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Home Improvement

2002-09-01 09:27:00

Attic ventilators

Question:  Why are ventilators important in the attic? How much good do they actually do? Should you install them if your home is older? How many do you need?Answers:  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. How much attic ventilation is required to provide proper temperature and moisture control? A number of studies sponsored by federal energy funds are under way to look at ventilation rates and methods. However, several studies previously made can help with this decision. The maximum ventilation rate is required to remove heat during the summer cooling months. Attics can reach temperatures of 150 to 160 degrees F during a summer day, although outside air temperatures are only 95 to 97 degrees F. The cooling load for a home air conditioner depends on the difference in temperature between the inside and outside air, and reduction of attic temperatures from 155 degrees to 105 degrees F will result in a significant reduction in cooling load. In a home with poor ceiling insulation, heat movement through ceilings may account for 30 percent or more of the total cooling cost. With a well-insulated ceiling, this source of heat may account for only 12 to 15 percent of the total cooling cost. Thus, high attic ventilation rates are most important for poorly insulated ceilings. A poorly insulated ceiling is one whose R rating is less than 14 or one with fewer than 4 inches of fiberglass, rockwool or cellulose insulation. Attic temperature depends on the amount of solar radiation, construction details and the rate of ventilation. Calculations indicate that on a July day in Texas, a ventilation rate of one air change per minute for a typical attic using 95-degree F air will lower the peak attic temperature to about 101 degrees F. Providing half air change per minute will lower the temperature to about 106 degrees F. Thus, the first half change per minute is most effective and a doubling of this rate only achieves about 5 degrees F additional cooling. Studies indicate that further increases in ventilation are not effective in significantly reducing attic temperatures. Winter attic ventilation must be sufficient to remove moisture vapor moving from the living space to the attic. In general, ventilation adequate for summer cooling is more than adequate for winter ventilation. Winter rates need not be more than about a tenth of the summer rate. Calculate the required summer ventilation rate by determining the volume of attic space and dividing by 2. This will be the cfm (cubic feet per minute) of ventilation air needed. The volume is determined approximately for a rectangular house by multiplying the height from the ceiling to the peak/ridge (H) times the width of the house (W) times the length (L) and dividing by 2 -- ( H x W x L / 2 ). Remember if you only want to provide one half change per minute, divide the above volume by two.  (you divide by 2 twice in this scenario, once to get the volume and again to cut it in half.)  For a gable roof, this will be reasonably accurate. For a hip roof house, the volume will be overestimated but adequate. Another good way to determine required ventilation is to consult the building code.  The Wichita code states that "the net free ventilating area shall not be less than 1/150 the area of the space ventilated.  There are two exceptions. (1.) The area may be 1/300 of the area of the space ventilated provided 50% of the required area is provided be ventilators located in the upper portion of the space to be ventilated at least 3 ft above the eave or cornice vents with the balance of the required ventilation provide by the eave or cornice vents.  (2.) The area may be 1/300 of the area of the space ventilated provided a vapor retarder having a transmission rate not exceeding 1 perm is installed on the warm side of the attic.On homes with gable ends, I like to use triangular vents which are installed as high as you can get them.  On hipped roofs, I like to provide at least half the required ventilation in the form of roof vents placed high on the roof and the remainder provided by soffit vents.  The Gable ends make use of the Kansas winds and the combination of roof vents and soffit vents causes convection currents to help do the job.