Controlling humidity through ventilation in commercial buildings

Why humidity is an operating issue, not just a comfort one

Every year, a commercial or institutional building in Greater Montreal swings between two extremes: very cold, dry winters and humid summers. Indoor humidity follows those swings, and ventilation decides whether it stays in a healthy band or drifts. For a facility manager, that is more than a comfort detail: poorly managed humidity shows up as occupant complaints, condensation on the envelope, mould risk, and eventually remediation costs.

A common mistake is to treat humidity as a standalone problem — a missing humidifier, a dehumidifier to add — when it is almost always a consequence of how the ventilation is set up: how much fresh air comes in, how it is distributed, and how heat and moisture are recovered or exhausted.

What range to target, and why it shifts in winter

Public-health guidance in Quebec recommends keeping indoor relative humidity between 30% and 50% to support comfort and limit biological contaminants. Sustained above 60%, you create conditions that favour mould, dust mites, and certain bacteria.

That target is not fixed, though. During deep cold, you often have to aim at the low end — around 30%, sometimes less — because humid indoor air meets cold surfaces (windows, frames, thermal bridges) and condenses there as soon as it drops below its dew point. An overly ambitious humidity setpoint in January means guaranteed condensation on glazing and, over time, envelope damage. The sound practice is to modulate the target with the outdoor temperature, not to impose a single number year-round.

How ventilation actually moves humidity

Ventilation acts on humidity in three ways.

Fresh-air change. Outdoor air in winter is cold and very dry. Once it is warmed, the resulting indoor air has a low relative humidity. Properly dosed, this air change carries off the moisture generated indoors (occupants, kitchens, showers, processes) and prevents build-up. Under-ventilate an occupied building and humidity climbs; over-ventilate in winter and it becomes uncomfortably dry and expensive to heat.

Heat and moisture recovery. Recovery systems — enthalpy wheels, plate exchangers — transfer part of the heat, and sometimes the moisture, from exhaust air to incoming fresh air. A well-maintained enthalpy wheel helps retain some humidity in winter while recovering energy. A fouled or poorly sequenced unit, by contrast, degrades both comfort and the energy bill.

Supplemental humidification. When dry fresh air is no longer enough, some buildings add central humidification. It is appropriate for specific uses, but it demands rigorous follow-up: a poorly tuned or poorly maintained humidifier becomes a contamination source rather than a fix.

The signals that point to an imbalance

Several symptoms reveal humidity that ventilation is not managing well:

• recurring condensation on windows or at the edge of walls in winter;
• occupant complaints about air that is too dry (throat, eyes, static) or too heavy;
• musty smells, stains, or localized marks;
• large differences from one room to another, a sign of poor fresh-air distribution.

Each of these points to a ventilation setting that should be reviewed before adding equipment. A reliable hygrometer, read at a few representative spots and at different times, beats a general impression for pinning down the problem.

Costs, timelines, and realistic priorities

The good news is that many humidity problems are corrected by tuning rather than replacement: adjusting fresh-air rates, sequencing recovery, balancing between zones, cleaning components. These are operating tasks, not capital projects.

When an investment is justified — an enthalpy recovery unit, central humidification, measurement instrumentation — it pays to scope it around real use: an office building, a school, and a healthcare facility differ in internal loads and requirements alike. Oversizing humidification “just in case” is expensive to run and complicates maintenance without guaranteeing a better outcome. The reasonable sequence is to measure, tune, then invest only where tuning falls short.

Where the Quebec framework fits

In Quebec, indoor air quality and building healthiness fall under both public-health best practices and, for workplaces, CNESST obligations. Comfort values (temperature, relative humidity) serve as benchmarks, but it is mainly the absence of condensation, mould, and occupant complaints that proves a building is healthy. For a manager, documenting humidity readings and ventilation interventions is also a way to demonstrate diligent operation.

Approaching the topic without overpromising

Controlling humidity through ventilation starts with understanding how a building breathes: where fresh air comes from, how it is distributed, how heat and moisture are recovered, and where the at-risk cold surfaces are. That whole-system reading is what lets you hit the right range year-round instead of reacting to complaints one at a time. Montréal Combustion supports this diagnosis across Greater Montreal, the North Shore, and the South Shore, to separate what is simply a matter of tuning from what genuinely warrants an investment.