Homeowners often ask us why some of their windows still look foggy or hazy no matter how well they are cleaned. Often the problem is not dirt at all. It is a failed seal, and it is something no window cleaner can fix. Here is what is happening and what your options are.
How double-pane windows work
Most modern windows are insulated glass units: two (sometimes three) panes of glass with a sealed space between them. That space is filled with dry air or an insulating gas such as argon, and a spacer around the edge holds a drying agent that keeps moisture out. The seal is what keeps the space dry and clear.
What a failed seal looks like
- Fog, haze or a milky film between the panes that you cannot reach from either side
- Water droplets or streaks inside the glass, often showing up on sunny mornings and fading later in the day
- White mineral trails or spots inside the glass (not to be confused with hard water stains on the outside surface) after the problem has gone on for a while
Over time, heat, cold and sun cause the seal to break down. Moist air leaks in, condenses on the inside surfaces, and leaves residue behind as it dries.
Is it a failed seal or just condensation?
Wipe the glass on both sides. If the moisture comes off, it is ordinary condensation:
- Inside your home (room side), usually in winter: indoor humidity meeting cold glass. Normal, and not a seal problem. Lowering indoor humidity helps.
- Outside, on cool mornings: dew on energy-efficient Low-E glass. Also normal, and it burns off.
- Between the panes: if you cannot wipe it away from either side, the seal has failed.
Why cleaning cannot fix it
Cleaning reaches the two outside surfaces of the glass only. The fog is on the inside surfaces, sealed off from both sides. A good cleaning will make the rest of the window look better, but the haze will stay. When our crews spot failed seals during a cleaning, we point them out so you know what you are dealing with.
Your repair options
- Check the warranty first. Many window manufacturers warranty the insulated glass seal for years, sometimes longer than the rest of the window. Look for the manufacturer name and date etched in a corner of the glass or on the spacer.
- Replace the glass unit only. A glass company or window repair company can usually replace just the sealed glass unit in your existing frame. This is often the most cost-effective fix when the frame is in good shape.
- Replace the window. If the frame or sash is failing too, or several windows of the same age are going, replacing the window may make more sense.
- Defogging services. Some companies drill, dry and reseal the space. Results vary, and it does not restore the insulating gas, so ask questions before choosing it.
Does a failed seal hurt energy efficiency?
Some. Once the seal fails, the insulating gas leaks out and the window insulates less well than it did. It is still a working window, so most people replace failed units when the look bothers them, or plan for it alongside other projects.
The short version
If the fog is between the panes, the seal has failed and the glass unit needs to be replaced by a glass or window company. Check your warranty first. If the moisture wipes off, it is normal condensation, and a cleaning and some humidity control will take care of it.
Related questions
- How much does window cleaning cost?
- Hard water stains on windows: what comes off and what does not
- Low-E windows: how to clean them without damaging the coating
- French pane and grid windows: what to know before they are cleaned
Want us to take a look?
We clean windows across Lake Minnetonka and the west metro. Get an instant estimate, call (612) 770-8221, or see our window cleaning packages and prices.