Insulated glass units, commonly called thermal windows or IGUs, sandwich two or three panes of glass around a sealed air space to slow heat transfer. They outperform single-pane glass in every climate, but the sealed space is also the weak point: when the edge seal fails, moisture works between the panes and condenses into fog that will not clear on its own. Before ordering a replacement, understand how failed double-glazed seals present, because the fix ranges from a low-cost defogging service to a full sash replacement. A single fogged window is not a reason to replace the whole window; in most homes the choice comes down to IGU replacement versus defogging, with full window replacement reserved for units that have frame damage or very old glass. This article explains how IGUs are built, why they fog, what each repair path costs, and the maintenance that extends seal life.
The age of the unit matters: seals typically last 10 to 20 years, so fog in a window older than that is rarely worth a repair that only delays the same failure.
How Insulated Glass Units Work
Anatomy of an IGU
An insulated glass unit is a factory-sealed assembly, not a field-made sandwich. The parts work together to keep the air space dry and still:
- Two or three panes of glass, usually tempered or laminated
- A spacer bar around the perimeter that sets the gap width
- A desiccant inside the spacer that absorbs residual moisture
- A primary seal that bonds the glass to the spacer
- A secondary seal that blocks water and gas loss
- An inert gas fill, typically argon or krypton, between the panes
The gas is denser than air, so it conducts less heat. Low-emissivity coatings on the glass reflect heat back toward the room in winter and away from it in summer.
Energy performance compared
Sealed units earn their keep on the energy bill. The table below compares typical performance ranges for the three common configurations.
| Configuration | Layers | Typical R-value | Best use |
|---|---|---|---|
| Single pane | 1 | About R-1 | Historic or unheated spaces |
| Double pane | 2 | R-2 to R-3.5 | Most homes |
| Triple pane | 3 | R-3.5 to R-5 | Cold climates, noisy streets |
Roughly 25 to 30 percent of residential heat loss happens through windows, so the jump from single to double glazing is one of the larger envelope upgrades available to a homeowner.
What the seal actually does
The seal has two jobs: keep water vapor out and keep the gas fill in. Desiccant inside the spacer absorbs whatever moisture slips past the primary seal during normal expansion and contraction. When the desiccant saturates, water stays in the air space and condenses on the glass, which is the fog homeowners see. Installation quality decides how long the seal lasts, and the causes and repair prevention guidance for fogged windows starts with keeping the frame dry and the unit square in its opening.
Why Fog Forms Between the Panes
Seal failure and moisture entry
Seal failure usually begins at the edge, where the glass, spacer, and frame meet. Sun exposure, freeze-thaw cycles, and wind-driven rain attack the secondary seal first. Once the seal cracks, humid air enters faster than the desiccant can absorb it. The first visible sign is a light haze at the bottom edge that grows upward over weeks or months.
Environmental stress on the seal
Temperature swings stress the unit from two directions. The glass expands and contracts with daily heating, and the frame transfers solar heat to the seal line. Frame color changes how much heat reaches the edge: black window frames absorb more solar radiation than white or light frames, which raises edge temperatures and accelerates seal breakdown, one of the trade-offs examined in the detailed comparison of black windows, including cost and energy behavior.
Altitude and barometric pressure
Elevation and weather fronts change the pressure difference between the inside of the unit and the outside air. Every pressure swing pushes and pulls on the seal, so units installed in mountain towns or regions with frequent storms tend to fail earlier than identical units in mild coastal climates.
Is it fog or condensation?
Not every water spot near the glass means the seal failed. Check before spending money:
- Fog between the panes that never clears is seal failure.
- Condensation on the room-side glass that dries by midday is normal humidity.
- A white film that reappears after cleaning indicates a failed unit.
- Water pooling at the bottom of the pane confirms seal failure.
Repair Option 1: Replacing the Insulated Glass Unit
What IGU replacement involves
IGU replacement keeps the existing frame and replaces only the glass assembly. A window company measures the unit, orders a matching IGU, and installs it in about an hour per sash:
- Remove the sash or stop beads that hold the glass.
- Lift the old unit out and clean the frame channel.
- Set the new unit on setting blocks.
- Reinstall the stops and apply new sealant.
- Allow the sealant to cure before operating the window.
Costs and when replacement wins
IGU replacement typically runs $150 to $500 per window installed, well below the $500 to $1,000 range for a full replacement window in a similar opening. Replacement wins when the frame is sound and the glass is worth saving, which covers most homes built after 1990. It also restores the gas fill and the low-e coating, so the repaired unit performs like new.
Ask the installer whether the quote includes removing the old sealant and cleaning the frame channel, since leftover adhesive shortens the life of the new unit.
A separate failure: spontaneous shatter
Fogging is the common IGU failure, but glass can also break on its own. Most units use tempered glass, and the reasons thermal windows spontaneously shatter include nickel sulfide inclusions and rapid temperature swings that create thermal stress. Spontaneous breakage is rare, but a cracked pane always needs replacement, and the same installer can handle both repairs.
Repair Option 2: Defogging
How defogging works
Defogging clears the fog without replacing the glass. The technician drills small holes in the outer pane, flushes the interior with a cleaning solution, installs small vents or tubes, and seals the unit:
- Drill two or three small holes at the bottom edge.
- Flush the space between panes with a cleaning solution.
- Rinse and dry the interior.
- Install small breather tubes or vents.
- Seal the holes and polish the glass.
What defogging does and does not fix
Defogging removes the visible fog and restores a clear view, but it does not restore the gas fill or the desiccant, and the unit loses part of its insulating value. It suits rental properties, secondary rooms, and windows where energy performance matters less than appearance. Homeowners weigh it the same way they weigh other cosmetic repairs: laminate countertop repair options range from quick fill sticks to full resurfacing, and the right choice depends on how long you plan to keep the surface. Fog can return within a few years if the original seal continues to fail.
Repair Option 3: Full Window Replacement
Comparing the three repair paths
The decision usually comes down to budget and how long you plan to stay in the home. The table below lines up the three options.
| Option | Typical cost per window | Time | Longevity | Energy improvement |
|---|---|---|---|---|
| Defogging | $100 to $200 | 1 to 2 hours | 2 to 5 years | Minor |
| IGU replacement | $150 to $500 | About 1 hour | 10 to 20 years | Full, restored |
| Full window replacement | $500 to $1,000+ | 2 to 4 hours | 20 to 30 years | Highest |
When to replace the whole window
Choose full replacement when the frame is rotted or warped, the hardware no longer works, the glass is single-pane, or the fog returns shortly after defogging. Before committing, rule out water entry from above, because water leaking above windows usually points to flashing failures at the head of the opening rather than a problem inside the glass. Fix the flashing first, then decide whether the unit itself needs attention.
Preventing Fogged Windows and Extending Seal Life
Maintenance habits that protect seals
Seal life depends more on how the window is treated than on the brand of glass. A short seasonal routine pays off:
- Keep weep holes and drain channels clear.
- Trim shade trees so the frame is not constantly baked.
- Recaulk the exterior bead when it cracks.
- Never pressure-wash directly at the glass edge.
- Keep indoor humidity below 50 percent in winter.
Choosing durable replacements
When you do replace, buy units with warm-edge spacers, low-e coatings, and an argon or krypton fill, and read the warranty, which typically covers seal failure for 10 to 20 years. Moisture damages sealed systems of every kind, and the same water movement that destroys window seals drives concrete deterioration and repair, because water carries chlorides and drives freeze-thaw cycles that crack rigid materials. Keeping water away from vulnerable edges is the shared rule for windows and slabs alike.