August 2003 Edition

How Much Attic Ventilation

Our attic heats up enough in the summer to bake a pizza. How much ventilation do we need and what is the best type to install?
If you’re like most homeowners, you know the importance of insulating and tightening up your house to conserve energy. However, what you may not know is that certain areas of a house need to breathe. One of these areas is the attic, which requires proper ventilation not only to maintain comfort below, but also to keep the very roof over your head solid and secure!
There are two particular villains that proper ventilation will fend off. These are heat and humidity.
Heat comes from the sun, and in summer a poorly ventilated attic can reach temperatures as high as 180°F - which means that even with insulation in the attic floor, the rooms below will be hotter than necessary, less comfortable, and more expensive to air condition. Excess heat can also shorten the life of some roofing materials.
Humidity comes primarily from within the house, drifting upward from showers, unvented clothes dryers, humidifiers and kitchen ranges. It also comes from other, not so obvious sources. The very act of breathing expels water into the atmosphere - at the rate of 1/2 pint per hour for the average family of four! Mopping the kitchen floor (about 150 square feet) releases 4 1/2 pints of water, and washing the dinner dishes - 1/2 pint. A windblown rain can also cause water to enter and evaporate into the attic area through roof leaks.
During cold weather, water vapor may condense in various areas of an insufficiently ventilated attic, seeping into wooden rafters or roof sheathing and rotting them. Moisture in the attic area can cause roof shingles to buckle and insulation to lose its effectiveness. It also creates an environment that is conducive to mildew.
In short, you need proper attic ventilation to help:
• Prevent structural damage caused by moisture
• Increase the life of the roofing material
• Reduce energy consumption
• Enhance the comfort level of the rooms below the attic.
Now that you know why it is crucial to maintain adequate ventilation in your attic, how do you do it? There are a variety of ways, and the right one will depend on the style and structure of your own roof.
First, it is wise to determine whether or not the existing ventilation is adequate. You can do this by placing a thermometer in the attic on a warm windless day to see if the temperature that is being maintained is more than 15° to 20°F warmer than the outside temperature. If it is, then more ventilation is needed.
The minimum attic ventilation recommendation, set by the Federal Housing Administration, is one square foot of free vent area for each 150 square feet of attic floor - if there is no vapor barrier under your insulation. Most homes in the Atlanta area do not have a vapor barrier.
Once you’ve determined your ideal total free vent area, then you need to divide it roughly in half for:
• Inlet vents, which should be located under the eaves (called the ‘soffit" area) or low on the roof face, and
• Outlet vents, which should be located at the roof ridge, in gables or cupolas, or otherwise near the top of the roof.
Since hot air rises, this type of system takes advantage of a natural "chimney effect," and air movement will be created through the attic even when there is no wind. (Wind will cause an even greater movement of air.)
The "ridge and soffit" vent combination can be applied to the majority of roofs in this country, which are gable style or pitched. In most cases, houses of this type feature louvered openings in the end walls of the roofs; however, unless these vents are perpendicular to the predominant breezes, their effectiveness is limited.
In regions of the country where the heat is extreme, attic ventilation can be enhanced by the use of a wind turbine exhaust vent. On a hot, still day, the heat rising up in the attic will start the turbine spinning - and the more heat going out, the faster it will spin. Add a little wind, and you’ve got almost a self-propelled vacuum cleaner!
• To maintain the most efficient attic ventilation, make sure that vents from your bath, kitchen and laundry are not routed to the attic but instead go directly to the outside.
• Never block off your attic ventilation in winter since moisture generated inside the house that rises to the attic can cause more problems in winter than in summer. With proper insulation between the attic floor and ceiling below, the ventilation will not lower the temperature in the house.
• If you are unsure about the correctness and efficiency of your attic ventilation, consult an ASHI home inspector. He is highly qualified to judge the current condition of your ventilation and to recommend methods to increase its effectiveness.

Source: American Society of Home Inspectors

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Theodore Roosevelt

A Tip Of The Hat To:

Patty Burke

Northside Realty

200 South Main Street

Alpharetta, Georgia 30004

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