Proper Ventilation a Must for a Healthy Attic

By Paul Bianchina

Proper attic ventilation serves two important purposes in your home - removing unwanted moisture and relieving the buildup of heat. Throughout the year, ventilation is necessary to allow moisture coming up from the house to be exhausted from the attic to the outside. Allowed to collect in the attic, moisture will do serious damage to the wooden framing members, and will also have a big impact on the effectiveness of your attic insulation. And during the summer months, properly installed attic ventilation will also exhaust hot, stale attic to the outside, which keeps the home's living spaces cooler and will help to prolong the life of wood and composition roofing materials.

Properly installed, attic ventilation works on the natural passive movement of air from a cool location to a warm one. For the typical attic, this means a combination of low vents along the eaves of the roof, and high vents along roof's ridge. Since the air in the attic is warmer at the ridge than it is at the eaves, the natural upward movement of air from cool to warm creates a current -- lower temperature air is drawn in through the low vents, pushing the higher temperature air out the high vents. While the movement of air is more dramatic in the summer when attic temperature differentials are higher, this movement actually occurs at all times and in all temperatures.

A simple formula for calculating ventilation requirements is to figure one square foot of ventilation area for each 300 square feet of ceiling area. Thus, for a 1,500 square foot house, 5 square feet of ventilation would be required. If the attic above the garage is continuous with the house attic, then be sure and add in the square footage of the garage as well.

Net Free Area

If you were to purchase a vent that is 12” x 12” (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, all types of attic vents, and also foundation 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 -- not the overall size of the vent -- in making your calculations for how many vents you need.

Types of Vents

A wide variety of different vents are available, depending on their intended location in the attic. These include:

• Soffit vents, which are a metal framework with louvers on one side and insect screen on the other designed for surface mounting on closed soffits.
• Eave vents, a rectangular screened metal frame for use between the rafters or roof trusses in open soffit construction. Because vents of this type are installed in a relatively protected area under the roof overhang, no louvers are required to prevent rain from coming in, so the vent's NFA is higher.
• Roof vents, which are covered, screened vents for use up on the roof, typically along the ridge. Because of their exposed location on the roof, the cover is designed to shed rain and snow, and the vents are available in brown or gray plastic or natural galvanized sheet metal to blend as much as possible with the color of the surrounding roofing.
• Gable vents. This are large screened vents for use in a gable end wall, allowing a large amount of NFA to be placed high up in the attic, near the peak of the roof. Gable end vents, since they're actually placed in the wall and not up on the roof, are the most visible of the vent types, and so need to be the most aesthetically pleasing. As a result, you'll find gable vents in galvanized and bronze-colored metal, natural and pre-primed wood, and maintenance-free vinyl, as well as in square, rectangular, triangular, round, oval and hexagonal shapes.
• Continuous ridge vents. These are long strips of open, screened vent, typically made from vinyl, that are installed along the ridge of the roof. The roof sheathing is cut back along both sides of the ridge to allow air to leave the attic, then the continuous ridge vent is installed over the sheathing.
• Custom and site-built vents. For open soffit construction, you can utilize simple site-built vents by drilling holes in the blocking between the rafters or trusses and covering them with insect screen, or by cutting wide slots in the panels used to cover closed soffits, again covering them with screen. Vents in custom sizes and configurations are also available from sheet metal shops and some cabinet or woodworking shops.

 

With whatever type of vents you use, remember to keep them free of insulation and other debris which 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.

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