VENTILATION REQUIREMENTS

How much ventilation your attic needs depends on the size of your house and, to some degree, its shape. To determine ventilation requirements, most building codes rely on a simple mathematical formula of 1 square foot of ventilation area for every 300 square feet of attic area. For example, if your home has 1,500 square feet of living space, you would need 5 square feet of vent area to provide an adequate amount of air flow (1,500 square feet divided by 300 = 5).

Since it is the passive movement of the air through the attic that creates the ventilation, the placement of the vents is a very important consideration in how effective they will be. They need to be installed so that roughly half are in high locations along the ridge or in the gable ends, and half are placed low along the eaves.

Attached garages can add to the ventilation load of the home as well. If your home has an attached garage and the attic of the garage is continuous with the attic of the house, then the square footage of the garage needs to be included as well. For example, if your 1,500 square foot home has a 500 square foot attached garage and the attics are continuous with one another, then the required vent area goes from 5 square feet to 6.67 square feet (1,500 square feet + 500 square feet = 2,000, divided by 300 = 6.67).

If the garage is attached to the house but the attics are not continuous, you have a slightly different situation. Because the attic of the garage is still going to get warm (even if the garage does not have a ceiling), that heat is still going to have an impact on both the garage roofing and the heat being transferred to the house, not to mention on the garage itself and all its contents. Therefore, the garage attic needs to be ventilated as well. You can use the same 1:300 formula, but the square-foot requirements and the layout of the vent locations for the garage should be considered independently of the house attic.

NET-FREE AREA

If you were to purchase a vent that is 12 inches by 12 inches (one square foot) in overall size, you would not actually be getting one square foot of ventilation area. The framework of the vent and especially the insect screening in it reduces the overall amount of area that the air can actually pass through -- sometimes by as much as half.

For that reason, vents are rated in net-free area (NFA), which is the actual amount of open ventilation area that the vent contains after deducting out all of the space taken up by the frame and the screening. The exact NFA will be printed directly on the vent by the manufacturer, and it's important to utilize this number as opposed to the overall size of the vent in making your calculations for how many vents you will need.

With whatever type of vents you use, remember to keep them free of insulation and other debris that reduce their effectiveness, and to be certain that all bathroom, kitchen and other exhaust fans in the house are vented all the way to the outside, not into the attic.

Remodeling and repair questions? E-mail Paul at paul2887@hughes.net.