Why are my windows foggy between the panes?
Updated August 2026
Haze or droplets sealed between the two panes of glass, the kind you cannot wipe off from either side, almost always means the insulated-glass unit's edge seal has failed and let humid air inside. Once moisture is trapped in that sealed cavity, no cleaning reaches it, and the fog comes and goes with the temperature. The good news: you usually replace just the glass unit (the IGU) for $150–500, not the whole window, and that restores the argon fill and insulating value the broken seal quietly leaked away.
Safety first: the fog itself is sealed inside the glass and is harmless, but do not confuse it with water that beads on the room-facing surface of the pane. That indoor condensation, common on a gas-depleted window in winter, can run down onto a wood sill and feed mold or rot within a season or two. If you see water pooling on the sill, dry it and address the indoor humidity before the frame is damaged.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Failed insulated-glass edge seal | Most common | Replace the sealed glass unit (IGU) alone, not the whole window. A glazier measures it and swaps the pane into the existing frame. | $150–500 per unit |
| Lost argon or gas fill from an aging seal | Common | The same fix: a new IGU arrives refilled with argon. A depleted fill is why a fogging window also feels colder near the glass. | $200–550 per unit |
| Age (seals dry out at 10–20 years) | Common | Expect more units to fog as a house full of same-age windows crosses 15 years. Replace them as they go, or budget a phased window swap. | $150–500 each, or full windows |
| Saturated desiccant in the spacer bar | Occasional | The drying beads inside the metal spacer are full and can no longer absorb humidity, so it condenses on the glass. Only a new IGU cures it. | $150–500 per unit |
| Standing water at the sill from clogged weep holes | Occasional | Water pooling against the glass edge cooks the seal early. Clear the weep holes and reseal the frame to protect the replacement. | $0–150 DIY |
| A previous "defogging" that did not hold | Rare | Drill-and-vent defog services clear the haze but rarely restore the seal or gas fill, so units often re-fog. Replace the IGU for a lasting fix. | $100–250 spent, refog likely |
Check these first
- Wipe both the inside and outside of the glass. If the haze stays put, it is sealed between the panes and no cleaning will ever reach it.
- Check whether the fog tracks temperature: clear on a mild afternoon, cloudy on a cold morning confirms a failed seal rather than surface dirt.
- Look at how old the windows are. Units past 15 years that fog one at a time are simply reaching the end of their seal life.
- Inspect the sill for standing water and the weep holes for debris. Trapped water shortens seal life and is worth clearing regardless.
- Note whether the glass feels colder than your other windows. A cold pane points to a lost argon fill on top of the failed seal.
- Count how many windows are affected. One is a spot IGU swap; several same-age units fogging together is a budgeting question.
Prevent it next time
You cannot un-fog a sealed unit, but you can slow the seal failure that causes it. The big lever is water management: reseal the frame and caulking on the recommended schedule and keep the weep holes clear so water never sits against the glass edge, which is what cooks a seal early. Beyond that, seals age out on their own. Track how each opening is doing with a seasonal window check, and once a majority of a same-age set are fogging, it is usually the point to plan a phased replacement instead of swapping glass units one at a time.
When to call a pro
This is a job to hand off early. Measuring an IGU to the millimeter and setting it into the frame without cracking it is glazier work, and a mismeasured unit is a wasted $150–500. Call a glass shop once you have confirmed the fog is between the panes; many will replace the sealed unit for far less than a full window. Ask for a quote on both the glass-only swap and a full window replacement if the frame is rotting, the sash binds, or the window is past 20 years, because at that point new windows may be the smarter spend. Check any original warranty first: insulated-glass seals are often covered for 10–20 years.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
Can I clear the fog between my double-pane windows myself?
Not durably. The haze lives in a sealed cavity between the panes, so wiping either surface does nothing. Drill-and-vent defogging kits and services do exist, running roughly $100–250 per window, and they can clear the visible cloud by venting the space. What they rarely restore is the airtight seal or the argon fill, so many treated units fog again within a year or two and still carry a worse insulating value. Replacing the sealed glass unit for $150–500 is the fix that actually lasts.
Do I have to replace the whole window if it is foggy?
Usually not. In most double- and triple-pane windows the glass is a separate sealed unit that sits inside the frame and sash, and a glazier can swap just that unit for $150–500 while keeping your existing frame, hardware, and trim. A full replacement window runs $450–1,200 installed for a standard size, so the glass-only route saves real money. You only need the whole window when the frame is rotted, the sash no longer seals or operates, or the window is old enough that everything is near the end of its life.
Does a fogged window actually cost me anything, or is it just ugly?
It costs you comfort and efficiency, not just looks. A failed seal means the insulating argon has leaked out, so that pane now transfers heat closer to a plain single sheet of glass than the sealed unit you paid for. You feel it as a colder draft near the window in winter and a hotter pane in summer, and it shows up as a small bump in heating and cooling bills across every affected opening. Replacing the sealed unit restores the gas fill and the original insulating performance without a full window job.
How long do window seals last before they fog?
Most insulated-glass seals hold up for about 10 to 20 years, and many manufacturers warrant the seal for a stretch inside that range. The quality of the original unit, sun exposure, and above all water management decide where a given window lands: a south-facing unit over a sill that pools water can fail before 10 years, while a well-drained, shaded window can pass 20. Because a whole house is often glazed at once, expect several windows to start fogging within a few years of each other once they reach that age.