Bath Venting

A common problem that I see on home inspections is the bathroom vent fan not properly installed. More accurately, it is the flexible duct for the vent fan that is usually the problem. Every manufacturer recommends the vent fan discharge to the outside but they don’t include the vent connector with the fan. The duct and vent hood are described as “accessories”. So it’s no wonder that bathroom vent fans are often venting into the attic. Venting warm moist air into the attic is about the worst thing you can do, especially in a cold climate. Warm moist air from the shower fills the attic space, goes up to the roof deck and condenses into water. Moisture in the attic can cause a whole host of problems.

Increased moisture and high humidity in the attic will collect on the bottom side of the roof deck.  In the winter months this moisture turns to frost in the attic that will eventually melt. When the frost melts, it “rains” inside the attic. This condition leads to wet insulation, wet drywall, and an environment for the mold growth. All of which have the potential to damage the structure of the home.

Excessive moisture in the attic from improper ventilation of the bathroom exhaust fan can often appear to be a roof leak. I have saved more than one homeowner the expense of a new roof after the roofing contractor concluded the water stains in the second story ceiling was from a roof leak. In one case, I found just four inches of fiberglass insulation and two bathroom vent fans venting into the attic.

Vent the bathroom exhaust to the outside either through the roof or sidewall of the house. The bathroom vent should NEVER exhaust into the attic. Venting by sticking the duct in the roof vent, continuous ridge vent, or soffit vent is also not recommended.  Bathroom vent fans should discharge at least three feet away from any other type of vent.

If the flexible duct is not insulated, it can also allow condensation of water vapor and drip water into the attic or the water can run back down and drip at the fan. All ducting that travels through a cold attic should be insulated with R-5. Flexible, insulated duct is available at most home improvement stores. It is also acceptable to cover the duct with insulation that is applied to the floor of the attic.

Bath vent example

Every bathroom should have a vent fan the discharges to the outside. If the flexible duct for the fan travels through a cold attic the duct should be insulated. The duct should leave the house through the roof or sidewall by way of a purpose built vent damper or hood.

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