Condensation on windows: when it's normal, when it's the seal, and when it's time to replace
Water on the glass can mean three very different things, depending on which side of which pane it's on. Here is how to tell them apart in about two minutes, and what each one means for your house.
First, find where the water is
Look at the glass first thing in the morning on a cold day. Touch the wet spot. Then decide which of these three you have, because they are not the same problem.
- On the room side of the glass: moisture in your house meeting a cold surface. Usually fixable, sometimes a sign the window is the weak point.
- Between the two panes: a failed seal. It cannot be wiped off, and it does not go away.
- On the outside of the glass: dew. Harmless, and the sign of a window that holds its heat.
Condensation on the inside: humidity, ventilation and cold frames
Mid-Island winters are long, wet and mild enough that nobody opens a window for long. Houses from the 1970s through the 90s were also built before ventilation was taken seriously. The result is a house full of moisture from showers, cooking, laundry on a rack and breathing, with nowhere to go but the coldest surface in the room.
The coldest surface is very often the window, and in older Island homes it is the frame as much as the glass. Aluminum frames are the worst offenders. Metal moves cold straight through from outside to in, so the frame sweats all winter, drips onto the sill and grows black mould along the bottom. Thin single-pane glass does the same thing.
What helps, in order of cost:
- Run the bathroom fan during and for 20 minutes after a shower, and run the range hood when you cook
- Open a couple of windows for five to ten minutes a day, even when it's raining
- Keep drapes and blinds open enough that air can reach the glass
- Vent dryers outside, not into the garage or crawlspace, and keep laundry racks to one room with a fan
- Check that the crawlspace has a ground cover and is vented, since that moisture rises into the house
If you've done all of that and the glass is still running with water, the window is the problem. A modern double-pane unit in a thermally better frame keeps the inside surface warmer, so the moisture has nothing cold to land on. Our draft guide covers the same frames from the comfort side.
Fog or droplets between the panes: the seal has failed
A double-pane window is two sheets of glass separated by a spacer and sealed around the edge. When that seal breaks, damp air gets into the gap. You'll see a milky haze, water beads, or mineral streaks that you can't reach to wipe. It often shows up first in the corner or along the bottom, and it often looks worse on a humid day.
This isn't a cleaning problem and it isn't fixable in the way people hope. The sealed gas fill and the dry gap are gone, so the window insulates less than it did when new. You can have the glass drilled and treated to clear the view, but it's a cosmetic patch. We explain that in full on the foggy windows page, including how to read the date stamp on the spacer to see how old the unit is.
Condensation on the outside: leave it alone
On a still, clear, cool morning, the outer pane can get colder than the air around it, and a film of water or light frost forms. It's the same reason the car windshield needs scraping. It means very little heat is travelling through the window, which is what you want. It burns off by mid-morning. There's nothing to fix, and if you see it on the new windows you just bought, that's them working.
When it's time to replace
- The fog is between the panes. That's a failed unit, whatever the frame looks like
- The frame sweats every winter morning, even with the fans running
- You see black mould, peeling paint or soft wood on the sill and trim beneath the window
- The frames are the original aluminum, or the glass is single pane
Whether the right fix is a retrofit or a full-frame replacement depends on what's behind the trim, and we look at that on the measure. If you want a rough idea of cost first, the online estimate will give you a range. The firm price comes in writing after the measure.
We work from Duncan to Qualicum Beach. If you're in Nanaimo, here's the Nanaimo page, with the details for the neighbourhoods around you.
FAQ
Common questions
Why do my windows have condensation on the inside every morning?
Warm indoor air holds moisture, and when it touches glass that is colder than the room, the moisture turns to water. On the mid-Island that happens most from October to April, because the outdoor air is cool and damp for months. Showers, cooking, drying laundry indoors and an unvented crawlspace all add moisture. Running the bathroom and range-hood fans, opening a window a crack for a few minutes, and keeping furniture and heavy drapes off the glass all help. If the water keeps coming back with every fan running, the window itself is probably the cold spot.
Is condensation on windows a sign of mould or rot?
Not by itself, but water that sits on a frame every day will eventually cause both. We see black mould along the bottom of aluminum and older vinyl frames, and soft, dark wood under the sill where the water runs down. If you can push a screwdriver into the sill or trim, the damage is already past the glass. We check this at the free measure and tell you before anything gets installed.
There is fog between the two panes of glass. Can it be fixed?
The fog means the seal around the sealed unit has failed and moist air has got in. The glass can't be properly resealed. Some companies drill and defog, or you can replace only the glass unit, but the fog tends to come back, and the unit has usually run out of its insulating value by then. Our straight answer is that a fogged unit is a replacement job. If the frame is still sound, the replacement can be a retrofit insert, which is quick and the lower price.
Is condensation on the outside of my windows bad?
No. On a clear, cool, still morning, the outside pane can drop below the dew point and collect a fine film of water or frost. It means the glass is not leaking heat to the outdoors, which is what a good double-pane window is supposed to do. It clears as the sun comes up. It is also worth knowing that older, leaky windows rarely show it, because the heat escaping through them keeps the outer glass warm.
Will new windows stop condensation completely?
No window can promise that, because it depends on how much moisture is inside the house. What new windows do is keep the inside glass and frame warmer, so condensation forms far less often and only in the worst conditions, such as a cold snap with a house full of steam. Pair them with working bathroom fans and you should not be wiping sills every morning.