savings
There is an additional two-fold benefit in a well-ventilated attic. First, by keeping your attic cooler during the summer months, you effectively increase the efficiency of your ceiling insulation. As a result, your air-conditioner will operate more efficiently—and not consume as much energy—thus reducing your energy costs. And because your home’s entire HVAC system is operating more efficiently, unnecessary and costly wear-and-tear on the equipment is avoided.
How Much Attic Ventilation is Enough?
The Federal Housing Administration and most state and local building codes recommend a minimum of one square foot (144 sq in) of ventilation per 300 square feet attic floor space. (Bear in mind that this is a minimum recommendation. A number of factors specific to your home—geographical location, roof style and pitch, orientation to prevailing winds and weather extremes, and so on—as well as local building codes may require even more attic ventilation.)
Here’s an example of calculating the minimum attic ventilation required:
your home has a 50 ft by 32 ft attic area
multiply length by width (50 ft x 32 ft = 1600 sq ft of attic area)
divide the total square footage of the attic area by 300 (1600 sq ft / 300 = 5.3 sq ft)
Your home requires 5.3 sq ft of attic ventilation.
What Vents Should You Choose?
As anyone who has looked at the variety of vents on the market today knows, there are many from which to choose. Different styles, shapes, and even colors (to match your roof) are available. How do you choose? To some extent, the style of vent you choose is a matter of personal preference and aesthetics.
The important consideration, however, is a vent’s “NFA” measurement. NFA stands for “net free area,” a measurement of the unobstructed area through which air can pass freely. The NFA formula takes into account not only the opening of the vent itself, but also considers any screening the vent has to protect the attic from the invasion of birds and rodents. NFA measurements are usually given in square inches. All Ventamatic, Ltd. “static” (i.e., non-powered) attic vents carry NFA measurements.
Here’s an example of how NFA is used in choosing the vents for your attic: your home requires 5.3 sq ft of attic ventilation. Most NFA measurements are usually reported in square inches. Multiply the square feet of attic ventilation needed by 144 sq in (5.3 x 144 = 763.2 sq in). Always round up. The vents you choose to properly ventilate your attic must add up to 764 sq in.
There’s only one more factor to consider: your attic ventilation must also be balanced.
The “Balancing” Act: Use the 50-50 Rule
A well-ventilated attic breathes-in (intake ventilation) and breathes-out (exhaust ventilation) through vents located at the lowest and highest parts of your attic. Properly placed and with the correct “balance” of breathing-in and breathing-out vents create a constant flow of air through your attic, cooling it in the summer and preventing moisture build-up in the winter.
The most efficient attic ventilation system—and the one recommended by roofing engineers—is a balanced system of 50 percent intake vents and 50 percent exhaust vents, known as the 50-50 Rule, which allows the most efficient and constant airflow through the attic. The outside air enters the attic through vents placed in the soffits or undereaves and exits the attic (through convection) through vents placed at or near the ridgeline of the roof.
The ventilation needed for proper attic ventilation in the example above is 764 square inches. To “balance” the vents for this attic, 50 percent (382 sq in) of the vents must be allocated for intake ventilation and 50 percent (382 sq in) for exhaust ventilation.
When choosing vents for the attic in this example, be sure to “balance” the 764 square inches of NFA by placing 382 sq in of intake ventilation at the soffits or undereaves and 382 sq in of exhaust ventilation at or near the roof ridgeline.