The short answer: if the fog is between the panes, the seal on the insulated glass unit has failed and moisture is now inside a sealed assembly. No cleaning product can reach it, because there is no accessible surface. The glass unit, or the whole window, has to be replaced.
First, work out which kind of fog you have
Three different things get called foggy windows and only one of them is a failure:
- Condensation on the inside surface, the one you can touch from indoors. Usually high indoor humidity against cold glass. Annoying, not a failure.
- Condensation on the outside surface, typically on a cool morning. This is actually a sign the window is working: the outer pane is staying cool because the glass is insulating well.
- Haze, streaking or droplets between the panes, which you cannot wipe from either side. This is seal failure.
The test is simple. Wipe both the inside and the outside of the glass. If the cloudiness is still there, it is between the panes.
What actually failed
A double-pane window is two sheets of glass held apart by a spacer around the edge, sealed, with the cavity filled with dry air or an inert gas such as argon. A desiccant in the spacer absorbs any trace moisture.
When the edge seal fails, outside air carrying water vapor gets into the cavity. The desiccant absorbs it until it is saturated, and after that every temperature swing condenses moisture on the inside faces of the glass. That is the haze. It usually appears and disappears with the time of day before it becomes permanent.
North Texas is hard on these seals specifically because of how often the temperature swings. A frame and its glass expand and contract at different rates, and every cycle works the seal a little. We went into that in our piece on frame materials and DFW heat, since the frame material has a lot to do with how long the seal survives.
What it is costing you
Once the gas fill is gone and the cavity is damp, the window has lost a large part of its insulating performance and is behaving much closer to single glazing. You will usually notice it first as a room that runs hot in the afternoon or cold near the window in January.
Mineral deposits also etch the inside of the glass over time. Past that point the haze is permanent even in dry weather.
Your options, honestly
- Replace the insulated glass unit only. If the frame and sash are sound, the glass unit can often be swapped without replacing the whole window. Cheapest route, and the right one on a newer window.
- Replace the window. Makes sense when the frame is also tired, when several windows have failed at once, or when the window is old enough that matching glass is difficult.
- Do nothing. A legitimate choice. A fogged window is not a safety problem. It is costing you comfort and some energy, and it will look worse over time.
What is not an option is the defogging service that drills the glass, flushes it and fits a vent. It clears the haze for a while, but the cavity is then vented to outside air, so the insulating value does not come back and the clearing does not last.
Preventing the next one
You cannot stop seal failure forever, but you can slow it. Lighter frame colors on sun facing elevations run cooler and cycle less. Fiberglass frames move much less than vinyl. And good installation matters: a window racked into an out of square opening is under stress from day one.
If you are not sure whether it is the glass unit or the whole window, we will tell you which, and we are happy to say when it is the cheaper one. Going Green Concepts is family owned and has been replacing windows and doors across DFW for more than twenty years. Estimates are free and there is no obligation. Call (682) 209-1449 or see our project cost calculator for a rough range first.