The short answer
Condensation on the inside of a window means warm, moist indoor air is meeting a cold surface. In a Calgary winter that is not exotic — it happens in perfectly sound houses, and in newer ones more than older ones. It becomes a problem when it is persistent, when it is happening on surfaces other than glass, or when it has been going on long enough to wet the framing and the drywall.
Mould is the consequence of sustained moisture, not a separate mystery. Health Canada's position is straightforward: control humidity, fix water damage promptly, and clean up any visible or concealed mould growth. Find the water source, fix it, clean it. That is the whole framework.
Why this shows up in Calgary specifically
Two things collide here and they pull in opposite directions.
Our outdoor air is very dry and very cold. Cold air holds almost no moisture, so on a −25°C morning the window glass in your house may be far below the dew point of the air inside. That is why the condensation appears overnight and in the coldest corners of the house first.
And our newer houses are built tight. Modern Calgary construction has good air sealing, high-performance windows and a controlled envelope, which is genuinely better — but it means the moisture people generate does not leak out the way it did in a 1960s bungalow. Showers, cooking, laundry, plants, aquariums and breathing all put water into the air, and in a tight house it stays there until a ventilation system removes it. A newer home that sweats in January is very often a newer home whose HRV is not running, whose bathroom fans are not being used long enough, or whose humidifier is set as if this were a leaky old house.
The other Calgary-specific twist is chinooks. A rapid swing from −20°C to +10°C and back changes surface temperatures and dew points sharply, which is why condensation can appear and vanish over a week and convince people the problem "fixed itself".
In a new Calgary home, winter window condensation is usually a ventilation setting, not a defect. In an old one, it is more often a window or an envelope.
Humidity problem, or building problem?
Here is how we read it on a walkthrough.
Likely a humidity and ventilation problem if: condensation is on the interior glass surface only; it appears on the coldest mornings and clears through the day; it is worst in bathrooms, the kitchen and bedrooms; it improves when fans run; a hygrometer reads high. Health Canada's guidance is to keep indoor relative humidity in the 30–50% range and to fix moisture problems immediately. In deep Calgary cold, the lower end of that band is where most houses need to sit.
More likely a building problem if: moisture is between the panes of a sealed unit — that is a failed seal, and the window or the unit needs replacing, not the humidity adjusted; condensation or frost appears on walls, in closets on exterior corners, or on the ceiling; there are stains, bubbling paint or soft drywall; there is a musty smell with nothing visible; it is concentrated in one room or one wall rather than house-wide; or the basement is damp regardless of season, which points outside to grading and drainage rather than inside to humidity.
That last one matters. A large share of what gets called a mould problem in Calgary is ultimately a grading and downspout problem or the causes covered in basement moisture in Calgary homes. Fixing drywall without fixing the water outside just buys you a repeat.
What Health Canada says — and what we will not say
We are REALTORS®, not health professionals, so on health we stay strictly inside what Health Canada publishes:
- Exposure to indoor mould is associated with an increased prevalence of asthma-related symptoms such as chronic wheezing, along with irritation and non-specific symptoms.
- Some people are more susceptible than others — children, seniors, and people with conditions such as asthma and severe allergies — and because sensitivity varies, Health Canada does not set a "safe" level of indoor mould.
- The recommendation is to control humidity, repair water damage promptly, and clean up thoroughly any visible or concealed mould growth.
- Health Canada does not recommend air testing for mould. An air test does not provide information that predicts health effects and often tells you little about the cause of the moisture damage.
That last point saves buyers money regularly. If an inspector finds suspect growth, the useful next step is a qualified professional identifying the moisture source and the extent, not a laboratory air sample.
If you are buying
- View in winter if you can. A January showing tells you things a July showing cannot. If you are viewing in summer, ask directly what the windows do in January.
- Look at the sealed units. Fogging between panes is a replacement cost you can count at the showing. Count them — on a large house it adds up.
- Check the mechanical room. Is there an HRV? Is it running? Is the furnace humidifier turned on and set high? Is there a bathroom fan in every bathroom and does it vent outside rather than into the attic?
- Trust your nose in the basement, and follow up a musty smell rather than dismissing it.
- Use the inspection condition. An inspector with a moisture meter and a thermal camera can distinguish a surface humidity issue from wet framing, and our guide to reading an inspection report covers what the language in the report actually means.
Surface mould from a corrected humidity problem is a cleaning task. Mould from an unresolved water source is an open-ended one. What you are buying, when you buy a house with visible growth, is either a known task or an unknown one — and the price difference between those two is the reason to find out which it is before you are firm.
If you are selling
Deal with the source before listing, and keep the invoices. Repainting over a stain without addressing the water is the version that reappears in the buyer's inspection and costs you far more in the negotiation than the repair would have. A known significant hidden problem also falls under a seller's Alberta disclosure obligations. And run the house at sensible humidity through a winter listing — dripping windows at a showing tell a story no photography fixes.
Sources
- Health Canada — Mould and the Residential Indoor Air Quality Guideline: Moulds (association with asthma-related symptoms, susceptible groups, no "safe" limit, control humidity and repair water damage, clean visible and concealed growth, and the recommendation against air testing).
- Health Canada — guidance to maintain indoor relative humidity between 30% and 50%.