Exhaust Fans

An exhaust fan does one job — pull moisture, odors, and stale air out of a space and push it outside before it builds up. A well-chosen fan pushes contaminated, hot, or humid air out of a space before it builds up. Get the selection wrong and you end up with a fan that runs continuously but doesn’t actually clear the air. That’s more common than it sounds.

For broader project planning, explore WBC’s building materials resources.

Here’s the mistake almost everyone makes: picking a fan by size or catalog number alone. Airflow isn’t something you choose in isolation. A fan creates a pressure difference that overcomes the resistance of the system. Airflow is the result of the balance between fan capability and system resistance. Put plainly — the fan and your ductwork meet at one operating point, and that point decides whether the room actually clears.

For Montana, this matters more than most places. Indoor moisture that never makes it outside condenses inside walls and ceilings. In a cold climate that means mold and rot. So sizing and venting aren’t optional details. They’re the whole game.

Sizing: Start With CFM, Then Account for Resistance

Airflow capacity gets measured in CFM. Start there.

A common baseline for bathrooms is 1 CFM per square foot, with at least 50 CFM for any bathroom. That covers a standard bath. For kitchens and showers, which require higher extraction, aim for 100 to 300 CFM.

Then add headroom. Use a 20% performance buffer to account for pressure resistance caused by ducting. The reason is simple — duct adds resistance, and that resistance eats into rated airflow.

Don’t oversize past that, though. A fan that is undersized will fail to deliver the required airflow at the actual system resistance. A fan that is oversized may create noise, waste energy, and operate far from its best efficiency point. The trap is treating selection as a catalog exercise. Pick an airflow, add a pressure margin, and move on — that approach is exactly why many systems fail to perform the way the designer intended.

There’s a physics reason a 20% buffer is enough and a wild guess is wrong. Small changes in airflow cause disproportionately higher pressure losses. If you increase airflow by 10%, pressure drop does not increase by 10%; it increases roughly by 21%. Overshoot “just to be safe” and you pay for it in energy and noise every hour the fan runs.

Noise matters in any room people actually occupy. Exhaust fans get rated in sones — lower is quieter. Use quiet models (0.3 to 1.0 sones) and consider features like humidity sensors, timers, and backdraft shutters for improved efficiency.

Application	Target Airflow
Bathroom (baseline)	1 CFM per sq ft, 50 CFM minimum
Kitchen / shower	100 to 300 CFM
Buffer for duct resistance	Add ~20%
Quiet operation (living space)	0.3 to 1.0 sones

Fan Types and Where Each One Fits

Where the unit mounts and how it’s built drives performance, noise, and where it can physically go.

By mounting: Ceiling-mounted units are the standard for baths and kitchens. Wall-mounted fans make sense when there’s no ceiling or attic access to work with. Inline fans sit in the ductwork itself — they offer the quietest operation and the ability to vent multiple rooms through branched ducts.

By blade design: This is where airflow versus pressure gets decided.

Axial fans, sometimes called propeller fans, move a great quantity of air but have very little static pressure capability. In HVAC products they’re almost always used for condenser fans. They may be used for light static exhaust fans. That’s the key limitation — high volume, low pressure.

For ducted runs with more resistance, the type steps up. Propeller and tube axial types work well up to about 250 Pa. Vane axial fans extend that range to 500 Pa. Beyond 500 Pa, you need a centrifugal fan.

A propeller fan is the right call when there’s no duct involved. Propeller fans are the simplest option and the right one when there’s no ductwork — wall-mounted exhaust in warehouses, side ventilation in agricultural buildings, roof openings in storage facilities. Low resistance, high airflow, easy to install. But add even a little duct and they fall off fast. Even a short 6-foot section raises static pressure enough to reduce airflow substantially on a propeller fan. If your exhaust path goes through any ductwork at all, move to tube axial.

Efficiency varies by blade type too. Comparing static efficiency, FC fans have a range of 60 to 80%, AF fans have a range of 75 to 80%, and axial fans have a static efficiency in the range of 70 to 72%.

Type	Static Pressure Limit	Best For
Propeller	Up to 125 Pa	Open wall/roof, no ductwork
Tube axial	125 to 250 Pa	Ducted factory/exhaust runs
Vane axial	250 to 500 Pa	Long ducts, multiple bends
Centrifugal	Above 500 Pa	High resistance, complex filtration

Drive type is one selection question most buyers skip. Direct drive versions have no belts to degrade in chemical vapor environments. In a damp or vapor-heavy airstream, that’s the practical edge — no belt sitting in the moisture.

Venting It Right — Especially in a Cold Climate

This is where good intentions cause rot. Avoid venting into attics; always vent directly outside. Dump warm, moist bathroom air into a Montana attic and it condenses on the cold sheathing. That’s mold and rot waiting to happen.

Use the right duct. Prioritize rigid metal or PVC ducting to minimize air resistance. Smooth, rigid duct means less resistance, which means the fan actually delivers its rated airflow.

Backdraft shutters earn their keep in winter. They stop cold outside air from flowing back through the fan when it’s off. Worth specifying on any unit that vents to the exterior.

If you’re supplying air through a wall opening — or mounting near a snow-prone roofline — water and snow ingress is a real concern. The amount of water captured depends on air velocity, water droplet size, length of the event, wind strength and wind direction. Because of these variables, some degree of water entrainment can occur. A few mitigations help: weatherhoods and louvers are recommended to reduce the likelihood of water entering, and installing the fan with a slight slope toward the outside will minimize water ingress. Snow is the bigger worry up here — air velocities below 500 ft/min reduce the risk of rain ingress; however snow can be captured at much lower rates.

Some of this work calls for a pro. If you are uncomfortable with roof penetrations, complex electrical wiring, or cutting exterior walls, hire a licensed contractor to ensure code compliance and prevent leaks or fire hazards.

What Western Building Center Carries

The exhaust lineup runs from straight fans to combo units that pull double duty. The BFQ50 is a high-performance bath fan, and the BFQ90 runs at 2.5 sones. For combination units, the BFQF70 pairs a bath fan with a light, the Air King LEDAK80 combines an exhaust fan with a light, and there’s a 70 CFM fan/light/heat unit for baths that need warmth too. For the duct side, a 4-inch adjustable aluminum elbow handles the turns. Moving-air needs beyond exhaust are covered by a 16-inch oscillating pedestal fan and a 24-inch two-speed direct drive drum fan.

Frequently Asked Questions

How much airflow does a bathroom fan need? Figure 1 CFM per square foot of floor area, with a 50 CFM floor for any bathroom. Add roughly 20% on top to cover the resistance your ductwork adds.

Can I vent an exhaust fan into the attic? No. Always vent directly to the outside. Venting into an attic dumps moist air against cold framing, where it condenses and leads to mold and rot — a serious problem in a cold climate.

What duct should I use? Rigid metal or PVC. Both keep air resistance low so the fan delivers the airflow it’s rated for. Flexible or undersized duct adds resistance and chokes performance.

Why does my fan run but the room still feels stuffy? Usually undersized for the actual system resistance. A fan that can’t overcome the duct resistance runs continuously without clearing the air. Recheck your CFM against room size and add the buffer for ducting.

What’s the difference between an axial and a centrifugal fan? Axial (propeller-style) fans move large volumes of air at low pressure — good for open or short-run exhaust. Centrifugal fans handle higher resistance, needed when static pressure climbs above about 500 Pa or when there’s complex ductwork or filtration.

Do backdraft shutters matter? In winter, yes. They keep cold outside air from flowing back through the fan when it’s off, which helps efficiency and comfort in a cold climate.