- Condensation on windows in Airdrie winters
- Airdrie winters
Condensation on Windows in Airdrie Winters: What It Means and How to Stop It
The three kinds at a glance
On the room side
Indoor humidity meeting cold glass; wipes dry. Fix: lower humidity, warm the glass.
- Worst along the bottom edge
- Humidifier, fans, blinds
- Check the humidity table
Between the panes
A failed sealed unit; wipes from neither side.
- Fix: replace the sealed unit
- No humidity setting clears it
- Frame, sash and trim stay
On the outside at dawn
Dew on a well-insulated window; gone by mid-morning.
- Fix: none
- A sign the low-E glass is working
- Clears on its own
Why do windows get wet on the inside in winter?
Warm indoor air holds water vapour. When that air touches glass that’s colder than its dew point, it can’t hold all of it, and the extra condenses onto the glass. Natural Resources Canada: “when air contacts a cold window, the air loses heat. The air can then no longer hold all the water vapour and some condenses out.” [1] It shows at the bottom edge first, the coldest part, next to the spacer and the sill.
Where it shows first
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How cold does Airdrie get, and why does that matter?
The nearest station with published 30-year normals is Calgary International Airport, 20 minutes south. Its 1991 to 2020 normals give a January daily mean of minus 7.6 C, an average low of minus 13.5 C, and an average coldest night of minus 27.9 C. [2] The humidity a window tolerates falls as the outdoor temperature falls.
−7.6 °C
January daily mean
−13.5 °C
Average January low
−27.9 °C
Average coldest January night
Record low minus 45 C
What humidity should an Airdrie house run in winter?
Natural Resources Canada recommends 30 to 50 percent indoor humidity, lower in winter. [1] The ceiling depends on the outdoor temperature and the glass. Table 9-1 gives the maximum indoor relative humidity at 21 C with no condensation: [3]
Outdoor | Standard double-glazed | Double-glazed low-E | Triple-glazed low-E |
0 °C | 58% | 70% | 81% |
-10 °C | 44% | 59% | 74% |
-20 °C | 34% | 50% | 67% |
-30 °C | 25% | 42% | 60% |
-40 °C | 18% | 35% | 54% |
Read it against the climate: on an average January night, around minus 13 C, a standard double-glazed window stays dry under about 40 percent; on a minus 28 C night it needs under 25 percent. Let your windows be the indicator; “occasional condensation does not pose a problem”. [3]
How do I stop condensation on the inside of the glass?
Lower the humidity, warm the glass, or both. Natural Resources Canada says excessive condensation “is an indication that you should reduce moisture production or increase ventilation”. [3]
Furnace humidifier
Turn it down below minus 15 C and off below minus 25 C; the table is your guide.
- Below −15 °C: turn down
- Below −25 °C: off
- Check the table each cold snap
Bathroom and kitchen fans
Run during the shower and 20 minutes after; a fan that won't hold a tissue isn't moving air.
- During the shower
- 20 minutes after
- Tissue test for airflow
Blinds and curtains
Open an inch at the bottom so room air warms the glass.
- An inch at the bottom
- Room air reaches the glass
- Worst rooms first
Dry the sill
Wipe each morning until the humidity is under control; standing water feeds mould.
- Wipe each morning
- No standing water
- Until humidity is under control
What does fog between the panes mean?
Fog, streaks or a milky haze you can’t wipe from either side means the sealed unit has failed. The edge seal has let moist air into the gap, the desiccant is saturated, and no humidity setting will clear it. Natural Resources Canada’s remedy: “replace the insulating glass unit”. [4] Research puts a typical window’s service life at 20 to 30 years. [5]
The wipe test
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What replacing one involves: foggy window and sealed unit replacement
Why is there condensation on the outside of my windows?
Droplets on the outside at dawn mean the window is doing its job: low-E glass keeps indoor heat in, so the outer pane stays cold overnight and collects dew. “If the night has been cool and calm and the air is humid, condensation may form on the outside surface of the glass around sunrise.” [4] It clears on its own.
Nothing to fix
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How much condensation on windows is normal?
A little, on the coldest mornings, along the bottom edge, gone once the room warms. “Occasional condensation does not pose a problem.” [3] Water that pools on the sill, drips, freezes into a ridge of frost, or shows up at humidity under 30 percent is not normal, and that’s where the humidifier, the fans and the blinds come in.
Not normal
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When should I call about condensation?
What you see | What it is | What fixes it |
Water or frost on the room side, worst at the bottom, wipes dry | Indoor humidity meeting cold glass | Lower humidity, fans, blinds; check the table |
Room-side glass wet at low humidity, cold air at the sash | Frame or perimeter seal | Weatherstrip, gaskets, sash alignment |
Haze or fog you can't wipe from either side | Failed sealed unit | Replace the sealed unit in the frame |
Droplets outside at sunrise, gone by mid-morning | Normal dew | Nothing |
Not sure which row you’re in? Send one photo of the whole window and one of the wet spot; most of the time the answer is a humidifier setting, not a bill: send two photos
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Sources cited on this page
[1] Natural Resources Canada, Keeping the Heat In, Section 2: How your house works (2.1.6 Condensation and 2.4.2 How much humidity?). natural-resources.canada.ca. Passages: the dew point is 100 percent relative humidity; air that contacts a cold window loses heat and condenses some of its water vapour onto the glass; a humidity level between 30 and 50 percent, lower in winter, limits humidity-related health risks.
[2] Environment and Climate Change Canada, Canadian Climate Normals 1991 to 2020, Calgary International Airport, as tabulated at wikipedia.org (daily mean January −7.6 °C; mean daily minimum January −13.5 °C; mean minimum January −27.9 °C; record low −45 °C in February). Applicability note: Calgary International Airport is the nearest station with published 30-year normals; Airdrie sits about 30 km north at a similar elevation and runs slightly colder on still nights.
[3] Natural Resources Canada, Keeping the Heat In, Section 9: Operating your house, Table 9-1 Maximum indoor relative humidity levels. natural-resources.canada.ca. Table rows: 0 °C 50% 58% 70% 81%; −10 °C 34% 44% 59% 74%; −20 °C 24% 34% 50% 67%; −30 °C 15% 25% 42% 60%; −40 °C 10% 18% 35% 54%. “Let your windows become your indicator”; “Occasional condensation does not pose a problem. Excessive condensation or frosting is an indication that you should reduce moisture production or increase ventilation.”
[4] Natural Resources Canada, Condensation (product energy ratings), modified 16 January 2025. natural-resources.canada.ca. Passages: condensation can form between the glass panes if the seal is broken (replace the insulating glass unit); on a cool, calm, humid night condensation may form on the outside surface of the glass around sunrise.
[5] Degradation of Insulating Glass Units: Thermal Performance, Measurements and Energy Impacts, Buildings 2023, 13, 551 (NREL and University of Colorado). docs.nrel.gov
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