Bathroom ventilation is often overlooked or poorly installed, leading to serious long-term issues like excess humidity, mold, and material deterioration. At AirGreen, we’ve repeatedly seen, through our service calls across Montréal, Laval, Longueuil, the North Shore and the South Shore, that a large number of HVAC installations in bathrooms do not follow proper technical guidelines. This article highlights the most common mistake we encounter during our HVAC maintenance and installation services — and the best way to correct it for good.
A widespread mistake: connecting the fan to the same exhaust duct as the kitchen hood or dryer
The most frequent mistake we identify during our service calls is connecting multiple ventilation devices to a single exhaust duct. Typically, a bathroom fan is connected to the same duct as the kitchen hood or the dryer, which is non-compliant, ineffective, and dangerous.
This type of setup, while it may seem cost-effective in the short term, creates a series of technical problems:
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Backdrafting: When multiple appliances share the same duct, air expelled by one can return through another, bringing humidity, grease, or heat where it shouldn’t go.
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Moisture buildup: Poor evacuation of moist bathroom air leads to condensation in walls and ceilings, accelerating mold growth.
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Cross-contamination risks: Odors or vapors from the kitchen or laundry room can end up in the bathroom, compromising indoor air quality.
Why is this mistake so common?
Often, non-specialized contractors or DIY homeowners try to cut costs or simplify the job by combining ducts. While it may seem practical, the Québec Building Code strictly prohibits this configuration. Each ventilation device must have its own independent duct leading directly outdoors.
Real consequences of poor bathroom ventilation
At AirGreen, we’ve handled numerous cases in Montréal and surrounding areas where poor ventilation caused major structural damage. Here are a few concrete examples:
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In Longueuil, a customer had connected their bathroom fan to the kitchen hood’s duct. The result: steam and grease built up in the shared duct, clogging the exit and causing serious moisture issues. We had to replace the entire duct network and treat mold in the attic.
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In Laval, a single-family home had persistent odors in the bathroom. Our technicians found that air was being drawn back in from the dryer due to a faulty backdraft damper. By redesigning the system with two separate, well-insulated ducts, we solved the problem instantly.
How can you fix this mistake effectively?
The solution is simple, but must be done professionally: each ventilation system must have its own sealed, insulated duct (if needed), with a proper backdraft damper at the outlet.
Key steps for a proper bathroom ventilation installation:
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Choose the right fan: The recommended airflow rate is around 1 CFM per square foot, with a minimum of 50 CFM.
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Use a dedicated duct: It should be as direct as possible, with minimal bends, and a sufficient diameter (usually 4 or 6 inches, depending on the airflow).
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Never vent into the attic or crawl space: The duct must exhaust outside the building, never into unventilated areas.
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Install an exterior damper: This prevents cold air from coming back in and stops moisture infiltration.
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Insulate the duct: Especially in unheated areas like attics or exterior walls, to prevent condensation.
Why call AirGreen to fix your ventilation?
At AirGreen, we have deep expertise in residential and commercial ventilation throughout Greater Montréal. Our team is fully versed in building codes, installation best practices, and local climate conditions.
Here’s what we offer during our bathroom ventilation services:
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Complete inspection of your current system
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Detection of improperly connected or blocked ducts
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Recommendation of a high-performance, quiet, energy-efficient fan
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Professional installation according to building code
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Installation warranty and post-service follow-up
Real project: Bathroom ventilation installation in Sainte-Thérèse (North Shore)
In an older triplex in Sainte-Thérèse, we were called to fix a malfunctioning bathroom ventilation system. The owner complained about persistent condensation on the walls, peeling paint, and lingering odors.
Our diagnosis: the original fan was outdated and connected to the same duct as the kitchen hood in the basement. We installed a Panasonic WhisperCeiling 110 CFM fan, a dedicated insulated aluminum duct, and a magnetic-seal exterior wall damper. Result: humidity was controlled within the first weeks, and indoor air quality noticeably improved.
Our practical tips for homeowners
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Have your ventilation system inspected if you notice persistent fog, black spots on the ceiling, or musty odors.
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Avoid temporary solutions like plug-in fans or poorly secured flexible ducts.
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Invest in a quality fan: a quiet, high-performance, well-installed fan will save you from costly repairs later.
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Call an HVAC expert before making any changes. A professional diagnosis will save you time and money.
Conclusion
Poorly designed bathroom ventilation can cause more damage than you think. A bad duct layout or the wrong choice of fan leads to humidity issues, mold, and reduced comfort. At AirGreen, we have the skills, experience, and tools to ensure a compliant, efficient, and long-lasting bathroom ventilation system. Whether you’re in Montréal, Laval, Longueuil, the North Shore or the South Shore, call on us for professional, customized service.
Contact us today for a professional bathroom ventilation inspection or installation.