What do you get when you mix warm moist air with something cold? 

Answer: Condensation.

While this may look cool on a nice cold beverage glass in the summertime, it’s not something you want happening in your attic.

Improper Ventilation to the Exterior

Exhaust fan venting into attic

One of the most common things we find in an attic during a home inspection in the Des Moines area is bathroom fans that vent directly into the attic. They usually are easy to spot – white plastic ductwork hanging from a rafter or laying on top of the insulation.

While this was a common practice when vent fans started to be installed in bathrooms and homes, it is certainly not an ideal situation. This often means that warm moist air from inside the house is sent into an enclosed space. It is a recipe for a multitude of problems.

Condensation staining in attic

Condensation Issues

Without adequate ventilation, this can lead to condensation staining on the roof sheeting, microbial growth in the attic space, and even deterioration of plywood and OSB. Over an extended period of time, this may result in soft roof sheeting that needs replaced or microbial growth that needs proper mitigation.

Another common problem that stems from unvented exhaust fans in the attic is condensation inside the vent pipes. When that warm moist air from a shower is sent into a cold attic on a winter day, condensation will occur. Sometimes this condensation happens inside the vent pipe. If conditions are right, this can result in water draining back down the vent pipe and staining the bathroom ceiling. 

Ceiling stains from attic not ventilated properly

Ceiling Water Stains

Anytime we see water stains on a bathroom ceiling around a vent pipe, we’re very likely to find exhaust fans in an attic that are not vented to the exterior. Under certain conditions, this condensation can collect into a notable amount of water.

During the slower inspection seasons of the fall and winter, we often fill our time with remodeling projects and handyman repairs. This past February we helped a client replace a couple of bathroom vent fans. There were telltale moisture stains on the ceiling around the fans because the old vent pipes were just hanging in the attic and not vented to the exterior.

During the removal process, it became very evident there was water that had collected in the horizontal portions of the ductwork. So much water in fact that we needed a bucket to collect it. One duct had a full quart of water in it and the other had an entire GALLON of water just laying in the pipe.

To avoid this scenario from happening in your house, here are some best practices for venting bathroom fans.

1. Exterior

Most important is to be sure the vent fan is exhausted to the exterior of the house and not into the attic. A best practice is to vent the fan out of the roof or gable when possible. Occasionally this ductwork is exhausted out of a soffit in a home, but this runs the risk of having that warm moist air being drawn back up into the attic space if the soffit is vented.

2. Insulate

To avoid condensation inside the vent pipe, the pipe should be insulated. Insulated flexible vent pipes are readily available at most home improvement stores. 

3. Separate

Each vent fan should have its own dedicated ductwork that goes to the exterior. While it may be tempting to combine exhaust fans into one pipe so there is only one roof penetration, this is not recommended. This typically reduces a vent fan’s effectiveness and often results in sending that air into one of the other rooms in the house.