Insulated Glass Unit Failure Signs Beyond Fog: Argon Loss, Distortion, and Seal Noise

Fog is the failure everyone recognizes. It is rarely the first one. An insulated glass unit — the sealed double-pane sandwich in most modern windows and patio doors — usually starts failing quietly, months or years before moisture shows between the panes. The room gets a little harder to keep comfortable. A reflection goes wavy. The window clicks on a hot afternoon. Owners notice these things and blame the weather, the house, or the glass itself, when all three signs are the same sealed unit telling you its seal, its gas fill, or its structure is giving up. Learning to read them early matters, because a unit caught early is a planned glass replacement. A unit ignored becomes fog, mineral staining you cannot clean, and eventually glass that has to come out anyway.

What a sealed unit is actually doing

An insulated glass unit, or IGU, is two panes held apart by a spacer around the perimeter, sealed at the edge, with the space between them filled with dry air or argon gas. That trapped space is the insulation. The low-E coating on one inside surface reflects heat, the gas slows heat transfer better than plain air, and the edge seal is what keeps all of it in and keeps outside moisture out. Every failure sign below is one of those three jobs breaking down: the seal letting gas out and moisture in, the gas fill gone, or the panes moving in ways a healthy unit does not.

Argon loss: the failure you feel, not see

Argon leaves a unit slowly through the edge seal as the seal ages — there is no hole to find and no hiss to hear. The window still looks perfectly clear. What changes is performance: the glass feels colder in winter and hotter in a summer afternoon than the window next to it, the room near a big slider or picture window drifts out of step with the rest of the house, and condensation starts forming on the inside room-side surface on cool mornings where it never used to. Compare panes side by side on the same wall — a failed unit stands out by touch and by comfort long before it fogs. Argon loss alone does not mean the seal has fully failed, but it means the unit is no longer doing the energy job it was sold to do, and the seal that let the gas out will let moisture in next.

Distortion: wavy reflections and panes that bow

Look at a long reflection in the glass — a fence line, a roof edge, the street. In a healthy unit it stays straight. In a failing unit it bends, ripples, or swims as you walk past. Some distortion is baked in at tempering and never changes. The kind that matters is distortion that is new, or that changes with temperature: panes bowing in or out as the trapped gas expands and contracts, a center that looks sucked in on a cold morning, or two panes getting so close under pressure change that they nearly touch in the middle. That movement means the unit is breathing with the weather instead of holding a stable sealed space, and panes that flex cycle the edge seal with every swing — which is exactly how the seal wears out. Persistent center contact can even leave a permanent interference pattern, a rainbow-like smudge that no cleaning will remove because it is between the panes.

Seal noise: clicking, popping, and ticking

Windows talk a little as materials expand, and a single soft tick from a frame on a hot day is normal. A pattern is different: repeated clicking or popping from the same window as the sun hits it, a ticking that follows temperature swings day after day, or a faint crackle when you press gently near the edge of the glass. That is the sound of a stressed unit — panes moving against a spacer and seal that are no longer holding them evenly, or a seal letting go in stages. Noise by itself is a reason to watch a window, not panic. Noise plus any other sign on this list — a comfort change, new distortion, the first haze at a corner — is a unit in active failure, and it will not reverse.

The early moisture signs people miss

• Fog that comes and goes: a faint haze that appears on a cool morning and burns off by noon is moisture between the panes in its early stage, not a dirty window. A healthy sealed unit never fogs inside, on any morning.
• Moisture at one corner or edge first: seal failure usually starts at a corner or along the bottom edge, so the first film, droplets, or white mineral trail shows there while the center still looks clear.
• Water marks that cleaning cannot reach: streaks and spots that survive cleaning both sides of the glass are inside the unit — minerals left by moisture that got in and dried, again and again.
• A sweet or musty look in the spacer: discoloration or corrosion visible in the spacer bar between the panes means moisture has been living in the unit for a while.

What does not fix it — and what does

A sealed unit cannot be resealed, refilled, or dried out in place in any way that restores it. Defogging services vent the moisture and make the window look better for a while, but the seal stays failed, the argon stays gone, and the fog returns — you have paid to hide the symptom on a unit that is still losing its insulating value. The real fix is also simpler than most people fear: in a sound frame, you replace the sealed glass unit only, not the whole window or door. The new unit is measured to the frame, built with the correct glass, coating, and gas fill for the orientation, and set into the existing sash. Our window and door service and repair work across Los Angeles is built around exactly that judgment — glass-only where the frame earns it, full replacement only where the frame, not just the unit, has failed.

Order matters if several windows are going at once, which they often do — units installed the same year tend to fail in the same season of their lives. Start with the worst performers: the fogged and mineral-stained units, the big west-facing panes driving your cooling load, and any unit in a bedroom or living space where comfort is the complaint. Clear but tired units can be planned next instead of treated as emergencies.

The short answer

Fog is the last obvious sign of IGU failure, not the first. Argon loss shows up on your comfort and your energy bill, distortion shows a unit breathing and flexing its seal apart, and repeating seal noise tells you the panes are moving under stress. Any one sign earns a watch. Any two together mean the unit is failing and a new sealed unit in the same frame is the honest fix. Send us a photo of the window and tell us which signs you are seeing — comfort, waves, noise, or haze — and we will tell you what we see and quote the exact work.