Homes have fan systems built into them to help prevent the build-up of condensation. These come in the form of fans in the kitchen and bathroom.
The purpose of a fan is the ventilate moisture created by steam or condensation. Many times fans in kitchens and bathrooms in older houses vent into attics, which are not well vented in and of themselves. When vents blow into an area that is enclosed and cold (like an attic), condensation will build up and the liquid created will soak into the insulation and framing materials in that area. As a result, mold will form and begin to rot the wood and materials. Similar results will happen when the attic has loose-fill insulation: the mold will eat through the insulation on the side closest to the fan vent. Condensation will form in this area since it will be cooler. The worst case scenario is when a bathroom does not have a vent as this can cause as much damage as a leaky roof.
Fans in the kitchen must have their hoods cleaned periodically to reduce the buildup of grease and the risk of fires. If a kitchen fan vents into an attic, not only will moisture buildup in that area, but grease will too. A kitchen fire can easily travel into the greasy fan hood and end up quickly catching an attic on fire.
A better solution is having fan systems ventilate outside. However, when vent fans are placed close to an outside wall of a home and under a roof, the moisture venting outside will be directed to the sheathing of the roof. When this happens the sheathing will form mold and begin to decay.
Dwight Barnett, a master inspector with the American Society of Home Inspectors, stated in a recent interview with The Seattle Times:
“A range fan must vent through approved metal pipe all the way to the exterior of the home. Whether vertical through the roof or horizontal through a wall or foundation, the range fan must vent to the outside.”
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