The bathroom exhaust fan is the least glamorous component in a house and one of the most consequential for roof longevity in this climate. A significant proportion of Seattle attic moisture problems trace back to a fan that is undersized, unused, badly ducted, or terminating somewhere it should not.
Fan capacity is measured in cubic feet per minute. The conventional guideline for a bathroom under about 100 square feet is roughly 1 CFM per square foot, with 50 CFM treated as a practical minimum.
For larger bathrooms, a fixture-based approach works better: allow roughly 50 CFM each for a toilet, a shower, and a bathtub, and around 100 CFM for a jetted tub, then total them.
Two things undermine the rating on the box. First, that figure is measured under ideal conditions; real duct runs with bends and length reduce delivered airflow substantially. Second, many builder-grade fans were selected on price rather than performance and were marginal when new, decades ago.
A simple field test: hold a single sheet of tissue paper against the grille with the fan running. It should hold firmly on its own. If it falls or barely clings, the fan is not moving meaningful air regardless of its label.
Diameter. Use the size the fan specifies, typically 4 or 6 inches. Reducing to fit an existing hole throttles the fan badly — the reduction in cross-sectional area is larger than it intuitively seems.
Length and bends. Shorter and straighter is dramatically better. Every elbow adds resistance equivalent to several feet of straight duct. A fan with a long, winding route through an attic may deliver a small fraction of its rated airflow.
Material. Smooth-wall rigid duct substantially outperforms flexible duct, because flex duct's corrugated interior creates turbulence and friction along its entire length. Where flex is used, it must be pulled taut — a sagging, compressed flex run is close to worthless.
Slope and sag. A duct that dips creates a low point where condensate collects. Water pools there, and eventually it either drips through the fan housing into the bathroom or waterlogs the duct. Runs should be supported and pitched slightly toward the termination.
Insulation. This one is specific to cold climates and constantly overlooked. Warm humid air passing through an uninsulated duct in a cold attic condenses inside the duct. That water then runs back down and drips out of the ceiling fan, which homeowners reliably diagnose as a roof leak. Ducts in unconditioned attic space should be insulated.
The duct must discharge outside the building envelope. Through the roof with a proper vent cap, or through a wall or gable with a hooded termination. Never into the attic, never into a soffit, and never simply pointed at a gable vent.
Two specific failures we find constantly.
The duct that has fallen off. It was connected properly at installation, the connection was made with tape rather than a clamp, and over years it separated. From the bathroom everything looks and sounds normal. In the attic, a duct is lying on the insulation blowing humid air at the sheathing. This is common enough that it should be the first thing checked in any attic moisture investigation.
Termination into a soffit. Seems reasonable — it is outside, after all. But the soffit is your ventilation intake. Air discharged there gets drawn straight back into the attic by the ventilation system it was supposed to be separate from. It is a closed loop that recirculates the moisture you were trying to remove.
The damper. The termination cap needs a working backdraft damper. Dampers stick open with age and debris, which lets cold air back in and can allow birds and rodents entry. Check it opens with the fan on and closes with it off.
A correctly sized fan run for two minutes accomplishes little. Moisture continues to evaporate from wet surfaces, towels, and the shower enclosure long after the water stops.
The practical guidance is to run the fan during the shower and for a good while afterwards — twenty minutes is a reasonable target. Since nobody reliably does that manually, a timer switch or a humidity-sensing switch that runs the fan until the room returns to a set humidity level is one of the highest-value small upgrades available in a Seattle house. It is inexpensive, it is a straightforward swap, and it removes the human factor entirely.
Everything above applies with more force to the clothes dryer, which moves far more moisture than any bathroom fan. A dryer venting into an attic or crawlspace is a serious moisture problem and a lint accumulation hazard. It must terminate outside, through rigid duct, with the shortest practical run.
Because in this climate, exhaust fans are a roofing issue. Deck sheathing rotted by attic condensation gets replaced during a re-roof at real expense, and if the duct that caused it is still discharging into the attic afterwards, the new sheathing is on the same trajectory as the old.
Any competent roofing scope on a house with condensation-pattern deck damage should include checking and correcting duct terminations. It is a small line item that determines whether the expensive part of the job lasts.
This post is part of our in-depth coverage of roof inspection in Seattle.
We walk the roof, check the attic, and give you a written fixed quote. Serving Seattle and the greater Puget Sound region, open daily 7AM–8PM.
