You clean a window, the inside shines, the outside shines, and yet a veil of fog remains — untouchable, as if trapped in the glass. That is exactly what it is. This guide explains why this kind of fog is not a cleaning matter, and how not to confuse it with ordinary surface condensation.
Three kinds of fog not to confuse
1. Condensation on the inner face. Droplets form on the room side, especially on winter mornings. It is ambient moisture settling on cold glass. It wipes off, and it mainly signals a need for airing or ventilation. Normal.
2. Condensation on the outer face. A veil appears outside in the early morning in clear weather. It is the sign of highly insulating glazing: the outer face stays cold and catches the dew. Counter-intuitive, but a good sign for insulation. It clears on its own and wipes off.
3. Fog between the two sheets of glass. It is inside the double glazing, in the sealed air gap. You cannot reach it: no cloth, squeegee or product passes between the two panes. This is the one that is a problem.
Why fog between the panes is a seal problem
Double glazing is two sheets of glass separated by an air or gas gap, closed by an airtight edge seal. That seal also contains a desiccant that captures residual moisture.
Over time, or after a defect, the seal can fail. Humid air then enters the sealed gap, and the desiccant eventually becomes saturated. The result: condensation forms between the two panes whenever the temperature changes. The glazing has lost its seal — and, with it, part of its insulating power.
It is not dirt: it is a failure of the glazing.
What cleaning can, and cannot, do
- Cleaning removes everything that is on the glass, inside and out.
- It removes nothing trapped between the two sheets, since that space is sealed and inaccessible.
No product, no washing technique, no pure water changes this. An honest provider will tell you before, not after: on this precise point, cleaning can do nothing.
What to do
- Confirm the diagnosis: wipe the inside and the outside. If the veil remains, it is between the panes.
- The solution is replacing the glazing (sometimes just the double-glazed unit, without changing the frame). It is a job for a glazier or joiner, not a cleaner.
- Check the warranty: depending on age and manufacturer, double glazing whose seal failed prematurely may be covered.
FAQ
Will the fog between my panes come off with cleaning? No. It is in a sealed space between the two sheets of glass, which no cleaning reaches. Only replacing the glazing makes it disappear.
How do I know if it is between the panes or on the surface? Wipe both faces. If the veil disappears, it was surface condensation, which is normal. If it remains, it is inside the double glazing.
Fog outside in the morning, is that serious? No, it is often even a sign of well-insulating glazing. It clears with the sun.
Does the whole frame need replacing? Not necessarily. In many cases only the double-glazed unit is replaced. A glazier will confirm depending on your joinery.