sgalbo
10-26-2004, 12:40 PM
Ventilation of the home attic is important for two reasons. During the summer, excess heat that builds up in the attic during the day results in high energy costs for cooling. Also, moisture produced within the home may move into the attic if ceiling vapor barriers are not used. If this moisture is not exhausted from the attic it can condense and cause insulation and construction materials to deteriorate. Thus, temperature and moisture control are the major reasons for providing attic ventilation.
Ventilation Methods
Attic ventilation can be accomplished by gravity ventilators, wind assisted ventilators or power ventilator.. Regardless of the method used, the purpose is to provide uniform ventilation of the attic for proper temperature and moisture control.
Natural ventilation is the most common and energy-efficient method of achieving attic temperature and moisture control. This method takes advantage of two principles. First, as air is heated it becomes less dense and rises. Second, wind movement over and around a home creates areas of high and low pressure. If a space has high air outlets in conjunction with low inlets, ventilation occurs as the air within the space is heated. The greater the vertical distance between the outlet and inlet, the greater the ventilation rate will be. Thus, roof outlets should be at or very near the ridge, and inlets should be under the roof overhang or near the ceiling line.
Ventilation caused by wind pressure differences requires less vent area to achieve the same ventilation rate as ventilation by gravity. One problem with wind ventilation is that the areas of high and low pressure change with wind direction, causing difficulty in locating inlets and outlets so that ventilation will take place regardless of wind direction. The best system is one in which the outlet is near the ridge and the inlets are in the soffit area. The hip roof is best suited to this system, since there is soffit area on all sides of the house. Outlet vents can be provided by roof louvers, gable end louvers, turbine ventilators or continuous ridge vents. These should be constructed so that rain and snow cannot enter. Water leakage is more likely with the turbine ventilator and gable end louver than with fixed roof or ridge ventilators.
Inlet vents may be of the manufactured type, either slotted or perforated, and installed as individual units or in a continuous strip. The slotted opening seems to resist clogging by fibers and dust better than the perforated opening. Screen wire is sometimes used for soffit vents, but again clogging is a problem. In some localities frequent cleaning may be necessary with either type.
Providing for Natural Ventilation
Natural ventilation caused by a chimney effect or by wind movement is the most economical ventilation method. The quantity of ventilation air depends on opening size, temperature rise and wind movement. Tests have shown that effective natural ventilation can be achieved by providing inlet and outlet open vent areas of approximately 1 square inch per square foot of attic area, when roof slopes of 3/12 to 5/12 are used. This vent area should be the net open area rather than gross vent size. Some prefabricated vent materials may have only 60 percent net open area. Thus; if a vent area of 10.4 square feet is required, the gross vent area would be 10.4 / 0.6 = 17.3 square feet. [National standards appear to require only 1/2 this amount of open vent area. The open vent area calculated by these formulas is to be divided roughly equally between inlet and outlet.]
Suppose a 1,500-square-foot home requires the above soffit vent area. If the total soffit length of the home is 100 feet, the vent width for a continuous soffit vent would be (17.3 x 12) / 100 = 2.1 inches. Ventilators should be purchased on the basis of net ventilation opening.
Roof overhangs of 12 inches or more provide ample space for inlet (soffit) vents. Outlet vents should have the same net area as inlet vents. This is easily provided by continuous ridge vents or individual roof vents for gable-roofed homes. The roof ridge length may not be long enough on a hip roof to use a continuous ridge ventilator. In this case, several individual roof vents, well spaced near the ridge, should be used.
Ventilation Methods
Attic ventilation can be accomplished by gravity ventilators, wind assisted ventilators or power ventilator.. Regardless of the method used, the purpose is to provide uniform ventilation of the attic for proper temperature and moisture control.
Natural ventilation is the most common and energy-efficient method of achieving attic temperature and moisture control. This method takes advantage of two principles. First, as air is heated it becomes less dense and rises. Second, wind movement over and around a home creates areas of high and low pressure. If a space has high air outlets in conjunction with low inlets, ventilation occurs as the air within the space is heated. The greater the vertical distance between the outlet and inlet, the greater the ventilation rate will be. Thus, roof outlets should be at or very near the ridge, and inlets should be under the roof overhang or near the ceiling line.
Ventilation caused by wind pressure differences requires less vent area to achieve the same ventilation rate as ventilation by gravity. One problem with wind ventilation is that the areas of high and low pressure change with wind direction, causing difficulty in locating inlets and outlets so that ventilation will take place regardless of wind direction. The best system is one in which the outlet is near the ridge and the inlets are in the soffit area. The hip roof is best suited to this system, since there is soffit area on all sides of the house. Outlet vents can be provided by roof louvers, gable end louvers, turbine ventilators or continuous ridge vents. These should be constructed so that rain and snow cannot enter. Water leakage is more likely with the turbine ventilator and gable end louver than with fixed roof or ridge ventilators.
Inlet vents may be of the manufactured type, either slotted or perforated, and installed as individual units or in a continuous strip. The slotted opening seems to resist clogging by fibers and dust better than the perforated opening. Screen wire is sometimes used for soffit vents, but again clogging is a problem. In some localities frequent cleaning may be necessary with either type.
Providing for Natural Ventilation
Natural ventilation caused by a chimney effect or by wind movement is the most economical ventilation method. The quantity of ventilation air depends on opening size, temperature rise and wind movement. Tests have shown that effective natural ventilation can be achieved by providing inlet and outlet open vent areas of approximately 1 square inch per square foot of attic area, when roof slopes of 3/12 to 5/12 are used. This vent area should be the net open area rather than gross vent size. Some prefabricated vent materials may have only 60 percent net open area. Thus; if a vent area of 10.4 square feet is required, the gross vent area would be 10.4 / 0.6 = 17.3 square feet. [National standards appear to require only 1/2 this amount of open vent area. The open vent area calculated by these formulas is to be divided roughly equally between inlet and outlet.]
Suppose a 1,500-square-foot home requires the above soffit vent area. If the total soffit length of the home is 100 feet, the vent width for a continuous soffit vent would be (17.3 x 12) / 100 = 2.1 inches. Ventilators should be purchased on the basis of net ventilation opening.
Roof overhangs of 12 inches or more provide ample space for inlet (soffit) vents. Outlet vents should have the same net area as inlet vents. This is easily provided by continuous ridge vents or individual roof vents for gable-roofed homes. The roof ridge length may not be long enough on a hip roof to use a continuous ridge ventilator. In this case, several individual roof vents, well spaced near the ridge, should be used.