Symptom page
Foggy windows: condensation between panes, what to do?
Fog on the inside, outside or condensation between two panes? Not all fog is the same. Learn to tell a foggy window caused by simple humidity from the real sign your thermos needs replacing.
The diagnosis
Three fogs, three very different causes
Before panicking, look at where the fog is. That's what distinguishes a normal phenomenon from a real problem.
Fog on the inside surface
It forms on the room-side pane when the indoor air is humid. It's a simple indoor-humidity phenomenon, not a window defect. A dehumidifier or better ventilation solves it.
Fog on the outside surface
This morning dew on the outside pane means your window insulates very well: the outside condenses, not the inside. It's perfectly normal, even reassuring.
Fog BETWEEN the panes
When the fog is trapped between the two panes, out of reach, it's a sign the seal has failed. It's irreversible by cleaning: the only solution is to replace the sealed unit.
Understanding
Why wiping makes no difference
When the fog is between the two panes, it's trapped inside the sealed unit. The space that once held the argon gas has now let moisture in, and that moisture stays confined there.
No product, no cloth, no cleaning can reach it. The fog may seem to disappear on a dry day, but it returns as soon as conditions change.
What this fog reveals is that the seal is broken: the argon gas has escaped and the insulation is lost. Over time, an opaque deposit may even form on the glass.
The durable solution
Replace the sealed unit, don't mask it
The only durable solution is to replace the faulty sealed unit with a new one, within your existing frame. Your frame, handles and hardware: everything stays in place.
Result: a perfectly clear pane, restored insulation and a written warranty. All at a moderate cost, well below a full window.
FAQ
Frequently asked questions
Why does wiping make no difference?
Because the fog is inside the sealed unit, between the two panes. You can't reach it from outside. The moisture is trapped in the space once filled with argon gas, and it will stay there until the unit is replaced.
Can fog between the panes disappear on its own?
It may vary with temperature and humidity, but it will always come back. The seal is broken: moisture enters and leaves with the conditions, but the underlying problem never goes away without replacement.
Is it dangerous to leave a foggy window?
It's not an immediate danger, but it's a sign of lost insulation. Over time, the unit can also become opaque (mineral deposit) and reduce clarity. Replacing the thermos restores insulation and transparency.
Can I fix this with "defogging"?
Injection defogging removes the fog temporarily (6–24 months), but repairs neither the seal nor the insulation. It masks the symptom. Replacing the sealed unit is the only durable solution.
Further reading
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