A tenant sends you a photo at 7 a.m. in January: a bedroom window running with water, and a thin dark line already forming where the glass meets the sash. That is one of the most common winter maintenance tickets in Canadian rentals, and it is almost never solved by telling someone to wipe the glass. Window condensation in winter Canada-wide is a symptom rather than a defect. Warm, moisture-loaded indoor air is meeting a cold surface and giving up its water. Balance the moisture, warm the surface, and mould never gets a foothold. Miss the root cause and you will scrape and repaint the same sill every spring, with a tenancy board application waiting at the end of it.
Why window condensation in winter across Canada is a building problem, not a tenant problem
Air carries water in proportion to how warm it is. When indoor air touches glass sitting near freezing, it cools past its dew point and the water drops out. On a mild October night in Victoria that almost never happens. During a -25°C stretch in Winnipeg, Saskatoon or Timmins, the inside face of an older sealed unit gets cold enough to condense fairly ordinary indoor air, and it will keep doing it every night until something changes.
So the useful question is not why the tenant is making so much moisture. It is which surface in the unit is coldest, and how much water is in the air reaching it.
Where the water appears tells you what is wrong
- Inside face of the glass, bottom edge first. The standard case. Humidity is too high for the coldest surface in the room, and the bottom goes first because the spacer bar conducts heat out while cold air pools at the sill.
- Between the panes. The insulated glass unit has lost its seal. Ventilation will not touch it; that is a glass replacement.
- On the frame or sash but not the glass. Thermal bridging, usually old aluminum frames with no thermal break, or a rough opening that was never properly foamed.
- On the exterior face in the morning. The window is doing its job. Nothing to fix.
- Frost in closet corners, damp at the base of exterior walls, or frost on attic sheathing. Escalate. That is air leakage and insulation, and it is a bigger repair than any window.
Set a humidity target before you change anything
CMHC has published the same sliding-scale principle for decades: the colder it gets outside, the lower indoor relative humidity has to be. In round terms, somewhere in the mid-thirties when it is near freezing outdoors, stepping down toward 20 per cent or below in a deep cold snap. Health Canada's general year-round guidance sits in the 30 to 50 per cent band. In a Canadian January, aim at the bottom of it.
Buy hygrometers. They cost almost nothing and a two-week log turns a subjective argument into a number. Nearly every case of window condensation in winter resolves into one of two findings. If a unit reads 48 per cent while it is -22°C outside, you have a moisture problem and no window on the market will out-run it. If it reads 28 per cent and the glass is still soaking, you have a cold-surface problem, and more ventilation will only dry out the tenant.
The five sources actually producing the water
- Occupancy. Showers, cooking, laundry, plants and breathing. A household of four puts a real volume of water into the air daily, and that is normal use, not misuse.
- Bathroom fans that do not exhaust. In older Canadian housing stock the duct often terminates in the attic, or sags through an uninsulated flex run that condenses and drips. The noise convinces everyone it is working.
- Recirculating range hoods. A charcoal filter does nothing for the litres of water released by boiling and simmering.
- Furnace-mounted humidifiers. Frequently left at a mild-weather setting all winter, or fitted with a dial nobody has touched since installation. This one alone explains a lot of winter window condensation calls.
- Bulk water. Exposed soil in an unsealed crawl space, a damp basement, laundry dried indoors, or a recent renovation still giving up construction moisture.
There is a sixth that catches owners out: you tightened the building. New windows, weatherstripping, blown-in attic insulation and spray foam all remove the accidental leakage that used to dry the place out. A unit with no history of condensation can develop one the winter after an energy retrofit, because the ventilation was never upgraded to match.
Fixes that prevent mould, in the order I would do them
1. Measure, then diagnose
One hygrometer in the affected room, one elsewhere in the unit, logged for two weeks including a cold snap. Photograph the window at 7 a.m., not at 2 p.m. once it has dried. Note whether the bedroom door stays closed overnight, whether furniture blocks a heat register, and whether heavy drapes are drawn. Drapes are a common trap: they cut the glass off from room heat, lower the surface temperature and make condensation worse.
2. Make the exhaust actually exhaust
Confirm every bathroom and kitchen fan terminates outdoors through insulated duct with a working backdraft damper, and that the exterior hood is not buried in snow or plugged with lint. Replace weak units with HVI-rated fans sized for the room, on a timer or humidistat switch so they run well past the end of the shower. A quiet fan that runs twenty minutes beats a loud one nobody turns on.
3. Commission the HRV
Most Canadian units built or renovated in the last two decades have a heat recovery ventilator, and a startling number have never been balanced, have a filthy core, a clogged filter, or a dehumidistat someone switched off during a cold snap and forgot. A properly running, correctly set HRV is the single most effective control on window condensation in winter in a tight Canadian unit.
4. Warm the cold surface
Air seal and insulate the rough opening with low-expansion foam. Replace failed sealed units with low-e, argon-filled, warm-edge-spacer glass meeting ENERGY STAR requirements for your climate zone. Where full replacement is not in this year's budget, interior storm panels or a well-fitted film kit raise the interior surface temperature enough to stop the daily soaking on single glazing and old aluminum frames. Weigh replace-now against staging the work over three winters properly; that kind of capital planning is covered well on Construction Arbitrage.
5. Deal with bulk water
Vapour barrier over crawl space soil, downspouts extended past the foundation, grading checked before the ground freezes, and a line in the tenancy package about drying laundry indoors. None of it is glamorous and all of it shows up on the hygrometer.
When condensation has already become mould
Surface mould on a painted sill comes off with detergent and water followed by a proper dry-out. Do not paint over it. Health Canada's position is straightforward: visible mould should be removed and the moisture source corrected, and species testing is not needed before acting.
Once growth extends into drywall, behind trim or into framing, treat it as a remediation job with containment rather than a cleaning job. Document everything: dated photos, humidity logs, work orders, contractor reports and the tenant communication trail. Obligations vary by province, but the through-line is consistent. Ontario's Residential Tenancies Act requires rental units to be kept in a good state of repair and fit for habitation, other provinces have equivalent standards, and municipalities layer property standards bylaws on top. A tenant who reported condensation three times and got nothing but advice to open a window has a strong file.
Making it a routine instead of a fire drill
Build winter prep into fall turnover: airflow check on every exhaust fan, HRV filter and core cleaned, humidifier setting confirmed, weatherstripping inspected, exterior vent hoods cleared, hygrometer readings logged by unit. Half an hour per unit in October removes most of the January tickets. The PlanaJob pages for Canadian property managers are built around exactly this kind of repeating seasonal work, and platforms like PlanaJob let you compare quotes from vetted contractors side by side instead of calling whoever answered last February. If you are on the trade side, ventilation and moisture retrofits are a dependable winter earner, and much of the practical debate happens in communities like Contractor Club.
Persistent condensation is a diagnosis, not a wipe-down. If the water keeps returning after you have measured, ventilated and sealed, bring in someone who will trace the whole moisture path from crawl space to attic. PlanaJob ventilation specialists find the root cause, and you can post the job in a few minutes. More seasonal maintenance walk-throughs are on the PlanaJob blog.
FAQ
Is window condensation the tenant's fault in a Canadian rental?
Rarely on its own. Normal occupancy produces moisture and the building is supposed to handle it. Habits can push a marginal unit over the line, so it is fair to ask that fans be used, drapes stay open overnight and laundry is not dried on radiators. But a fan venting into the attic or a failed sealed unit is a repair, and provincial tenancy law puts that on the landlord.
What indoor humidity should I target during a Canadian winter?
Use the sliding scale rather than one fixed number: drop indoor relative humidity as the outdoor temperature falls. Near freezing, the mid-thirties is usually comfortable and safe. In a hard cold snap you may need the low twenties or below to keep glass dry. Health Canada's 30 to 50 per cent range is a year-round guide, not a January target.
Will new windows stop window condensation in winter for good?
Better glazing raises the interior surface temperature, so condensation only begins at a higher humidity level, which solves most borderline cases. What replacement cannot do is remove water from the air, and new windows tighten the building, which can push moisture to the next coldest surface such as a closet wall or the attic. Always pair a window upgrade with a ventilation review.