April2017

loss as this will cause the homes air con- ditioning system to work harder to make up for the lost cool air. In extreme cases, such as when the attic has very limited open soffit ventilation, the pressure creat- ed by the fan to draw of air into the attic creates a negative pressure environment within the home. Negative pressure in a home can be harmful and ideally, the home should have a slightly posi- tive interior pressure. Negative pressure within the home can pull excessive mois- ture from the exterior into the building envelope through the floors, walls and roof assembly. Negative pressure can also cause a backdraft of exhaust air from combustion appliances, such as older non-direct vent gas fired furnaces and water heaters. Most attic fans now have warnings in their installation guide- lines regarding potential back drafting issues that may occur when the fans are installed near combustion appliances. Many older model homes have make up air ductwork between laundry and mechanical closets that provide a direct pathway from the combustion applianc- es to the attic space. Passive Ventilation System In a “passive” ventilation system, air movement is provided by the natural convection of air that causes warm air to rise. A properly designed passive ventilation system consists of an equal amount of upper ridge/dome vents and lower vents at the eave or soffit. As the warm air escapes out of vents installed at the ridge, cool air is pulled in through the lower soffit vents. This creates an even amount of ventilation throughout the entire attic space. Passive ventilation is often preferred as it can function through all sea- CONT I NU E S ON PAGE 70

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