Key takeaways
- A bath fan has to exhaust to the outdoors. Venting into the attic moves the moisture, it does not remove it.
- Fan capacity is rated in CFM, and the usual starting point is roughly one CFM per square foot of bathroom floor for a standard-height room.
- Sone rating decides whether the fan gets used. A loud fan is a fan people switch off.
- Price turns on whether ducting and a circuit already exist, the ceiling access above, and whether you want a plain fan or a heat-vent-light combination.
A bathroom exhaust fan is the least glamorous thing in the house and one of the few that quietly prevents expensive damage. Its job is to take the moisture a shower puts into the air and move it outside before it condenses on drywall, window frames, and the back side of paint. Skip it, or run one that does not work, and the result shows up months later as peeling finish, a musty smell that never clears, and eventually mold in places nobody looks. The fan at the store is not expensive. Almost everything you pay to install one is about where the air goes.
The duct is the job
A fan has to exhaust to the outdoors, through the roof or through a sidewall, with a damper that keeps outside air from coming back in. What we find instead, regularly, is a fan that dumps straight into the attic. That is not ventilation, it is relocation: the moisture leaves the bathroom and condenses on the underside of the roof deck and in the insulation, where it does slower and more expensive damage than it would have done downstairs. Sometimes the duct is there but has come loose from the housing, so it has been venting into the attic for years while the fan hummed away and everyone assumed it was working.
Flex duct also needs a run that does not sag. A long horizontal run with a belly in it collects condensate, and the pooled water strangles the airflow until the fan is moving a fraction of its rating. The fix is a short, supported, sloped run to a proper exterior termination, which is why the ceiling above the bathroom, and what is up there, is the first thing a technician looks at.
Sizing, in the only two numbers that matter
Capacity is rated in cubic feet per minute. The common starting point for a standard eight-foot ceiling is about one CFM per square foot of floor, so a typical hall bath lands near fifty CFM and a larger primary bath considerably more, with enclosed toilet compartments and jetted tubs counted separately. Undersize it and the room never clears; oversize it badly in a tight house and the fan struggles to find makeup air.
The second number is the sone rating, which is loudness, and it is the one that decides whether the fan gets used at all. A three-sone builder-grade unit is loud enough that people turn it off before it has done anything, or never turn it on. A unit at one sone or below disappears into the background and runs for the twenty minutes after a shower that actually matters. If you are paying a licensed hand to be in your ceiling anyway, the quiet unit is where the upgrade dollar does the most work.
- Ducting already present and properly terminated outside, or a new run and a new exterior termination.
- An existing circuit and switch at the location, or a new circuit, which is the biggest single swing.
- Attic access above the bathroom, versus a second-floor bath with a finished ceiling above it and no way in.
- A plain exhaust fan, a fan-light, or a heat-vent-light combination, which draws more and often needs its own circuit.
For a bath exhaust fan supplied, installed, ducted, and tested, our flat rate in the book is the Bath exhaust fan task, from $629. You approve that before the work starts, and a technician confirms it on site, because what is above the ceiling decides whether this is a swap or a duct run. Starting price. Your tech confirms the final number on site before any work.
If you want the heat-and-light combination instead of a plain fan, that is a different unit with a different draw and often a dedicated circuit of its own; it sits in the book as its own task and gets quoted as its own number rather than as a surprise on this one.
Replacing one that already exists
A straight replacement is the best case and it is genuinely quick: the opening is cut, the duct is there, the circuit is there. Two things still turn a swap into more than a swap. Housing sizes are not standard, so a new unit may not fit the existing ceiling opening, and a slightly larger housing means cutting and patching drywall. And the existing duct is worth inspecting before the new fan goes in, because a fan is the one appliance where nobody notices it has been failing.