The need for make-up air
When exhaust fans operate they pull air from the house-air which needs to be replaced, somehow. In the past, this was done with air that leaked into the building around windows, doors, or foundation cracks. In a well-sealed home, however, there are very few of these leaks. Without an additional source, the fans pull air down chimneys, clues, or other exhaust fans, leading to the entry of dangerous combustion products, or other contamination. The solution is to provide sufficient air to make-up the air that is being exhausted. This is usually done with an insulated duct that enters the house (often in the basement). When properly installed, this dust will only allow the entry of air when needed.
Building code rehires bathrooms have either an operable window or a fan to remove odor and excess moister. A bathroom fan is often quicker and more effective than opening a window – especially during heating or cooling season. In order to minimize moister buildup and mold, it is essential that bathroom fans be installed properly, using short, insulated ducts with no bends or kinks. Additionally, fans must be air sealed at the ceiling and insulated to prevent heat loss into the attic, and ducts must be sealed at all joints and connections, all though to the exit point at the roof or wall. ENERGY STAR rated fans are not only much more efficient, they also are quite and have a longer estimated life.
Many people confuse the ventilation requirements of an attic with those of the living space below. The two types of ventilation are not rated and the systems are completely separate. Attic ventilation is needed to protect the insulation and building materials in the attic from excess moister, caused primarily by air leakage from the moister-laden living space. Although sealing air leaks will reduce the need for attic ventilation (and reduce energy use and ice dam risks), minimum attic ventilation is still required. The ventilation required for the living space, however, is all about providing a safe and healthy environment for the occupants by replacing contaminated air with fresh air.