Why is my bathroom always humid even with the fan running?

Your bathroom fan is likely undersized for the space, not running long enough, or there may be ventilation issues preventing proper moisture removal. Even with a fan running, persistent humidity indicates the system isn't effectively moving moist air out of the room.

The most common culprit is an undersized exhaust fan. Many Toronto homes, especially older ones, have builder-grade fans rated at only 50-80 CFM (cubic feet per minute), which isn't adequate for most bathrooms. The proper sizing should be calculated at 1 CFM per square foot of bathroom space, with a minimum of 50 CFM. For a typical 8x10 foot bathroom, you need at least 80 CFM, but 100-110 CFM is better for effective moisture removal. If your bathroom has a separate toilet area, shower, and vanity, you may need 150+ CFM or multiple fans.

Ductwork problems are another major issue in Toronto's older housing stock. The fan may be properly sized but the ductwork could be too long, have too many bends, or terminate improperly. In many century homes and post-war bungalows, bathroom fans were retrofitted with inadequate ducting that doesn't provide a clear path to the exterior. The duct should be as short and straight as possible, typically 4-6 inches in diameter, and must terminate outside - never into an attic or soffit area. Flexible ducting should be avoided in favor of rigid metal dubing for better airflow.

Runtime duration is critical but often overlooked. The fan should run during the shower and for at least 30-60 minutes afterward to remove residual moisture. Many homeowners turn the fan off immediately after showering, leaving humidity trapped in the room. Consider installing a humidity-sensing switch or timer switch that automatically runs the fan for a preset duration.

In Toronto's climate, exterior conditions can also affect performance. During humid summer months or when there's little wind, even properly sized fans struggle against back-pressure. Winter conditions can cause ice buildup in exterior vents, blocking airflow entirely.

Professional assessment is recommended if you've verified the fan runs properly but humidity persists. An HVAC contractor can measure actual CFM output, inspect ductwork, and check for proper exterior termination. Electrical work for fan upgrades requires ESA permits in Ontario, and any ductwork modifications may need building permits depending on scope.

Next steps: First, check your fan's CFM rating (usually on a label inside the unit) and measure your bathroom to confirm proper sizing. Run the fan for longer periods and see if humidity improves. If problems persist, have a qualified contractor assess your ventilation system - browse contractors in our Toronto Construction Network directory for local HVAC professionals who can properly diagnose and resolve bathroom ventilation issues.