A bathroom fan does one job most homeowners never see: it moves warm, wet air out of the house. Where that air ends up decides whether the fan is helping or quietly loading your attic with moisture. In winter, the problem shows up as frost on the underside of the roof deck, stained insulation, or a damp smell near the ceiling. Tracing the duct now, while the attic is dry and the weather is mild, is far easier than guessing on a freezing morning.
Find the Termination First
Start at the fan housing and follow the duct with a flashlight. The run should end at an exterior hood, a roof cap, or a wall vent. It should never end in the attic space, in a soffit cavity, or in a gap between the joists. A duct that stops short of the outside is the most common reason a fan adds moisture instead of removing it. Note the duct type as you go. Flexible duct is easy to install but sags between supports, and every low point is a place where condensation pools and drips back toward the fan.
Check the Joints, the Sag, and the Damper
Look at every joint. Seams should be sealed with foil tape or the approved duct sealant, not cloth tape, which lets go as the duct heats and cools. Pull gently on the flex to see whether it hangs in a long curve between supports. If it does, add support straps so the duct slopes steadily toward the exterior. Next, go outside and look at the damper flap on the termination. It should open when the fan runs and close fully when the fan stops. A stuck, cracked, or missing damper lets cold air back into the duct, and in cold weather that air condenses on the duct wall.
Check the fan housing too. Insulation packed tight against the housing can cause overheating, so look for the clearance note printed on the fan itself and follow it. Keep insulation off the motor and out of the housing opening.
Read the Frost Like a Gauge
Frost on nail tips near the duct, dark staining on the roof sheathing, or a wet spot on the insulation below a low duct run all point to the same thing: moist air reaching cold surfaces. Mark the spot with a pencil on the sheathing or take a dated photo. The mark tells you whether the problem is getting worse over the season or holding steady.
ACHR News published a story on a contractor who used his own home as a heat pump test lab, and the summary describes contractors discussing what a "real-world heat pump installation reveals about sizing, training, comfort, and efficiency." The same idea applies to a bath fan. Your attic is the test bench, and a short written log beats memory. Read the full piece in the ACHR News coverage of a home heat pump test lab.
Fix What You Can, Call for the Rest
A homeowner can usually handle the attic side of the job. Reseal the joints, add straps to lift the sagging run, clear any debris from the exterior hood, and replace a damper that no longer seals. If you open the fan housing or touch the wiring, shut off the breaker first and confirm it is dead with your multimeter before you put a hand inside.
If the duct exits through the roof, the work changes. The penetration has to be flashed and sealed so that water cannot follow the pipe into the structure. A roofing contractor such as Keys Roofing can seal a roof cap or replace a failing boot. Ask them to check the flashing as well as the cap, since a neat cap over a leaking boot still leaves you with a wet attic.
Log It Before the Cold Arrives
Write down the termination location, the duct type, the date, and what you changed. Take a photo of the damper in the open and closed positions. Then go back into the attic after the first hard freeze and again after a stretch of long showers. Compare the new frost, staining, or drip marks against your notes. If the marks have not moved, the fix worked. If they have spread, the duct is still leaking air or the run is still sagging, and that is the place to start the next look.