How Do You Size and Vent a Bathroom Exhaust Fan in New Orleans?

If your bathroom mirror still holds fog ten minutes after your shower ends, or the ceiling paint above the shower has started to show faint gray spots, your exhaust fan is not doing its job. In a New Orleans bathroom, that is not a small annoyance. It is standing water vapor with nowhere to go, and it starts working against your paint, your grout, and your framing within hours of every shower.

The building industry has settled on a clear standard for how much air a bathroom fan actually needs to move. ASHRAE 62.2, the residential ventilation standard most local codes point to, sets the bar at a minimum of 50 CFM for a fan that runs only while you shower, or 20 CFM for a fan built to run continuously. A large share of the fans already installed in New Orleans bathrooms fall short of matching the room they sit in, because a builder or a prior owner picked a fan by price rather than by the numbers.

At TurnKey Bath Remodel, we size and route ventilation correctly on every shower and bathroom project across the New Orleans area, because a beautiful remodel that traps moisture behind the tile is a problem waiting to surface.

If you are remodeling and want the ventilation handled correctly the first time, contact us for a free consultation.

A fan rated below the CFM a bathroom actually needs will run constantly and still lose the moisture battle, while an oversized fan wastes energy and can pull conditioned air out of the rest of the house. Getting the number right is most of the difference between a bathroom that dries out after every shower and one that never quite does.

How Bathroom Exhaust Fan Sizing Actually Works

Every bathroom fan is rated by two numbers, and most homeowners have only ever noticed one of them. Getting both right is what separates a fan that actually clears a room from one that just makes noise.

The Two Numbers That Actually Matter: CFM and Sones

CFM stands for cubic feet per minute, the volume of air the fan physically moves. It is the number that determines whether the fan can keep up with the moisture a shower produces. Sones measure how loud the fan is in operation, on a scale where 1.0 sone is about as quiet as a refrigerator hum and anything above 3.0 sones starts to sound like a small vacuum. In plain terms, CFM tells you whether the fan works, and sones tell you whether anyone will actually leave it running long enough to matter. A fan that is powerful but loud gets switched off early, which defeats the sizing exercise before it starts.

A surprising number of bathrooms in the New Orleans area are still running the fan that came with the house decades ago, sized for a code minimum that has since been raised, or sized for a smaller room before an earlier remodel expanded it. Neither situation is anyone’s fault in the moment, but both leave the fan quietly undersized for the job it is actually being asked to do today. A remodel is the natural point to correct that mismatch, since the ceiling is already open and the duct run is already accessible, and it pairs well with choosing materials that actually hold up to New Orleans humidity for the rest of the room.

Step 1: Measure Your Bathroom’s Square Footage

Start with a tape measure, not a guess. Multiply the room’s length by its width to get the square footage, and round up rather than down if the room is an odd shape.

• For a bathroom under 100 square feet, the standard rule is 1 CFM per square foot of floor area.
• A 50-square-foot bathroom needs at least 50 CFM, even though the square-footage math alone would suggest less, because code sets 50 CFM as the intermittent-use floor regardless of room size.
• A small powder room can sometimes use a smaller fan under 50 CFM if it runs continuously at the lower 20 CFM continuous minimum, but for a full bathroom with a shower or tub, plan around the 50 CFM floor.

This step alone corrects the most common mistake we see in older New Orleans homes: a fan that was sized for the powder room it was originally installed in, still doing that same job in a bathroom that has since been expanded. Measuring first, before assuming the existing fan is adequate, is what catches that gap before new tile and drywall make the mismatch expensive to fix later.

Step 2: Add Extra CFM for Each Fixture in a Larger Bathroom

Once a bathroom crosses roughly 100 square feet, the simple square-footage rule stops being accurate, because a big room with one toilet does not need the same airflow as a big room with a separate shower, a soaking tub, and a toilet. The Home Ventilating Institute’s fixture method fills that gap.

• Toilet: add 50 CFM.
• Standard shower or bathtub: add 50 CFM each.
• Jetted or whirlpool tub: add 100 CFM, since the larger water surface and agitation release more moisture into the air.
• Add each fixture’s number together rather than using the room’s square footage once the room is large enough to need this method.

A primary bathroom with a separate shower stall and a soaking tub, for example, calls for meaningfully more airflow than the square-footage math would suggest on its own. Skipping this step is the second most common undersizing mistake, right behind reusing an old fan without checking the numbers. We see this most often in primary suites that were expanded at some point, where the fan was never upgraded to match the added fixtures.

Step 3: Decide Between Continuous and Intermittent Ventilation

Every fan is either sized to run only while someone is in the room, or sized to run quietly in the background all day. New Orleans humidity makes this choice matter more than it does in a drier climate.

• Intermittent fans need to hit the full 50 CFM minimum, since they have a short window to clear the room before someone flips the switch off.
• Continuous fans can run at the lower 20 CFM floor, because they never stop working, but ASHRAE 62.2 also calls for a certified sound rating of 3.0 sones or less so the fan is quiet enough to leave running around the clock.
• A continuous fan is the stronger choice for a bathroom that stays humid for hours after a shower, which describes most bathrooms here for a good part of the year.
• An intermittent fan paired with a wall switch is still code-compliant and often the simpler retrofit when a continuous fan is not practical.

Neither choice is wrong on its own. The mistake is picking a fan’s CFM rating without first deciding which mode it will actually run in, since the two modes have different minimums. We generally recommend continuous ventilation for a primary bathroom that gets daily use, and reserve intermittent switches for guest bathrooms and powder rooms that see lighter, more occasional moisture load.

Step 4: Match Your Duct Size to the Fan’s CFM

The fan is only half the system. The duct carrying air away from it has to be sized to match, or the fan will fight against its own ductwork and move far less air than its rating promises.

• A 4-inch round duct is the standard fit for fans rated up to roughly 50 to 75 CFM.
• Fans rated higher than that need a 6-inch duct to avoid excessive friction loss, which quietly robs a correctly sized fan of a large share of its rated performance.
• Rigid metal duct outperforms flexible duct on friction loss and is worth the extra labor on a full remodel.
• If flexible duct is used, keep the run as short and straight as possible, fully extended with no sagging, since every bend and sag in flex duct reduces effective airflow.

This is the step homeowners are least likely to check themselves, because the duct is hidden in the wall or attic. It is also where a correctly rated fan most often underperforms in practice.

Step 5: Route the Duct Straight Outside, Never Into the Attic

Where the duct terminates matters as much as its size. Code is direct on this point, and it is one of the most commonly skipped steps in older New Orleans homes.

• The duct must terminate outdoors, through a wall or roof cap, not into an attic, soffit, or crawlspace.
• A duct that dumps into the attic simply relocates the moisture problem instead of solving it, since that warm, humid air then condenses on the coldest surfaces it finds up there.
• Roof or wall caps should have a damper that closes when the fan is off, to stop outside air and insects from coming back in.
• If your current bathroom fan vents into the attic, correcting that single detail during a remodel is one of the highest-value fixes available, even when the fan itself is otherwise the right size.

We routinely find this exact issue when we open up a ceiling during a remodel here. It rarely shows up as an obvious problem until the humidity has had months or years to do its work, and by the time it does, the fix usually involves more than just the fan itself. Our guide on how to prevent mold in a New Orleans bathroom covers what tends to happen once ventilation has been missing that long.

Step 6: Add a Timer or Humidity-Sensing Switch

Even a correctly sized, correctly vented fan only helps if it runs long enough. A switch that automates the run time removes the guesswork.

• A basic timer switch lets you set the fan to keep running for 10 to 20 minutes after you leave the bathroom, which is typically enough to clear residual humidity.
• A humidity-sensing switch goes a step further, turning the fan on automatically once moisture in the air crosses a set threshold, whether or not anyone remembers to flip a switch.
• Either option is a straightforward add during a remodel, since the wiring is already open.
• For a continuous-ventilation setup, a boost switch is the equivalent tool, letting the fan step up to a higher CFM temporarily during and after a shower.

Pairing the right switch with the right fan is what actually closes the loop between sizing the system on paper and getting the humidity control it was designed to deliver in daily use.

Work With a New Orleans Remodeler Who Gets Ventilation Right

Ventilation is one of those details that never shows up on a mood board and rarely gets asked about before a remodel, yet it decides how well every other choice in the room holds up over time. A fan sized and vented correctly protects the tile, the paint, and the framing you are paying to install, and it does that work quietly in the background for as long as the room is in service.

At TurnKey Bath Remodel, we calculate the right CFM, size the duct to match, and route it straight outside on every bathroom we build across the New Orleans area, so the finished room performs as well behind the walls as it looks in front of them. If you are planning a remodel and want ventilation handled correctly from the first measurement, call us today at (504) 513-6366 to schedule your free consultation.

Frequently Asked Questions About Bathroom Exhaust Fans

What size exhaust fan do I need for my bathroom?

For most bathrooms under 100 square feet, use 1 CFM per square foot with a 50 CFM minimum for intermittent use, or 20 CFM if the fan will run continuously. For larger bathrooms, size by fixture instead: 50 CFM for a toilet, 50 CFM for a standard shower or tub, and 100 CFM for a jetted tub, added together.

Should a bathroom fan run continuously or only while I shower?

Either is code-compliant, but continuous ventilation tends to serve New Orleans humidity better, since the room stays humid well after the shower ends. A continuous fan only needs to meet the lower 20 CFM minimum, as long as it is quiet enough to run all day.

What size duct does a bathroom exhaust fan need?

A 4-inch duct fits fans rated up to about 50 to 75 CFM. Fans rated higher than that need a 6-inch duct, since a duct that is too narrow creates friction loss that quietly reduces the fan’s actual performance below its rating.

Can a bathroom exhaust fan vent into the attic instead of outside?

No. Code requires the duct to terminate outdoors through a wall or roof cap, not into an attic, soffit, or crawlspace. Venting into the attic just moves the moisture problem to a space that is even harder to inspect and dry out.

How loud should a bathroom exhaust fan be?

Under 1.0 sone is considered whisper-quiet and is the right target for a fan meant to run continuously, since a loud fan usually ends up switched off early. Fans intended for shorter, intermittent use have more room to be louder without being a nuisance.