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Sizing local exhaust fans for Canadian homes
Local exhaust ventilation removes moisture, odours, and pollutants right at the source — bathrooms, kitchens, and laundry rooms — before they spread throughout the home. This is distinct from whole-building ventilation, which provides general dilution air for the entire dwelling. ASHRAE 62.2 requires both: a whole-building rate plus local exhaust capacity for these high-moisture and high-pollutant rooms.
For bathrooms, ASHRAE 62.2 sets a minimum of 50 CFM intermittent or 20 CFM continuous local exhaust. Canadian provincial building code amendments often align with or exceed this. A simpler area-based rule of thumb commonly used by contractors is 1 CFM per square foot of bathroom area, with an absolute minimum of 50 CFM regardless of room size — useful as a quick cross-check against the ASHRAE minimum.
Kitchen exhaust for residential applications
Residential kitchen range hoods are typically sized based on cooktop BTU output or simply by CFM rating matched to hood width: a 30-inch hood commonly uses 150-300 CFM, while professional-style ranges may need 400-600+ CFM. ASHRAE 62.2 sets a minimum kitchen exhaust rate of 100 CFM intermittent or 5 ACH continuous, whichever the homeowner or designer chooses to provide. For ducted range hoods serving high-BTU professional ranges, makeup air becomes a critical consideration — see the makeup air calculator for sizing requirements once exhaust exceeds about 400 CFM in most Canadian jurisdictions.
Duct sizing and static pressure considerations
The fan's nameplate CFM rating is measured at zero static pressure in a lab setting. Real-world performance drops as duct length, elbows, and termination hood resistance add static pressure. A fan rated at 110 CFM free-air might only deliver 70-80 CFM through a long, elbow-heavy 4-inch duct run. Always select a fan rated for higher airflow than your calculated requirement to account for this derating, and verify the fan's published performance curve at your actual system static pressure, not just the maximum free-air number on the box.
Noise (sones) and occupant compliance
A fan that's too loud often gets switched off by occupants, defeating its purpose entirely. ENERGY STAR and most Canadian ventilation guidance recommends bathroom fans rated 1.5 sones or quieter for spaces where people spend time, reserving slightly higher sone ratings only for quick-use powder rooms. Higher-CFM fans tend to be louder unless using a remote inline blower design that relocates the noisy motor into the attic or duct run away from the occupied space.
Frequently Asked Questions
ASHRAE 62.2 requires at least 50 CFM intermittent or 20 CFM continuous local exhaust for bathrooms. Many Canadian codes require a minimum 50 CFM intermittent for bathrooms with a tub or shower. An alternative area-based method uses 1 CFM per square foot with a 50 CFM minimum. A typical 5×8 ft bathroom (40 sq ft) needs approximately 40-50 CFM by either method. Use the calculator above to get your exact requirement and recommended duct size.
1 sone is roughly equivalent to a quiet refrigerator hum. ENERGY STAR and most Canadian professionals recommend 1.5 sones or lower for comfortable use — occupants are far more likely to actually use a quiet fan. Standard builder-grade fans often run 3-4 sones. Higher-CFM fans (over 80 CFM) tend to be louder unless using a remote-mounted inline blower design that moves the motor noise away from the room.