Ventilation management: fresh air, safely, in a tighter house

Why tighter houses need planned ventilation

Older houses got fresh air by accident, through gaps. NRCan explains that this leakage depends on wind and the indoor-outdoor temperature difference, so it peaks on cold, windy days, falls off in summer and does not reach closed bedrooms evenly. As NRCan's Keeping the Heat In ↗ puts it, “Air leakage does not make for good ventilation.”

As houses get tighter, that accidental supply shrinks. Health Canada ↗ notes that construction improvements have reduced unintentional leakage areas by up to 90% in recent decades, and that tighter houses are likely to be under-ventilated unless a mechanical system is present. The National Building Code (NBC) has called for mechanical ventilation in all new houses since its 1985 edition, which set a rate of 0.5 air changes per hour; the 1990 edition changed it to 0.3.

How much is enough? Health Canada says guidelines generally call for about 5 to 10 litres per second (L/s) of outdoor air for each occupant, or roughly a complete house air change every three hours. NRCan's target is similar: a system able to change all the household air once every three hours, plus a separate supply of combustion air for fuel-burning appliances.

Fans: code minimums and how long to run them

For houses with five or fewer bedrooms, the National Building Code 2025 ↗ describes three parts: a principal ventilation fan, supplemental exhaust fans in kitchens and bathrooms, and protection against depressurization. Provinces adopt the national code on their own schedule and sometimes change it, so check local rules. The principal fan's normal operating capacity is set by the number of bedrooms:

• Principal fan: approved for continuous operation, with a manual switch in the living area marked “VENTILATION FAN”. The exhaust side of an HRV can serve as this fan.
• Bathrooms: a fan rated at least 25 L/s (about 50 cubic feet per minute, or cfm) unless the principal fan already draws from that room. Fans mounted at the ceiling or wall are capped at 2.5 sones on the CSA scale or 3.5 on the HVI scale. A sone is a loudness rating; lower is quieter.
• Kitchens: a fan rated at least 50 L/s, ducted directly outdoors in non-combustible, corrosion-resistant duct, with a grease filter.
• Clothes dryers: exhaust directly outdoors.

Installed fans only help when they run. Health Canada's review found that bathroom fans operate under 30 minutes a day on average, and range hoods 15 to 45 minutes. Its advice: run the bathroom fan during every shower or bath and for at least 30 minutes afterward, and use the range hood whenever you cook, preferably on the highest setting.

NRCan adds that noisy fans tend not to be used, so buy quiet ENERGY STAR certified models designed for continuous use, make sure every fan exhausts fully outdoors, and avoid range hoods that recirculate air back into the room. For a fan meant to run continuously, NRCan's ventilation fact sheet ↗ suggests one using less than 25 watts, rated 1.0 sone or less, moving 25 L/s (50 cfm). A timer or humidity control can switch a bathroom fan on and off automatically.

Makeup air, backdrafting and signs of trouble

Every litre of air a fan pushes out has to be replaced. Without a planned supply, the house is depressurized and air is pulled in through any opening, including a chimney. Health Canada notes that depressurization of around 5 pascals (Pa) can make appliances that vent through a chimney, such as open fireplaces and natural-draft water heaters, backdraft. It adds that fans larger than roughly 90 to 180 L/s (200 to 400 cfm) need added venting or ductwork to supply makeup air.

The NBC's explanatory notes name naturally aspirated gas furnaces with draft hoods, oil furnaces with barometric dampers and almost all fireplaces as susceptible to spillage. In a home with a fuel-fired furnace, boiler or water heater that is not direct-vented or mechanically vented, the code requires every exhaust device other than the principal fan to have a makeup-air fan that switches on with it and supplies at least as much outdoor air as the device exhausts, but no more than 10% extra. A depressurization test to the CAN/CGSB-51.71 standard can show that it is not needed, and the need disappears if those appliances are replaced with spillage-resistant types.

Signs of trouble. NRCan links stuffy air, lingering odours and high humidity in fall and winter to inadequate ventilation. Signs of combustion problems include soot or staining around a furnace, boiler or water heater; melted plastic fittings on top of a tank water heater; odours or hot, muggy air near the appliance; frequent headaches or nausea; and unexplained carbon monoxide alarms. If you notice any of these, book a combustion safety test and check that your carbon monoxide alarms work. If a CO alarm sounds, Health Canada ↗ says to leave immediately and call 9-1-1 from outside.

What an energy advisor checks

• Airtightness, with a blower door test. Health Canada recommends measuring it before adding a ventilation system, since the result helps choose the system and predict whether depressurization could become a concern.
• Fan performance, during a ventilation assessment: whether bathroom and kitchen fans actually move air outdoors, and whether air exchange suits the number of occupants.
• Depressurization and spillage, in a combustion safety test that checks whether exhaust devices can pull combustion gases back into the house. NRCan ↗ notes that many air-sealing companies also test ventilation and combustion air needs, including backdrafting.

For design and installation, HRAI certifies people in residential ventilation to Section 9.32 of the building code (RV9.32), design to CSA F326 (RVF326), HRV and ERV installation and balancing (VIBF) and depressurization testing (DTRS). Its certification database ↗ lets you check a name.

Fixes, from simplest to biggest

Mistakes to avoid

• Treating a bathroom fan as whole-house ventilation. NRCan says relying on a bathroom fan alone is not an effective strategy: it depends on outdoor air leaking in through random gaps.
• Adding a powerful range hood or downdraft cooktop without makeup air in a house with a chimney-vented furnace or water heater. CMHC ↗ warns against combinations such as a natural-draft furnace with a range-top barbecue exhaust fan, and against running several powerful exhaust devices at once.
• Ending exhaust ducts in the attic. The NBC says exhaust ducts may not discharge into heated or unheated enclosed spaces.
• Switching ventilation off in winter. Health Canada says that in newer houses the system should run continuously at low to medium capacity for most of the year.
• Under-ventilating a new or renovated house. NRCan calls for high ventilation rates in the first fall and winter to clear construction moisture, and during drywalling, painting or floor refinishing.

Rebates and next steps

NRCan describes attention to controllable whole-house ventilation and combustion-air supply as an important part of comprehensive air sealing. If you are planning air sealing, ask for a ventilation and combustion safety check in the same visit.

Rebates vary by province and change often. Check current offers in our rebate program tracker, estimate the return on an upgrade with the upgrade payback calculator, and use the home energy planner to sequence ventilation with air sealing and heating upgrades.