Moist air in your bathroom can lead to water or mold damage, but improperly venting the bathroom's moist air through the soffit on your house can also damage your house's attic. You need to weigh the different options for the duct that connects the bathroom vent to the soffit vent, as well as the placement of the vent in the soffit.
Duct Types
Use either a flexible vent duct similar to the one attached to the exhaust on the back of your clothes dryer, or use a rigid steel duct. The flexible can expand to the size of the gap between the bathroom fan and the soffit vent, making the installation process fairly easy. Use hose clamps on either end of the flexible duct to attach it to the bathroom vent exhaust port and the soffit vent. If using a rigid steel duct, measure and cut the various sections of the duct to the size of the space. The attachments between the duct, the bathroom fan exhaust port and the soffit vent must be made with sheet metal screws.
Insulation
Insulating the bathroom vent duct inside the attic helps prevent condensation during the winter months. Since cold outside air flows into the attic, making the temperature of the attic lower than the inside of your house's living space, the hot air flowing through the duct heats up the walls of the duct. When the cold air contacts the hot duct, the moisture in the air condenses in the duct. The condensation can collect and drip in the attic, leading to water damage. Either slide an insulation sleeve over the duct inside your attic, or use spray foam insulation to prevent condensation.
Soffit Dangers
Attaching your bathroom fan to a vent in your house's soffit can present dangers to the attic. According to home improvement expert Tim Carter, the moist and warm air flowing out of the vent you install in the faucet may be drawn back into the attic by the soffit intake vents. The intake vents are designed to allow for air flow through the attic, preventing mold growth, but if the bathroom air flows in the vents, the attic can become moist and mold may grow. Openings in the soffit can also allow the bathroom air to flow into the attic.
Vent Location
You do not have to place the bathroom vent in a specific part of the soffit. The further you place the vent from the bathroom fan, though, the longer the duct must be. Place the exhaust vent as far from the soffit intake vents as possible. Seal any cracks or gaps in the soffit around the exhaust vent using silicone caulk.
Tags: bathroom vent, soffit vent, your house, intake vents, into attic, vent duct