A foggy window that will not wipe clean is one of the most common callouts in older homes. The cloud sits between two panes of glass, which means no amount of glass cleaner touches it. What failed is the edge seal that binds the insulated glass unit, or IGU, together, and the repair decision usually comes down to cost, age, and how much the fog bothers you.
Seal failure is a glass and gas problem rather than a mechanical one, so the window regulator clip repair routine used for vehicle power windows does not apply here. Home windows have no moving parts inside the sealed cavity; the fix is either a warranty replacement, a defogging treatment, or a full unit swap, and each path carries a different cost.
How an Insulated Glass Unit Works
Insulated windows, also called IGUs, sandwich two or three panes of glass with the air removed from the space between them. That space is refilled with a heat-resistant gas, usually argon or krypton, which slows heat transfer far better than plain air. A spacer frame around the perimeter holds the panes apart, and the edge seal locks the whole assembly together.
The edge seal is the weak point. When it fails, the inert gas escapes and outside air and moisture migrate into the cavity, which is when condensation appears on the inside face of the glass. Unlike a pipe joint where a wrap of self-fusing seal tape can stop an emergency pipe leak repair, an IGU edge seal cannot be patched from the outside, because the gap sits between the panes, out of reach.
The Edge Seal and the Gas Fill
Seals are usually built from polyisobutylene backed by a structural silicone or butyl secondary seal. Manufacturers assemble them in climate-controlled conditions, because the seal has to keep a dry gas fill stable for years. Argon is the common fill because it is cheap and roughly one-third less conductive than air. Krypton performs better in thin cavities but costs more, so it shows up mainly in high-end triple-pane units.
How Long Seals Last
A well-made IGU survives 10 to 20 years before the seal starts to leak. Temperature swings, direct sun exposure, and frame flex all shorten that service life. South- and west-facing windows usually fail first because the glass expands and contracts more through the day, working the seal like a flexing joint until it gives.
Signs a Window Seal Has Failed
The classic symptom is condensation between the panes that will not wipe away. Fog forms first on cool mornings, then lingers all day as the leak grows. Reflections look distorted, and in advanced cases a white mineral film builds up where moisture evaporated and redeposited salts on the glass.
Windows present the same when to repair and when to replace decision that roofs do: both systems fail gradually, and both reach a tipping point where replacement beats another patch. Knowing where that line sits starts with reading the symptoms correctly.
Condensation Between Panes
Condensation on the room-side surface of the glass is a humidity problem and usually has nothing to do with the seal. Condensation between the panes, visible through the glass, is a seal problem. The distinction matters because the two have completely different fixes, and treating the wrong one wastes a service call.
Other Clues Beyond Fog
- A film or droplets that reappear hours after you wipe the inside pane
- Distorted, wavy reflections caused by mineral deposits on the inner surfaces
- A visible crack or gap in the sealant along the spacer edge
- A cold draft near the window even when it is closed and latched
Any one of these signs means the insulating gas is already gone or going. The window still keeps weather out, but it no longer insulates the way it was designed to, and the energy bill starts to notice.
Energy Loss and Moisture Damage
A failed seal does not just look bad; it changes the heat balance of the room. The gas that provided the insulation has leaked away and been replaced by ordinary air, so the pane conducts heat roughly as well as a single layer of glass. Heating and cooling systems run longer to hold the same temperature.
What the Numbers Say
| Glazing type | U-factor range | Relative insulation | Typical use |
|---|---|---|---|
| Single glass | 1.0 to 1.2 | Baseline | Old homes, storm windows |
| Double pane, air fill | 0.45 to 0.55 | About 2x single | Older IGUs |
| Double pane, argon fill | 0.25 to 0.35 | About 3x single | Standard new windows |
| Triple pane, krypton fill | 0.15 to 0.22 | About 5x single | Cold climates |
Lower U-factor numbers mean better insulation. The jump from a failed unit back to an argon-filled double pane typically cuts heat loss through that opening by 40 to 60 percent compared with the air-filled state, which is the difference between a drafty wall and a comfortable one.
The Moisture Chain
Fog inside the cavity is liquid water sitting in the coldest part of the building envelope. Left alone, that moisture migrates into the frame and sill over years, feeding rot and mildew in wood windows and corrosion in steel frames. The damage is slow, which is why many homeowners tolerate the fog for a season or two before acting. The opening itself matters as much as the unit; a window is only as dry as the flashing around it, and leakproof window flashing is the detail that makes a watertight window installation shed water instead of wicking it into the wall.
Reading a Deteriorating Seal
If the fog comes and goes with the weather, the leak is small and the decision can wait. If the cavity stays wet through dry spells and mineral film builds up, the seal has failed completely and the unit is effectively a single pane with a water pocket trapped inside it.
Repair Path One: Warranty and Tolerance
Before spending money, check whether the window is still covered. Most IGU manufacturers back their seals with 10- to 20-year warranties, and many prorate the credit: you pay for the years used, the maker covers the rest. The warranty applies to the sealed unit rather than the frame, so a claim usually ends with a glass replacement instead of a whole new window.
Checking the Warranty
- Find the manufacturer label, usually etched in a corner of the glass or printed on the spacer.
- Note the model, date code, and company name, then call or email with photos of the fog.
- Ask whether the warranty is prorated and who pays for labor and the service call.
- File the claim before hiring anyone, because the credit disappears if you replace the unit yourself.
Warranty terms vary by manufacturer, but the claim process is short and the payoff is a new sealed unit at a fraction of retail price.
When Fog Is Tolerable
If the window faces a porch, sits behind furniture, or is rarely looked at, doing nothing is a legitimate option. The unit still blocks drafts and weather; it just insulates poorly. Budget for replacement in a future renovation and move on, but plan to address the moisture before it works its way into the frame.
Repair Path Two: Replacing the IGU or the Window
The complete fix is replacing the sealed unit. A glass shop measures the IGU, builds a new one with a fresh gas fill, and installs it into the existing sash. This keeps the frame and the exterior look while restoring the insulation value. Costs run roughly $100 to $300 per opening depending on size, gas fill, and whether the unit is stock or custom.
Part of diagnosing the job is confirming where the moisture comes from. Water leaking above windows usually traces to flashing failures at the head of the opening rather than to the edge seal, and the two problems need different trades and different materials, so checking the history of the leak before ordering glass saves a wasted install.
Measuring and Ordering an IGU
- Remove the sash or access the unit and measure width, height, and thickness to the nearest 1/16 inch.
- Note the glass type: tempered, laminated, or annealed, plus any coatings visible in the corner label.
- Order from a local glass fabricator, which typically turns units around in one to two weeks.
- Have the installer verify the new unit fits the opening before removing the old one.
Full Window Replacement
When the frame is old, the hardware is worn, or the window sits in a spot where insulation matters, replacing the whole window beats swapping the glass. New units come with modern low-E coatings, warm-edge spacers, and better frames, and the labor overlaps with the IGU route because the opening work is similar. The extra cost buys a fresh warranty and stronger long-term performance.
DIY vs. Professional Repair
Defogging kits exist for homeowners who want a cosmetic fix. They drill a small hole, flush the cavity, and install a vent plug so moisture evaporates instead of collecting. The result is clear glass at a fraction of replacement cost, but the gas fill is gone for good, so the unit stays at single-pane insulation levels. Treat the kit as a visibility fix, not an energy fix.
What a Professional Brings
A glazier brings the tools to remove and reinstall sashes without cracking the glass, plus access to wholesale IGU pricing that usually beats retail. Labor for a typical unit runs $150 to $400 depending on accessibility, floor level, and whether scaffolding is required. For second-story windows and oversized units, the professional route is often the cheaper one once you count your own time and the risk of handling large glass.
Moisture that has been migrating for years leaves marks on the surrounding structure, and those materials need their own inspection. Concrete elements, for example, develop distinct concrete deterioration and repair patterns that have nothing to do with the window and everything to do with how long water was allowed to sit against them.
Making the Call
Match the fix to the unit’s age. Under 10 years, chase the warranty. Between 10 and 20, price both an IGU swap and a full window and let the frame condition decide. Over 20, replacement is almost always the better spend, because the frame and hardware are near the end of their service life anyway.
Whatever path you choose, the installation details decide how long the repair lasts. A proper opening drains water outward, and sill pan flashing techniques, whether site-built or prefabricated, are what make a durable window installation shed water instead of trapping it, so ask the installer how the new unit will shed water before you sign.