The bathroom exhaust fan is the least glamorous piece of mechanical equipment in your house and one of the highest-return upgrades available. A fan that’s undersized, poorly ducted, or vented into the attic instead of outdoors doesn’t just fail to clear steam — it deposits moisture directly into your building assembly, where it grows mold, rusts fasteners, and ruins insulation.
We open a lot of Glenview and Northbrook attics, and the single most common defect we find is a bathroom fan duct that terminates six inches above the insulation, blowing warm humid air into a cold attic all winter. Here’s how to size, duct, and install a bathroom fan correctly, and what it costs.
Why It Matters More Than People Think
A single ten-minute shower releases roughly half a pound of water into the air. Multiply by two showers a day, add a family of four, and you’re generating pounds of moisture daily that has to go somewhere. If the fan doesn’t move it outdoors, that moisture:
- Condenses on cold surfaces — window frames, exterior walls, and attic sheathing
- Feeds mold on grout, ceiling paint, and drywall paper
- Degrades attic insulation performance when it soaks in
- Causes peeling paint and swollen trim
- In extreme cases, rots roof sheathing from below — a repair that runs into five figures
Chicago-area winters make this worse than in milder climates, because the temperature difference between the humid indoor air and the attic sheathing is large enough for condensation and frost to form readily.
Sizing: How Much CFM Do You Need?
Two standard methods, and you use whichever gives the larger number.
Method 1 — By floor area (bathrooms under 100 sq ft)
1 CFM per square foot, with a 50 CFM minimum. A 7×8 bathroom (56 sq ft) needs at least 56 CFM, so an 80 CFM fan.
Method 2 — By fixture count (bathrooms over 100 sq ft)
| Fixture | CFM |
|---|---|
| Toilet | 50 |
| Shower | 50 |
| Bathtub | 50 |
| Jetted tub | 100 |
A large primary bath with a toilet, a shower, and a jetted tub needs 200 CFM — often better delivered as two separate fans, one over the shower and one serving the toilet area, than one very loud unit.
Common sizes
| Bathroom Size | Recommended Fan |
|---|---|
| Powder room / under 50 sq ft | 50–80 CFM |
| 50–80 sq ft | 80–110 CFM |
| 80–100 sq ft | 110–150 CFM |
| 100+ sq ft or multiple wet fixtures | 150–250 CFM, often two fans |
Important: rated CFM is measured at 0.1″ static pressure on a test bench. A fan on 15 feet of flexible duct with two elbows may deliver 60% of its rating. Size up one step, or specify a fan with an ECM motor that holds its airflow under real duct loads.
Sone Rating: The Number That Determines Whether You Use It
A fan you find unbearably loud is a fan you turn off. Sones are a loudness scale:
- 4.0 sones — old builder-grade fan, sounds like a jet engine
- 2.0 sones — average TV volume, tolerable
- 1.0 sone — quiet refrigerator hum
- 0.3 sones or lower — genuinely near-silent; you check whether it’s on
Spend the extra $60 on a sub-1.0-sone fan. It’s the difference between ventilation that happens and ventilation that theoretically exists.
Ducting: Where Most Installations Fail
The fan is rarely the problem. The duct usually is. Rules that matter:
- Terminate outdoors. Through the roof with a proper cap, or through a gable or soffit. Never into an attic, soffit cavity, or crawlspace. Never into a soffit vent directly beneath an intake — you’ll pull the humid air right back in.
- Use rigid or semi-rigid duct where possible. Insulated flex is acceptable but must be pulled taut. Sagging flex collects condensate and drips back through the fan.
- Match duct size to the fan’s collar. Necking a 6″ fan down to a 4″ duct destroys its airflow. 4″ for fans up to 100 CFM, 6″ for 100–200 CFM.
- Keep it short and straight. Under 15 feet is ideal. Every 90° elbow costs roughly the equivalent of 15 feet of straight duct.
- Insulate any duct in unconditioned space. R-6 minimum, with the vapor barrier intact. Uninsulated duct in a cold attic condenses inside.
- Slope slightly downward toward the exterior so any condensate drains out, not back into the housing.
- Use a backdraft damper and an exterior cap with a functioning flapper — and check that the flapper isn’t painted shut.
Control Options Worth Considering
- Humidity-sensing (dehumidistat) fans — turn on automatically when RH rises, run until it drops. The best option for households where nobody remembers the switch, especially kids’ and guest bathrooms.
- Timer switches — 20 to 30 minutes after the shower ends is the right run time. A simple countdown timer is a cheap, high-value upgrade.
- Occupancy sensors — convenient, but they shut off when you leave, which is exactly when you need the fan running.
- Combination units — fan/light, fan/light/heater, or fan with Bluetooth speaker. Heater models need a dedicated 20-amp circuit; verify capacity before ordering.
- Continuous low-speed operation — some ECM fans run at 30–50 CFM continuously and boost on demand, which doubles as whole-house ventilation in a tight home. Pairs well with an indoor air quality strategy.
Installation Costs in Glenview & Northbrook
| Scope | Typical Cost |
|---|---|
| Replace fan, existing duct & wiring reused | $275 – $500 |
| Replace fan + new duct to exterior | $500 – $950 |
| New fan installation where none exists | $650 – $1,400 |
| Fan/light/heater unit + dedicated circuit | $900 – $1,800 |
| Reroute duct improperly terminated in attic | $350 – $750 |
| New roof penetration & cap | $250 – $500 |
| Humidity-sensing upgrade | +$75 – $200 |
| Timer switch installation | $125 – $250 |
| Inline remote-mount fan (quietest option) | $800 – $1,600 |
| Two-fan large primary bath | $1,200 – $2,400 |
Cost drivers: attic access, whether the ceiling is drywall or plaster, whether existing wiring can carry the load, roof vs. sidewall termination, and second-floor vs. first-floor bathroom (a first-floor bath under living space is the hardest duct route).
Signs Your Current Fan Isn’t Working
- The mirror stays fogged more than five minutes after a shower
- Mold or dark spotting on the ceiling near the fan or on grout lines
- Peeling paint or bubbling drywall tape in the bathroom
- The fan runs loudly but a tissue held to the grille doesn’t stick
- Frost or damp insulation in the attic above the bathroom in winter
- A musty smell that returns no matter how much you clean
The tissue test is the fastest diagnostic in home maintenance: hold a single square of toilet paper to the running fan’s grille. It should hold firmly. If it flutters or falls, you have a duct problem, a clogged grille, or a dead motor with a spinning-but-useless blade.
Simple Maintenance
Twice a year, pull the grille and vacuum the blade and housing. Dust loading on the blower wheel can cut airflow in half. While you’re up there, look at the damper and the exterior cap. Ten minutes of work restores performance that people typically replace a fan to fix.
Frequently Asked Questions
Can a bathroom fan vent into the attic?
No. It violates code and it damages your roof structure. If yours does, it’s worth fixing this season regardless of anything else on this list.
How long should the fan run after a shower?
At least 20 minutes. A timer switch handles this without anyone thinking about it.
Do I need a fan if my bathroom has a window?
Code sometimes permits an operable window as ventilation, but practically, nobody opens a window in January in Northbrook. Install the fan.
Can I put the fan on the light switch?
You can, and many homes do, but you lose the run-on time after the shower. Separate switching or a timer is better.
Why is my new fan louder than expected?
Usually duct restriction — undersized duct, crushed flex, or too many elbows — forcing the fan to work against pressure. Occasionally it’s loose mounting transmitting vibration into the joists.
What’s an inline fan?
The motor mounts remotely in the attic with only a grille in the ceiling, so almost no noise reaches the room. It’s the premium solution for a primary bathroom and worth the upcharge if quiet matters to you.
Get Ventilation That Actually Works
MMDS handles bathroom ventilation from a simple fan swap to full duct rerouting and dedicated circuits, across Glenview, Northbrook, Skokie, Evanston, and the surrounding North Shore. Because we do HVAC and electrical under one roof, we size the fan, run the duct properly, and handle the wiring without coordinating two contractors.
Call (847) 221-6280 or schedule an evaluation. If we’re already in the attic, we’ll check every bath fan in the house and tell you what we find. Financing through Sunbit and PowerPay is available on larger projects.