What Size Exhaust Fan Does Your Bathroom Need?
What Size Exhaust Fan Does Your Bathroom Need?
A bathroom exhaust fan is one of the most important moisture-control tools in your home. If the fan is undersized or poorly vented, humidity lingers and damages paint, drywall, and framing.
Sizing Basics
Most fans are rated in CFM (cubic feet per minute). A simple rule of thumb:
- Match fan CFM to bathroom square footage (for example, a 50 square foot bath gets a 50 CFM fan).
Larger bathrooms, long duct runs, or high ceilings often need more.
Features That Improve Real-World Performance
- Low noise rating so people actually use it
- Humidity sensor for automatic operation
- Timer switch to keep it running after a shower
Vent to the Outside
The fan should vent to the exterior, not into the attic. Moist air in the attic can create mold and roof issues.
Placement and Ducting Tips
Fan performance depends on the duct run. Keep ducts short and straight when possible, and vent to an exterior wall or roof cap.
Place the fan close to the shower or tub, but not directly above the shower unless it is rated for wet locations.
Signs Your Fan Is Undersized
- Mirrors stay foggy for a long time
- Paint peels or the ceiling stains
- The bathroom smells damp after use
A Simple Upgrade Checklist
- Quiet fan rating so people use it
- Timer switch or humidity sensor
- Proper exterior vent cap
These small upgrades make a big difference in moisture control.
Fan Placement for Better Results
Aim to place the fan near the main moisture source. If your bathroom is long or has a separate toilet area, consider a second fan or a higher-capacity unit.
Simple Operating Habits
Even the best fan will not help if it is never used. A timer switch makes it easy to run the fan for a set time after a shower.
Final Thought
If you are remodeling, upgrade the fan. It is a low-cost improvement that prevents expensive moisture damage.