Your windows are fine. Your indoor air is holding too much moisture.
When warm, humid indoor air meets a cold surface — like a window pane in January — it cools rapidly. Once it drops below what's called the dew point temperature, it can no longer hold all its water vapour and that moisture deposits as liquid on the glass. This is the same physics as a cold drink sweating on a summer day. The window is acting as a symptom display, not as the source of the problem. The source is the relative humidity inside your home.
Modern homes — especially those built after 2000 or recently retrofitted with insulation and draught-proofing — are significantly more airtight than older homes. That's mostly a good thing for energy bills, but it means moisture generated inside (from breathing, cooking, bathing, houseplants, tumble dryers, even newly laid concrete) has nowhere to escape. Humidity accumulates until the coldest surfaces in the room — your windows — reveal the problem every morning. In older, draughtier homes the same moisture simply leaked out through gaps before it built up.
The outdoor temperature matters too. The colder it gets outside, the colder your glass surface becomes, and the lower your indoor humidity needs to be before condensation forms. This is why the problem appears in winter and vanishes in spring — it is not seasonal damp, it is a humidity-to-temperature imbalance that only becomes visible when temperatures fall.
Inside-window condensation isn't always the same problem
The underlying cause is the same — too much indoor moisture — but the pattern of where and when it appears tells you a lot about which specific moisture source is driving it in your home.
It won't fix itself — and the damage it causes is cumulative
Light condensation that evaporates by mid-morning is mostly a cosmetic annoyance. But persistent condensation — the kind that pools on timber sills, soaks into plaster reveals, or drips behind curtains — does quiet, expensive damage over a winter season. Timber window frames can begin to rot within one or two seasons of sustained soaking. More immediately, the persistently damp, cold microclimate at the window edge is one of the most reliably productive environments for black mould growth in a home. The same elevated indoor humidity that is fogging your glass is also raising the moisture content of wall surfaces, ceilings, and soft furnishings throughout the house.
The US EPA and UK Health Security Agency both identify sustained indoor relative humidity above 60% as the primary driver of mould growth in homes. Condensation on glass in a heated room is a reliable indicator that your humidity is exceeding that threshold. A basic digital hygrometer costs under $15 and will confirm your indoor RH in minutes — that single piece of data is the starting point for every effective fix.
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What others have experienced
47 community experiences-
RK
We had this every winter for four years and assumed we needed new windows — quotes were coming in at $8,000+. Then a friend suggested we just measure the humidity first. It was sitting at 68% in January. We got a whole-house dehumidifier and within two weeks the condensation was basically gone. I'm still a little annoyed we almost spent that money on windows.
34 found this helpful -
DM
Ours was specifically the bedroom windows — everywhere else was fine. Turned out we were drying clothes on a rack in the bedroom overnight and not realising how much moisture that releases. Moved the drying rack to a ventilated utility room and cracked the window trickle vent at night. Problem dropped by about 80%. The remaining condensation cleared up when I adjusted the humidistat on the furnace humidifier down for winter.
28 found this helpful -
SP
I want to add a less positive experience because I think it helps. I tried everything — dehumidifier, trickle vents, extractor fan in the bathroom — and still had heavy condensation on the ground-floor windows. Eventually found out we had a slow leak under the kitchen that had been saturating the subfloor for who knows how long. The window condensation was the only visible sign. If you've tried the humidity fixes and nothing is shifting, it might be worth checking for a hidden moisture source.
41 found this helpful
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