By the AMG Proline Team

This is the least glamorous decision in a bathroom remodel and one of the few that can quietly damage a house. A bathroom exhaust fan does not just move air. It moves warm, saturated air out of a heated room, and where that air ends up in a Chicago January determines whether you have a working bathroom or a slow moisture problem in your attic.

We vent bath fans through the roof or through a gable wall, with insulated duct, to a capped exterior termination with a damper. We do not vent into the soffit and we never leave a duct dumping into the attic. Here is the reasoning.

What the fan is actually removing

A hot shower produces air at roughly 70 to 75 degrees carrying a large amount of water vapor. Air holds water in proportion to its temperature. Cool that air and it reaches its dew point, at which the water it is carrying condenses out onto whatever surface it touched.

In January in Chicago, a properly ventilated attic is close to outdoor temperature. Push warm saturated bathroom air into that space and it does not disperse. It condenses on the underside of the roof sheathing, and when it is cold enough it does not even condense, it deposits as frost. You can find a rime of it across the nail points and the plywood.

Then a warm week arrives, the frost melts all at once, and it rains inside the attic. That shows up as a brown ring on a bedroom ceiling, wet insulation that has lost most of its R value, and over repeated seasons, mold on the sheathing and rot at the framing. The homeowner blames the roof. The roof is fine. The bathroom fan did it.

Why the soffit is the wrong target

Soffit vents are not exhaust points. They are the intake side of the attic’s ventilation system. Air is designed to enter at the soffit, travel up the underside of the roof deck, and leave at the ridge. That airflow is what keeps the deck dry and, in winter, keeps the roof cold enough to reduce ice damming.

Terminate a bath fan at or near the soffit and you are injecting moist air directly into the attic’s intake. The ventilation system then does its job perfectly and carries that moisture up across the entire roof deck. It is a more efficient version of the same failure.

There is a second problem specific to older Chicago housing stock with shallow or no overhang: there simply is not enough soffit to terminate into. The path of least resistance for a hurried installer is to point the duct in that direction and staple it down, which is not a termination at all.

How it should be built

The details are simple and they are all skippable, which is why they get skipped.

  • Insulated duct. Bare metal or bare flex duct running through a cold attic will condense on the inside, and that water runs back down to the fan housing and out of your ceiling. Insulated or wrapped duct keeps the interior surface above dew point.
  • As short and straight as the framing allows. Every elbow and every foot of sag costs airflow. A fan rated at 80 CFM buried in twenty feet of crushed flex may be moving a fraction of that.
  • Rigid smooth wall duct where it is practical. Flex duct is easier to install and worse at moving air. If flex is used, it should be pulled taut and supported, not left in loops.
  • A slope toward the exterior. Any condensate that does form should run out of the building, not back into the housing.
  • A dedicated exterior cap with a backdraft damper, sized to the duct, so cold air is not free to flow back in when the fan is off. Do not reduce the duct size at the cap.
  • Sealed connections. Mastic or foil tape at the housing and at every joint. Standard cloth duct tape fails within a couple of seasons in an attic.
  • No sharing. A bath fan does not tie into a dryer vent, a kitchen hood, or another bathroom’s duct.

Sizing and run time

For a bathroom under 100 square feet, a common sizing guideline is roughly 1 CFM per square foot with a practical minimum around 50 CFM. Larger bathrooms and bathrooms with a separate enclosed toilet or a steam shower are sized by fixture instead. An undersized fan in a tiled shower room simply cannot keep up.

Run time matters as much as capacity. A fan that runs only while someone is in the room leaves the moisture behind. We install a timer switch or a humidity sensing fan so it continues running after the shower ends and shuts itself off. If the door seals tightly to the floor, the fan also needs a path for replacement air, usually a small undercut at the door.

It is also worth choosing a quiet fan. Fans are rated in sones, and a loud one gets switched off, which defeats the entire system. The best ventilation is the ventilation people are willing to leave running.

What we do on a remodel

Every bathroom we gut gets its ventilation looked at, even when the client did not ask, because the walls and ceiling are already open and it will never be cheaper to fix. We check where the existing duct goes, whether it is insulated, whether it terminates outside, and whether the fan is sized for the room. Frequently the existing fan was never ducted anywhere at all.

It adds a modest amount to a project and it protects everything above it. That is the kind of decision we would rather make correctly than cheaply, which is the approach described on our about page.

Learn more about our bathroom remodeling in Chicago services, review our licensing and accreditation on the certificates and awards page, or contact AMG Proline for a free in-home consultation.