Quick Answer
To install an exhaust fan in a bathroom window, buy a window-mount fan sized at about 1 CFM per square foot of bathroom, remove the window sash or pane, and replace the glass with a polycarbonate or glass-shop panel that has a round cutout for the fan. Mount the fan, seal it with exterior silicone, and connect it to a switched GFCI-protected circuit. Most installs take 2-4 hours.
Key Takeaways
- Size the fan at roughly 1 CFM per square foot for bathrooms up to 100 square feet; 50 CFM is the common minimum.
- Never try to drill or cut tempered glass, which shatters; replace the pane with polycarbonate or order glass pre-cut by a glass shop.
- Plug-in window fans need no wiring, while hardwired fans should be connected by a licensed electrician if you are not experienced.
- Seal every gap with exterior-grade silicone so the fan does not let in rain, drafts, or insects.
- Run the fan during showers and for about 20 minutes afterward to clear moisture and prevent mold.
A bathroom without a ceiling duct still needs a way to push humid air outside. When the only exterior opening is a window, a window-mounted exhaust fan is often the simplest fix. It removes steam, odors, and moisture that would otherwise feed mold on grout, ceilings, and window frames.
Why Install an Exhaust Fan in a Bathroom Window?
Running a fan through the window avoids cutting into the roof or running ductwork through an attic. That makes it a practical choice for condos, older homes with plaster ceilings, basement bathrooms, and rentals where major remodeling is not allowed. It also keeps the window usable for light while ventilating far more reliably than cracking the sash open in winter.
Moisture control matters because bathroom humidity regularly spikes above 70 percent during a hot shower. The EPA recommends keeping indoor humidity below 60 percent, ideally 30-50 percent, to limit mold growth. A properly sized fan brings the room back down quickly.
What Size Exhaust Fan Does Your Bathroom Need?
Ventilation is measured in cubic feet per minute (CFM). The Home Ventilating Institute guideline is 1 CFM per square foot of floor area for bathrooms of 100 square feet or less, with 50 CFM as a typical minimum. Larger bathrooms are sized by fixture count instead.
| Bathroom size | Recommended fan | Notes |
|---|---|---|
| Up to 50 sq ft (powder room) | 50 CFM | Common minimum rating |
| 50-80 sq ft | 70-80 CFM | Typical full bath |
| 80-100 sq ft | 80-110 CFM | Add capacity for tall ceilings |
| Over 100 sq ft | Add 50 CFM per toilet, shower, or tub; 100 CFM per jetted tub | HVI fixture method |
Also check the sone rating, which measures noise. A fan rated 1.0 sone or less is quiet enough that people actually use it. Window fans tend to be louder than ceiling units, so compare models carefully.
Which Type of Window Exhaust Fan Should You Choose?
- Through-the-glass fans: a round unit, usually 6 to 9 inches, mounts through a hole in a glass or polycarbonate panel. Clean look and permanent, with backdraft shutters on the outside.
- Window-insert fans: sit in the open sash with adjustable side panels, similar to a small window air conditioner. Easiest for renters, but they block the window opening.
- Plug-in vs. hardwired: plug-in models connect to a nearby GFCI outlet; hardwired models connect to a wall switch, timer, or humidity sensor.
Choose a fan with a reversible or exhaust-only setting, gravity or automatic shutters to block drafts, and a motor rated for damp locations.
What Tools and Materials Do You Need?
- Window-mount exhaust fan with manufacturer template
- Replacement panel: 1/4-inch polycarbonate sheet, or a new glass pane pre-cut by a glass shop
- Exterior-grade silicone sealant and caulk gun
- Glazing points, glazier’s putty or the window’s original glazing strips
- Jigsaw with a fine-tooth plastic blade or hole saw (for polycarbonate only)
- Drill, screwdriver, tape measure, marker, utility knife
- Safety glasses and cut-resistant gloves
- Voltage tester if hardwiring
Safety: Glass near a tub or shower is often tempered safety glass, which cannot be cut or drilled; it shatters into pieces. Look for a small etched stamp in a corner of the pane. If you want glass rather than plastic, have a glass shop make a new tempered pane with the hole cut before tempering.
How to Install an Exhaust Fan in a Bathroom Window
Step 1: Measure the Window and Choose the Location
Measure the pane you plan to replace, usually the upper sash, so the fan sits high where warm, moist air collects. Confirm the fan diameter plus its mounting flange fits with at least 1-2 inches of panel on all sides. Check that the window can still lock and that the exterior shutter will not hit screens or storm windows.
Step 2: Remove the Sash and the Existing Pane
Remove the sash from the frame if possible and lay it on a padded work table. Wearing gloves and safety glasses, remove the glazing strips, points, or putty and lift the old pane out. Keep the original glass if you may want to restore the window later, which is especially important in rentals.
Step 3: Prepare the New Panel With the Fan Cutout
Cut polycarbonate to the pane size, then trace the manufacturer’s template and cut the hole with a jigsaw and fine plastic blade or a hole saw, keeping the protective film on to avoid scratches. Drill pilot holes for the mounting screws. If you are using glass, give the glass shop the exact pane size and template so they cut and temper it for you.
Step 4: Mount the Fan in the Panel
Fit the fan through the hole with the shutter facing outside and the grille facing the room. Add the included gaskets, run a bead of silicone around the flange, and tighten the screws evenly without over-torquing, which can crack the panel. Wipe away excess sealant.
Step 5: Reinstall the Panel and Seal the Sash
Set the new panel in the sash with glazing points or the original strips, then seal the perimeter with exterior silicone or glazing putty. Reinstall the sash, check that it opens and locks smoothly, and caulk any gaps on the exterior side so rain cannot drip in.
Step 6: Connect Power and Test
Plug-in fans connect to a GFCI outlet; route the cord neatly with adhesive clips and keep it away from water. For a hardwired fan, shut off the breaker, confirm the circuit is dead with a tester, and connect to a switch box following the manual, or hire a licensed electrician. Turn the fan on and hold a tissue near the grille to confirm air is being pulled out.
How Do You Wire a Bathroom Window Fan Safely?
Bathroom circuits have stricter rules than other rooms. Receptacles in bathrooms must be GFCI protected under the National Electrical Code, and many local codes also require it for fans installed over or near a tub or shower. Most local jurisdictions require a permit for new hardwired circuits.
- Use a fan listed for damp or wet locations if it sits near the shower.
- Pair the fan with a countdown timer or humidity-sensing switch so it keeps running after you leave.
- Never run extension cords to a bathroom fan.
- If you are unsure about adding a switch or box, call a licensed electrician; wiring mistakes in wet rooms create shock and fire risks.
Troubleshooting Common Problems
| Problem | Likely cause | Fix |
|---|---|---|
| Cold draft when fan is off | Shutter stuck open or missing | Clean the shutter hinges; add a backdraft damper kit |
| Water dripping inside | Gaps in sealant | Re-caulk the exterior flange and sash perimeter |
| Rattling or buzzing | Loose screws or thin panel | Tighten evenly; add rubber washers; use thicker polycarbonate |
| Mirror still fogs | Fan undersized or door sealed shut | Upsize the fan; leave a 3/4-inch gap under the door for makeup air |
| Fan trips the outlet | Moisture or ground fault | Stop using it and have an electrician inspect it |
Fans also need occasional cleaning. Dust and lint on the grille and blades can cut airflow noticeably. Unplug or switch off the breaker, remove the grille, and wipe it with warm soapy water every few months. A disinfectant such as the one covered in this guide to using Lysol disinfectant spray can help on surrounding surfaces where mildew starts.
How Can You Reduce Bathroom Moisture Besides a Fan?
- Run the fan during every shower and for about 20 minutes afterward.
- Squeegee glass doors and tile walls after showering.
- Hang towels to dry outside the bathroom or on a heated rack.
- Keep caulk and grout sealed; repair cracked tub caulk promptly, as covered in this guide to fixing a crack in a plastic bathtub.
- Use a small dehumidifier in windowless or basement bathrooms.
When Should You Call a Professional?
Call a licensed electrician for any new hardwired circuit, switch, or junction box, or if you have aluminum wiring or an older panel without GFCI protection. Call a window or glazing contractor if the window is double-pane insulated glass, since replacing an insulated unit with a single panel reduces energy efficiency and may void the window warranty. If the bathroom already shows widespread mold on drywall, consult a remediation professional before installing the fan.
Tip: If you rent, ask your landlord first. Many will approve a window-insert fan or will install a proper ventilation fan themselves, because building codes generally require bathrooms to have either an exhaust fan or an openable window.
Frequently Asked Questions
Can you put an exhaust fan in a bathroom window?
Yes. Window-mounted exhaust fans are made for bathrooms without ceiling ducts. They mount through a replacement glass or polycarbonate panel or sit in the open sash as an insert. They vent moisture straight outside, which meets the goal of most building codes for bathroom ventilation.
Is a window fan as good as a ceiling exhaust fan?
A ceiling fan usually captures steam better because warm air rises, and it runs quieter. A well-sized window fan mounted high in the upper sash works well, especially when it runs during showers and for about 20 minutes after. It is often the most practical option without attic access.
Do I need a permit to install a bathroom window fan?
A plug-in window fan usually needs no permit. Adding a new hardwired circuit, switch, or junction box often does, depending on your city or county. Check with your local building department, and use a licensed electrician for any permanent wiring in a bathroom.
How do I stop cold air coming through a window exhaust fan?
Choose a model with backdraft shutters and keep them clean so they close fully. Seal all gaps around the flange and sash with exterior silicone. In very cold climates, some owners add an insulated cover over the interior grille during long periods when the fan is not used.
How much does it cost to install an exhaust fan in a bathroom window?
Costs vary widely by fan model, glass versus polycarbonate panel, and whether an electrician is needed. A plug-in DIY install mostly costs the fan and materials. Custom tempered glass and hardwired switches add glass shop and electrician fees, so get local quotes before starting.
