Proper installation of bath and kitchen fans is important to their effectiveness and the structural integrity of the house. Bath and kitchen fans should always vent to the outside and never in to the attic or other unconditioned space. In our northern climate for most of the year the air being vented from the house is warm and moist compared to the outside air. This warm, moist air should always be vented to the exterior. Imagine that you are venting your bathroom fan into the attic of the house in the winter.
As this warm moist air enters the cold attic it begins to condense on any and all cold surfaces. This leads to the degradation of the homes structural components and can result in mold growth. In extreme cases it can also lead to the condensation dripping back in to the house through the ceiling. To prevent these issues, vents should also be continuous to the exterior of the house where this condensation presents no problems.
Proper use of the fan is also important to reducing condensation. Bath fans should always be used when taking a shower to expel the moist air from the house and not trap the moisture in the bathroom. But you also need to run the fan for several minutes after the shower is complete to continue to expel the moisture from the bathroom and also to continue to move the moisture through the vent to the exterior. If a fan is turned off prematurely moisture can accumulate in the exhaust fan ductwork and can cause condensation issues.
The same is true for kitchen exhaust fans. Don’t simply turn off the exhaust fan when you are finished cooking the pot of spaghetti. Let the fan run for several minutes after the pot has been taken off the stove to allow for the moisture to be completely expelled from the exhaust vent.
By having bath and kitchen fans installed correctly and using them correctly, you can reduce the risk of mold or condensation issues in your home.