Putting Natural Forces to Work | |
Like just about everything else in life, a properly designed ventilation system requires balance. That balance is achieved in two ways: 1. Air flow capacity must be balanced between intake and exhaust vents. In general, the net-free area ( i.e., the total unobstructed area - usually measured in square inches- through which air can enter or exhaust a non-powered ventilation component) of intake venting should equal the net-free area of exhaust venting. 2. Intake and exhaust vents must be positioned to create a proper high-low balance. That balance is achieved when two conditions are met: a) Half the vent area must be high in the attic, with the other half low in the attic. Without that balance, the area of effective ventilation is limited to the lesser of the two vent areas. For example, if 75 percent of the venting is high and 25 percent low, ventilation is limited to the air moving through the lower vents. b) The vents placed high must act as exhaust vents, while the low vents act as intake vents. That placement assures a continuous flow of air, moving in the desired direction. In planning the location of intake and exhaust vents, two other factors must be considered: 1) Intake and exhaust vents must be positioned so they assure continuous air flow along the underside of the roof sheathing. During summer, air flow along the sheathing removes heat before it can radiate to the attic floor. During winter, air flow along the sheathing removes moisture before it can condense into water droplets or frost. 2) Intake vents must be located so there is little possibility of rain or snow infiltration. Obviously, if wind-driven moisture were allowed to enter an attic, one reason for a ventilation system would be negated. We would simply exchange one problem for another, similar problem. To prevent this from happening, intake vents should be placed in protected areas, the most convenient being in the soffit (area underneath the eave of the house). Placing intake vents in soffits doesn't assure that a strong wind won't drive moisture into the openings. But should that happen, the area around the soffit is less likely to suffer major damage. For one thing, insulation isn't installed in the soffit, so the problem of wet insulation is avoided. In addition, rain or snow entering a soffit vent is more likely to drain back through that opening. At worst, the moisture would be confined to the soffit area, where it can evaporate quickly without causing permanent damage. Note: To assure optimum performance of intake vents, you must make certain the area above the intake opening isn't blocked by dirt, building debris or attic insulation. Source: Air Vent Inc. A Certainteed Company Reprinted with permission, HouseNet, Inc. |