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A properly functioning ventilation system removes excess moisture in the air, controls the circulation of air inside and removes unpleasant odors in a home. It’s a necessary component of any modern home and one of the most important factors for maintaining a healthy environment. With stagnant air and wet conditions, mold can easily grow. Having a fully functioning system is essential to the comfort and health of all members of the home. 


Condensation is water that forms from moisture vapors in the air. As moisture vapors move through a wall surface, and the temperature is lower on the other side of the wall, there is a possibility of the vapors changing to liquid behind the wall. When this occurs, mildew develops in the wall and grows to the surface. The mildew behind the surface of the wall (colder side) will always be worse than the mildew that is visible on the surface (warmer side).

The following are examples of summer condensation and mildew. Remember, all condensation is caused by the amount of moisture in the air and temperature differential.

  • Moisture on the outside of air conditioning ductwork in a basement or crawl space
  • Water forming on the outside of the cold water service pipe and supply piping in the basement or crawl space
  • Mildew forming on the paneling in a basement with high relative humidity
  • Condensate from the air conditioner’s air handler. This is a normal process, which removes moisture from the air
  • Condensation and mildew in below-grade areas, such as basements and crawl spaces
  • Most likely places would be on the walls or the concrete floor; depending on the amount of moisture present, it could develop almost any place. The denser the material, the more likely it is that condensation will develop
  • Condensation may develop inside of walls and cause paint to peel. This could occur in a room above a crawl space with a dirt floor or water penetration
  • Bathroom showers where the ceiling or a wall is adjacent to an exterior (colder) wall
  • At colder, usually masonry walls, typically on the north side. Mildew may be found at metal windows, electrical outlets, behind pictures and dressers
  • In attics

Exterior Paint Peeling
Typically on the north side. Moisture only moves from the warm to the cold side. Paint can act as a vapor barrier and stop the moisture from passing through in a vapor state. If the vapors turn to liquid before it can get out of the wall, the paint will peel or blister.

Other Potential Condensation Problem Areas
Fossil fuel heating appliances all have exhaust. Much of this exhaust is moisture. Heat from the appliance keeps the moisture in a vapor state. If the chimney is too large, the exhaust gases will expand and cool, and the vapors will turn to water in the chimney. If the chimney is too small, blocked or partially blocked, the exhaust will be restricted and spill back into the utility area. The moisture vapors may accumulate and excessive humidity may be evident. This is also an indication that an unacceptable amount of carbon monoxide may be present. A large number of plants will give off a considerable amount of moisture, and contribute to elevated relative humidity in the home.

Other water sources can contribute to the relative humidity in the home (i.e. standing water in a sink, improperly vented dryer, a large number of people, water penetration in the basement or crawl space, unvented showers, cooking, dishwasher, a humidifier, etc.).

Humidity in the Home
The relative humidity in a home should be between 40% and 60%, preferably around 45%. When the relative humidity rises above 60%, mold or mildew may begin to develop and grow. As it gets closer to 100%, the mold growth becomes more extensive. When the relative humidity is below 40%, we exhale more moisture than we inhale. This can cause our mucus membranes and esophagus to dry out and make us vulnerable to respiratory problems, such as catching colds. If you control the relative humidity, you can eliminate mildew and create a healthier environment.

Any bathroom without a window is required to have a vent fan. Bathroom ventilator fans, if available, should be used when baths/showers are taken; or open the window 1 to 2 inches to permit the moisture to escape.  It the bath vent exhausts into the attic, the duct should be extended to the exterior or to within a few inches of the ridge vents, to avoid discharging moisture into the attic.

Use of a range hood vented to the exterior will also assist in removing moisture from a house.  Cooking can add more than 5 pints of water to the air in a house during one day.  Range exhaust fans should never discharge into the attic, as the fumes may be combustible.

Recommended Ventilating Capacities areGiven in Cubic Feet per Minute (CFM)
For kitchen ventilation, the recommended CFM equals forty times the linear feet of range hood, if located above a peninsula or island range. (Example: a 36" long hood should expel 120 CFM if mounted on a wall, or 150 CFM if mounted over an island arrangement). Bath ventilation in CFM is 1.07 times the floor area. Laundry room ventilation in CFM is 0.8 times the floor area. Exhaust fan capacity ratings usually increase in increments of 10 CFM if it is determined through above calculations that a 128-CFM fan is required.

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