Foggy Windows? Condensation Between the Panes Explained

You have cleaned the glass twice and the haze is still there. That is because the fog in a double-pane window is not on the glass—it is between the two panes, inside a sealed unit you cannot reach with a rag. Once you understand why it happens, the fix becomes straightforward.

What a Double-Pane Window Actually Is

A double-pane window (also called an insulated glass unit, or IGU) consists of two panes of glass separated by a spacer and sealed around the edges. The space between the panes is filled with either dry air or an insulating gas—usually argon or krypton—and the whole assembly is hermetically sealed. That sealed gas pocket is what gives the window its energy-efficiency advantage.

The seal exists to keep moisture out of that interior space. When the seal fails, outdoor air—and its moisture content—begins to infiltrate the gap. When temperatures change, that moisture condenses on the inside surfaces of the glass where you can see it but cannot wipe it off.

Failed Seal vs. Surface Condensation: How to Tell

These are two completely different problems with completely different fixes.

Failed seal (moisture between the panes). The fog, haze, or water droplets appear inside the glass, not on the surface. You can run a dry finger across the glass and feel nothing—the moisture is trapped. The fogging may come and go with temperature changes (worse in the morning, clears on a warm afternoon) but returns persistently. Over time it may leave a white mineral residue or staining on the inner glass surfaces as the water cycles through.

Surface condensation (on the inside of the glass). This is a humidity problem in your home, not a window defect. When warm, humid indoor air hits a cold glass surface, water condenses—just like on a cold drink on a hot day. This type of condensation wipes off easily with a dry cloth. It tends to appear in winter on the coldest nights, especially in rooms with poor air circulation or high moisture sources like cooking and showers. The fix is reducing indoor humidity with better ventilation or a dehumidifier, not replacing the window.

Surface condensation on the outside. On cool mornings in summer, you may see condensation on the exterior surface of energy-efficient windows. This is actually a sign the window is working well—the exterior glass is cool because little heat is escaping through the IGU. This kind of condensation evaporates on its own as the morning warms up and requires no action.

What Causes the Seal to Fail

Window seals do not last forever. Most manufacturers rate IGU seals for 10 to 25 years under normal conditions. Several factors accelerate failure:

Thermal cycling. Glass and frames expand and contract with temperature changes. Over years, this repeated stress can degrade the sealant and the flexible spacer material, allowing air infiltration.

Water pooling. Windows installed without proper flashing or sloped sills can trap water at the base of the frame, which accelerates sealant breakdown.

Physical damage. A pressure washer aimed directly at a window, a crack from impact, or improper installation can compromise the seal prematurely.

Age. Most double-pane windows installed before 2005 were made with aluminum spacers, which conduct cold to the seal edge more aggressively than the warm-edge spacers used in modern IGUs. Older aluminum-spacer units tend to fail faster.

Repair or Replace?

Once a seal fails, the fogging will not resolve on its own. You have three options:

1. Replace just the IGU (the glass unit). In most cases, the frame and hardware are still perfectly functional. A glazier or window company can remove the failed IGU and install a new sealed glass unit in the existing frame. This is the most cost-effective option when the frame is in good shape.

IGU-only replacement typically costs $150 to $450 per window for standard sizes, with larger or specialty glass (tempered, Low-E, laminated) running $280 to $550 or more.¹

2. Replace the full window (sash + glass). If the frame is warped, the hardware is broken, or you want to upgrade to a more energy-efficient product, a full window replacement makes sense. Expect to pay $300 to $800 per window installed for standard residential windows, more for large or custom sizes.

3. "Defogging" services. Some companies offer a process that involves drilling a small hole in the glass, injecting a drying agent, and plugging the hole. This can temporarily clear the fog, but it does not restore the insulating gas, does not reseal the unit, and most reports suggest the haze returns within 12 to 24 months. Defogging also typically voids any remaining manufacturer warranty. This option is worth skipping.

How Much Energy Are You Losing?

A failed seal does not just look bad—it reduces the window's insulating performance. The U.S. Department of Energy notes that heat gain and loss through windows accounts for 25 to 30 percent of residential heating and cooling energy use.² A double-pane IGU with a functioning argon fill can have a U-factor (heat flow rate) roughly twice as good as a single-pane window. When the seal fails and the gas escapes, performance drops back toward that of a basic single-pane.

In climates with significant heating or cooling loads, restoring a failed IGU can meaningfully reduce energy bills, not just improve appearance.

When to Call a Pro

Replacing an IGU requires removing the glass stop or glazing tape, removing the old unit without breaking it, fitting and sealing the new unit, and re-glazing. It is possible for a skilled DIYer on a ground-floor window, but one cracked pane can quickly erase any cost savings. Call a professional if:

The window is on an upper floor
The frame shows signs of rot, warping, or water damage
The window has special glass (tempered, laminated, frosted, historic)
You are replacing multiple units at once (volume pricing from a glazier makes sense)

Common Mistakes

Replacing windows before diagnosing the condensation type. If the fog wipes off, you may just need a dehumidifier, not new windows. Spending $500 per window on a humidity problem is an expensive misdiagnosis.

Waiting too long. A failed seal lets in not just moisture but also outdoor contaminants. Over time, mineral deposits from the condensation cycles can etch the inner glass surfaces, making replacement of just the IGU no longer practical. Act before staining sets in.

Using a pressure washer on windows. High-pressure water forced against window seams is one of the fastest ways to blow out a seal. Always clean windows with low-pressure water and a soft cloth.

Ignoring frame damage. If the frame has rot or water damage, replacing only the glass will not stop the moisture problem. Check the frame, sill, and surrounding trim before deciding on IGU-only repair.

How to Extend Window Seal Life

Keep window frames painted or sealed to prevent moisture absorption, especially at the corners and sill.
Ensure proper drainage at the sill—most windows have small weep holes at the base of the frame that should be kept clear.
Maintain caulk around the exterior window perimeter and replace it when it cracks or pulls away.
Avoid using a pressure washer aimed directly at window seams.
Keep indoor humidity in the 30 to 50 percent range to reduce thermal stress cycling on the seals.

Frequently Asked Questions

Can I fix foggy windows myself? Not reliably. The DIY defogging kits sold online involve drilling holes in the glass and are not a permanent fix. For a proper repair, the IGU needs to be replaced by a glazier or window company.

My window fogs in the morning and clears by afternoon. Is the seal gone? Yes, almost certainly. This cycling behavior—fog when cool, clear when warm—is a classic sign of a failed seal with moisture trapped inside. The moisture evaporates temporarily as the glass warms but returns with the next cold cycle.

How long do replacement IGUs last? Modern IGUs with warm-edge spacers and high-quality sealant are generally rated for 15 to 25 years. Proper installation and frame maintenance have a significant impact on longevity.

Does homeowner's insurance cover failed window seals? Generally, no. Seal failure is considered gradual deterioration, not sudden damage. However, if a window seal failed because of a covered event (such as hail or impact), you may have a claim. Check your policy.

Is argon gas refilled when only the glass is replaced? Yes. A properly manufactured replacement IGU comes pre-filled with argon (or krypton) and sealed from the factory. As long as it is installed correctly, the new unit will have its full insulating gas fill.

Get a Free Quote

If you have foggy windows and want to know whether an IGU swap or full window replacement makes more sense for your home, a local window pro can give you a clear answer. Request your free estimate today.

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Airtasker — Window Seal Repair Cost in the US (2025) ↩
U.S. Department of Energy — Energy Efficient Windows ↩