If moisture is trapped between the panes of a double-glazed window, the sealed glass unit has failed, and no cloth or cleaner can reach it. Here is why it happens, how to confirm it in five minutes, and which repair options are worth asking about.
In This Article
Where the moisture sits decides what is wrong
Condensation can show up in three places, and only one of them is a seal problem.
| Where it appears | Usual cause | What to do |
|---|---|---|
| On the inside face of the glass | Humid indoor air meeting cold glass. | Lower humidity, use exhaust fans and keep air moving. Wiping clears it, but it returns until the air is drier. |
| On the outside face | Dew forming on efficient glass on a calm, clear night, because the outer pane stays cold when little indoor heat escapes through it. | Usually nothing. It burns off by mid-morning. |
| Between the panes | A failed seal in the sealed glass unit. | No wipe removes it. The unit needs replacing. |
What sits between the panes, and how it fails
A sealed glass unit is two panes of glass held apart by a metal or composite spacer around the edge. Inside the spacer is a desiccant, a drying agent that absorbs trace moisture. A layer of sealant around the perimeter keeps outside air out, and many units are filled with argon gas to slow heat loss.
That seal lives a hard life. It swells and shrinks with every hot afternoon and cold night, absorbs heat from the sun, and can sit in water if a sash drains poorly. Over the years it can crack or pull away from the glass. Humid air then seeps in and the desiccant fills up, and moisture condenses on the inner glass on cold mornings. When it dries, it leaves a film of minerals and dirt on surfaces that no cloth can reach. The gas fill drifts out too, so the window insulates less than it did.
None of that is a dirt problem. The space between the panes is closed to the outside world, which is exactly why cleaning cannot fix it.
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Confirm it in five minutes
- Wipe both faces of the glass with a dry cloth. If the moisture stays put, it is inside the unit.
- Note when it appears. Early fog often shows on cold mornings and fades by afternoon, then stays longer as the failure progresses.
- Check whether it is one window or several. A single window points to one failed unit, and several of the same age suggest wear across the house.
- Photograph the fog from the same spot each time, with the date, so you can see whether it is spreading.
- Look for a stamp along the spacer or a label on the frame with the maker's name and date, and dig out any paperwork that mentions a warranty.
Repair options, and what to ask
- Replace the sealed unit only. If the frame and sash are sound, a glazier or window company can measure and order a new unit. Ask what the new unit is made of (low-E coating, gas fill, spacer type), and how long the order will take.
- Replace the whole window. A better choice if the frame is rotting, the hardware is failing or several windows are going at once.
- Defogging services. Some businesses drill small holes, clean and vent the unit. Ask exactly what they guarantee, because venting does not rebuild the seal or restore the insulating gas.
- Warranty claims. Many window makers cover sealed units for a set period, subject to conditions. Look for it before you pay for anything.
Fog between panes is not an emergency, and it does not harm the house. But it tends to spread, and the window insulates less. Plan a repair in the coming months instead of leaving it for years.
What cleaning can still do for you
Cleaning cannot remove fog between panes, but it is a good way to diagnose one. When both faces are clean, you can see exactly where the haze lives, and surface film stops hiding a real seal problem or being mistaken for one. A cleaner who wipes both sides can usually tell you which it is.
D&D Home Services quotes window cleaning from $125 for typical Kitchener-Waterloo homes, confirmed in writing before work begins. If the haze turns out to be on the surface, read cloudy windows: dirt or seal failure. To keep other seals healthy, see extending the life of your window seals, and for what a visit includes, window cleaning.
Key Takeaways
- Condensation between the panes means the sealed glass unit has failed, and no cleaning can reach it.
- Fog on the inside face is a humidity issue, and dew on the outside face is usually harmless.
- Wipe both faces to confirm, photograph the fog, and check the maker's stamp and warranty.
- Replacement of the sealed unit or the window is the real fix, so ask what any defogging service actually guarantees.
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