Commercial Building Humidification: Setting a Winter Target

Three hundred and fifty litres of water a day. That is what commercial building humidification costs in water for a four-storey office on a -20 °C January day, just to hold 30% relative humidity indoors. The arithmetic is short: 2,000 L/s of outdoor air, a gap of about 4 grams of water per kilogram of air between outside and inside, ten occupied hours. At 0.75 kWh per litre for an electric steam unit, that is roughly 26 kW running continuously — a load comparable to a large piece of heating equipment, and one that almost never appears as its own line in an operating budget.

That is the first reason the winter setpoint deserves more thought than a number typed once into a controller and left there. The second reason is less comfortable: the wrong number quietly damages the building it was supposed to make pleasant. Getting it right is squarely a ventilation and air quality question, not an equipment purchase.

What humidity level should a commercial building hold in winter?

Target 40–45% relative humidity above 0 °C, 35–40% between 0 and -10 °C, and 30% or less below -10 °C. No fixed value works all winter: the setpoint must fall as outdoor temperature falls, with a regulatory floor of 20% during business hours in Quebec workplaces.

That gradation is not an operator preference. The ranges compiled by the INSPQ place comfort and health between 30% and 50%, but CMHC guidance attaches a temperature condition: 30% or less below -10 °C, as low as 25% in extreme cold, and never more than 45% during the heating season. In Greater Montreal, where a -22 °C stretch can be followed by a +5 °C thaw inside the same February week, that upper limit moves constantly.

The ceiling is not comfort — it is the coldest surface you own

Indoor humidity becomes a problem the moment air touches a surface colder than its dew point. At 21 °C and 40% relative humidity, that dew point is about 7 °C. Drop the setpoint to 30% and it falls to roughly 3 °C. In between sits most of Montreal’s 1960s-to-1980s commercial stock: aluminium frames with no thermal break, window sills, poorly insulated exterior corners, structural columns that bridge the envelope. At -20 °C outside, those surfaces commonly sit between 2 and 8 °C.

What follows is mechanical. At a 40% setpoint, water forms on the aluminium, runs onto the sill, then settles into drywall and carpet. Six weeks later you are no longer handling a comfort complaint but a stain, an odour, sometimes an air-quality test. Which makes the instinctive response — raise the humidifier because tenants say the air is dry — the most expensive move available: it shifts the spending from comfort to restoration.

What a litre of steam actually costs

Humidification is one of the few HVAC loads you can cost almost exactly. Turning a kilogram of supply water into steam takes roughly 0.75 kWh, so a 40 kg/h output means close to 30 kW of electrical demand. That figure has nothing to do with the brand or the age of the unit — it is physics.

What does vary is how much water you have to evaporate, and that is governed by three levers a building manager controls:

A sensible-only heat recovery ventilator does nothing for humidity: it recovers heat, not vapour. The distinction is easy to miss when reading a quotation, and it decides roughly half of the humidification bill.

Building the sliding setpoint

A sliding setpoint is a four-line table in the control sequence, not an automation project. It needs three things, in order.

A humidity sensor where the air is representative. In the main return duct, away from a loading dock, an exterior door or a kitchen. A badly placed sensor reads air that exists nowhere in the building.

A trustworthy outdoor sensor. It drives the whole reset. If the controller already has one for the economizer or for hot-water reset, that sensor does the job.

A hard ceiling in the controller. Program 45% as the heating-season maximum. That prevents a manual “just for today” override from surviving until April.

Verifying the humidity sensor belongs on the maintenance calendar: a 10-point drift on a sensor a few years old is unremarkable, and it translates straight into a 10-point error on the setpoint you think you are holding.

From the field: Longueuil, 1,400 litres a week and wet sills

A four-storey office building in Longueuil, original aluminium glazing, electric steam humidifier mounted on the rooftop outdoor-air unit. Setpoint: 40%, fixed, since the controller was commissioned. The call came in January after a week below -18 °C — water on the sills on the third and fourth floors, stained drywall under two windows, tenants convinced the roof was leaking.

Readings: 41% relative humidity in the return, surface temperatures between 3 and 6 °C on the north-facing aluminium frames, and the humidifier’s water meter showing about 1,400 litres over the week. No leak anywhere. The indoor dew point had simply been above the frame temperature every night for six days.

The fix: a sliding setpoint — 42% above 0 °C, 35% at -10 °C, 28% below -18 °C — plus moving the return sensor three metres away from the fitness-room exhaust duct, which had been pushing its reading two points high. Condensation stopped within two days. Humidifier water use dropped below 900 litres a week, and no dry-air complaints came back. Total cost: half a day of controls work and one relocated sensor.

Three cheaper checks before you add humidification

Plenty of buildings that feel dry do not have a humidification deficit — they have an outdoor-air surplus. Before buying equipment or raising a setpoint, check the economizer damper stuck open, the ventilation schedule running at full flow overnight and on weekends, and the exhaust balance. An oversized washroom exhaust puts the building under negative pressure and pulls cold, dry air through the envelope all day long. These three items are verifiable in one visit and usually cost less than the kilowatts they are burning. The logic follows the same path as our article on controlling humidity through ventilation.

What Quebec requires, and what it leaves to you

The only regulated number is a floor: 20% relative humidity during business hours under the Regulation respecting occupational health and safety, for office buildings and commercial establishments built or put into operation after 19 December 1979. No upper target is prescribed.

Above that floor, good practice governs. The RBQ lists ASHRAE standards and manuals among the acceptable technical references for heating, ventilating and air-conditioning installations, and specifies that a chosen standard must be applied in full rather than mixed with another. For a building manager, that means the humidity setpoint is a documented, defensible decision rather than a compliance checkbox. Work that permanently lowers the load — moisture recovery added when an air-handling system is replaced, for instance — falls under the Énergir and Hydro-Québec efficiency programs and is worth examining alongside a broader energy optimization review.

Humidify, or fix the envelope?

Settle the question with an infrared thermometer and one hour on a -15 °C day: read the coldest surface temperature on each façade, then compare it with the dew point of your current indoor air. If the surfaces are warmer, your setpoint has room to rise. If they are colder, no comfortable humidity level will hold as long as those surfaces exist — and the money belongs to the envelope, not to the humidifier.

That reading is the first thing Montréal Combustion takes when a building is caught between the two options, before anyone opens the humidifier cabinet. It costs one visit, and it prevents committing to the wrong solution for the next decade.