The haze that spreads across a double-pane window is water vapor that has condensed inside the glass, in a space you cannot wipe clean from either side. It is not dirt, and it is not a film on the surface. It is moisture that has worked its way into the sealed gap between the two panes and can no longer get out. Understanding how it got there explains why cleaning never helps and why the fog seems to come and go with the weather.
Inside a Sealed Pane
A modern insulated window is built from two panes of glass held apart by a spacer bar around the edges. That spacer creates a cavity, usually somewhere between a half and three-quarters of an inch wide. The cavity is filled with dry air or an inert gas like argon, and the whole assembly is bonded shut with a continuous seal so nothing moves in or out.
Tucked inside the spacer is a material called desiccant, the same kind of drying agent packed into shipping boxes. When the unit is manufactured, a tiny amount of moisture is always sealed in with the gas, and the desiccant absorbs it so the cavity stays clear. As long as the seal holds, the air between the panes stays dry, and light passes straight through both sheets of glass without scattering.
When Seals Fail
Seals do not last forever. Every day the sun heats the trapped gas, which expands and pushes outward against the seal; every night it cools and contracts, pulling back in. That constant flexing works on the bond year after year. Temperature swings are especially hard on windows in places like Houston, where hot afternoons and cooler nights repeat the cycle almost daily. Add ordinary aging of the sealant, poor drainage that lets water sit at the frame, or a manufacturing flaw, and eventually a gap opens.
Once that happens, the cavity is no longer isolated. Outside air carrying humidity gets drawn in and out with each pressure change. The desiccant absorbs the incoming moisture at first, but it has a limited capacity. When it becomes saturated, it can no longer keep up, and free water vapor starts collecting between the panes. From that point the fog will only get worse, which is the moment most homeowners begin looking into foggy window repair rather than trying yet another round of cleaning. Replacing the failed insulated unit, or the glass within it, is the only way to restore a dry sealed space.
Reading the Fog
The way the haze behaves tells you a lot about what is happening. On a warm, humid morning you may see visible droplets or a heavy cloud, and by a dry afternoon the same window can look nearly clear again. That is because the trapped moisture condenses when the inner glass is cool and evaporates back into vapor when it warms, exactly the way a bathroom mirror clears once the room dries out. The water never leaves the cavity; it just changes state.
Over time you may also notice faint white streaks or a mineral-looking residue on the inside surfaces. Those are deposits left behind as moisture repeatedly evaporates, a sign the seal has been failing for a while. A window that fogs only in the corners is usually in the early stages, while one that clouds edge to edge has lost most of its desiccant capacity.
If you are trying to confirm the problem, watch one suspect window across a full day with changing conditions. If the haze shifts with the temperature and sits between the panes where no cloth can reach it, you have a failed seal rather than a surface issue. The practical next step is to have that unit measured and quoted for glass replacement, so it can be swapped before the fog spreads to the panes beside it.