What size exhaust duct does BC code require for a bathroom fan vented to the exterior?

The BC Building Code requires bathroom exhaust fans to vent to the exterior through a duct that matches the fan's outlet size — typically 4 inches (100 mm) in diameter for most residential bathroom fans rated at 50–110 CFM. Never reduce the duct size below the fan's outlet diameter, and never vent into an attic, soffit, crawl space, or wall cavity. Exterior venting is mandatory.

Most residential bathroom exhaust fans in the 50–110 CFM range come with a 4-inch round outlet, and the duct connecting the fan to the exterior wall or roof cap should be the same 4-inch diameter throughout its run. Some higher-capacity fans (150+ CFM, used in large master ensuites) have a 6-inch outlet and require 6-inch ducting. Using a duct smaller than the fan's outlet creates back pressure, reduces airflow, increases noise, and causes condensation inside the duct — all problems you want to avoid in Metro Vancouver's already humid climate.

Rigid smooth-wall metal duct is the best choice for bathroom exhaust runs. It has the lowest airflow resistance, does not trap lint and moisture like flexible duct, and lasts decades without degradation. In Metro Vancouver, where bathroom exhaust systems work harder and longer than in drier climates due to 75–85% outdoor humidity, rigid duct pays for itself in better performance and lower maintenance. A typical 4-inch rigid duct installation from the bathroom fan to an exterior wall cap costs $200–$500 for materials and labour, depending on the run length and accessibility.

Flexible duct (flex duct) is commonly used because it is easier and faster to install, especially in tight ceiling and attic spaces. If flex duct is used, it must be the insulated type to prevent condensation on the outer surface of the duct in cooler weather. Flex duct should be pulled as straight and taut as possible — sagging flex duct traps condensation and restricts airflow. Every bend and sag in flex duct reduces effective CFM. The BC Building Code does not prohibit flex duct for bathroom exhaust, but rigid duct is the better-performing option.

Insulation on the duct is important in Metro Vancouver, particularly for duct runs through unconditioned spaces like attics or exterior wall cavities. When warm, moist exhaust air from the bathroom passes through a cold duct, condensation forms inside the duct. Over time, this condensation drips back toward the fan or accumulates in low spots, potentially causing water staining on ceilings or feeding mould growth. Insulated duct (R-4 minimum) prevents this condensation and is required for runs through unconditioned spaces.

The exterior termination must be a proper wall cap or roof cap with a damper flap that opens when the fan runs and closes when it stops. This prevents cold air, rain, and pests from entering the duct when the fan is off. In Metro Vancouver's rainy climate, a quality exterior cap with a spring-loaded damper is essential — cheap plastic caps deteriorate in UV exposure and rain, and their flaps stick open or break off within a few years. A quality stainless steel or painted galvanized wall cap costs $25–$60 and is well worth the investment.

Duct length and routing affect performance significantly. Every foot of duct and every elbow reduces the effective CFM of your fan. A general rule: each 90-degree elbow is equivalent to approximately 5 feet of straight duct in terms of airflow resistance. If your bathroom is far from an exterior wall — common in interior bathrooms of larger Metro Vancouver homes — you may need a higher-CFM fan to compensate for the long duct run. Most fan manufacturers publish performance charts showing CFM delivery at various duct lengths and configurations.

For a bathroom renovation in Metro Vancouver, budget $300–$800 for a complete exhaust fan installation including the fan unit ($150–$400), ducting to the exterior ($100–$300), and the exterior cap ($25–$60). If you are upgrading from an existing fan, the duct run may already be in place, reducing the cost to $200–$500 for the fan swap and any duct improvements. This work involves electrical connections and should be done by a licensed electrician, with the duct installation handled by either the electrician or your general contractor.