Can I vent two Calgary bathrooms into one exhaust duct?

No, you should not vent two bathroom exhaust fans into a single shared duct — this is a code violation under the Alberta Building Code and creates serious performance and moisture problems. Each bathroom exhaust fan requires its own dedicated duct run to its own exterior vent cap. When two fans share a duct, the operating fan can push moist air backward through the non-operating fan into the other bathroom, defeating the purpose of ventilation entirely.

The reason this matters goes beyond just code compliance. When one fan runs and the other is off, the path of least resistance for the exhausted air is often backward through the idle fan rather than all the way to the exterior vent. This means the running fan is effectively pumping warm, moisture-laden air from one bathroom into another — exactly the opposite of what you want. Even if both fans run simultaneously, they compete for airflow in the shared duct, reducing the effective CFM of each fan significantly. In Calgary's winter, this reduced performance means moisture lingers longer on cold surfaces, accelerating condensation, mould growth, and deterioration of caulk and grout.

There is one code-compliant alternative: an inline fan system. Rather than two separate exhaust fans, you can install a single inline exhaust fan (mounted in the attic or between joists) that pulls air from two bathrooms through separate intake grilles, each connected to the inline fan with its own dedicated branch duct. The inline fan then exhausts through a single duct to the exterior. This is an engineered system — not the same as simply Y-connecting two bathroom fans into one duct. Brands like Fantech and Panasonic make inline ventilation systems designed specifically for multi-room exhaust. Each intake grille has a backdraft damper to prevent air from one bathroom from flowing into the other when only one room is producing moisture. An inline fan system for two bathrooms costs $400 to $800 for the fan unit plus $500 to $1,500 for installation including ductwork, grilles, and electrical connections in Calgary.

If you are renovating and need to vent two bathrooms that are close together, the inline fan approach is often the most practical solution because it requires only one roof or wall penetration for the exterior vent — important in Calgary where every roof penetration is a potential point for ice damming and water intrusion during our freeze-thaw chinook cycles. A single, well-sealed exterior vent cap is better than two marginal ones.

Any new exhaust duct installation or modification requires a permit from the City of Calgary. This includes adding new ductwork, rerouting existing ducts, or installing an inline fan system. The permit ensures a Safety Codes Officer inspects the installation to verify proper duct sizing, slope (ducts must slope slightly toward the exterior to prevent condensation from pooling), insulation of ducts in unheated spaces like attics, and proper exterior termination with a backdraft damper. Permit fees for ventilation work typically run $75 to $200.

Hire a licensed contractor for any exhaust duct work. Improperly installed ductwork in a Calgary attic — where winter temperatures can drop well below -30°C — leads to condensation inside the duct that freezes, eventually thawing during chinooks and dripping back into your bathroom or into the attic insulation. Proper duct insulation (minimum R-8 for ducts in unheated spaces) and sealing at all joints with foil tape (never cloth duct tape, which deteriorates) are essential for Calgary installations.

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