Why Ventilation Reduces Condensation
Ventilation reduces indoor humidity by diluting the moisture-laden interior air with drier outdoor air. In Canadian winter conditions, outdoor air is typically much drier than indoor air even at 100% relative humidity outdoors — because cold air holds far less absolute moisture than warm air. When outdoor air at −15°C is brought inside and warmed to 21°C without adding moisture, its relative humidity drops to approximately 10–15%. This makes cold-weather ventilation an effective means of reducing indoor humidity and, consequently, window condensation.
The challenge is that bringing in large volumes of cold outdoor air in winter is energy-intensive. This is why heat recovery ventilators (HRVs) are the standard approach in modern Canadian residential construction: they exchange heat from outgoing exhaust air to incoming fresh air, recovering a significant portion of the energy while still lowering indoor humidity.
Exhaust-Only Ventilation
In older homes, ventilation often consists of exhaust fans in kitchens and bathrooms, with makeup air entering through leaks in the building envelope. This approach removes humid air from the highest-moisture rooms directly but does not recover heat. Running kitchen and bathroom exhaust fans consistently — particularly during and after cooking, bathing, and dishwashing — is a low-cost first step in addressing window condensation without modifying the ventilation system.
Continuous low-speed exhaust ventilation (running a bathroom fan on a timer for a portion of each hour) maintains a steady slight negative pressure that draws in drier outdoor air through envelope leakage. This can be effective in homes with moderate air leakage but is less effective in newer, tighter homes where envelope leakage is minimal.
Heat Recovery Ventilators (HRVs)
An HRV brings in a measured volume of fresh outdoor air while exhausting an equal volume of stale indoor air through a heat exchange core. The exchange core transfers heat from the warm outgoing air to the cold incoming air, typically recovering 70–85% of the heat energy. The fresh air enters the house at a much warmer temperature than outdoor air, reducing the heating load compared to uncontrolled infiltration.
From a condensation standpoint, the key benefit is that the HRV introduces dry outdoor air at a controlled rate, lowering indoor humidity throughout the house rather than only in high-moisture rooms. A well-sized and properly operating HRV can maintain indoor humidity within the target range (30–50% RH) through the winter heating season in a typical Canadian home.
Balanced Ventilation vs. Exhaust-Only
Exhaust-only systems create negative pressure, which draws in outdoor air through whatever gaps exist in the envelope. This is uncontrolled and may introduce air through paths that carry odours, radon, or moisture from crawlspaces or wall cavities. Balanced systems (HRVs or energy recovery ventilators) supply and exhaust equal volumes, maintaining neutral pressure and directing fresh air intake through a filtered, controlled port.
In newer homes built to current Canadian building codes, an HRV or ERV is typically required as part of the mechanical ventilation system. In older homes, retrofitting an HRV is a common approach to both improving air quality and addressing persistent winter condensation problems.
Ventilation Rate and Humidity Control
The ventilation rate required to control humidity depends on the moisture generation rate of the household and the tightness of the building envelope. ASHRAE Standard 62.2 provides ventilation rate guidelines based on floor area and number of bedrooms. Natural Resources Canada’s guidance on residential mechanical ventilation, available through the NRCan website, covers sizing considerations relevant to Canadian climates.
An HRV with a humidistat control will automatically increase ventilation when indoor humidity rises above a set threshold, which is the most direct approach to controlling condensation without constant manual adjustment.
Kitchen and Bathroom Fan Selection
For kitchens, a range hood exhausted to the exterior (not recirculating) removes the highest single source of indoor moisture in most homes: cooking steam. The capacity should be sufficient to capture steam at the cooking surface rather than allowing it to disperse. Bathroom fans rated for quiet continuous operation (0.3–1.0 sone) are suitable for extended or continuous use without occupant annoyance, which is relevant if continuous ventilation is the goal.