Fog between your window panes means the seal on the insulated glass unit has failed. It cannot be repaired, but it usually does not require a new window either — replacing the glass unit inside the existing sash runs $150 to $400 when the frame is sound.
Before you do anything else, check the warranty. Insulated glass units frequently carry 10 to 20 year manufacturer coverage, and a surprising number of people pay for a replacement they were entitled to for free.
What actually failed
A double-pane insulated glass unit is two panes of glass separated by a spacer, sealed around the perimeter, with a desiccant packed inside the spacer to absorb any residual moisture. The cavity is usually filled with argon.
Every day that unit breathes. The sun heats it, the air inside expands and pushes on the seal; night comes, it contracts and pulls. Thousands of cycles a year. Eventually the sealant loses its grip somewhere along the perimeter.
Once that happens, humid outside air moves in and out with the cycle. The desiccant absorbs moisture until it is saturated, which is why a seal can fail eighteen months before you see anything. Then the water has nowhere to go, and it condenses on the inside faces of the glass. That is the haze, and the mineral deposits that etch into the glass over time are why an old failure often leaves a permanent ghost even after the water clears.
Why it happens sooner on some windows
Not random. The pattern is consistent.
- South and west exposures fail first. More heat cycling, more UV on the sealant.
- Bottom edges fail first, because that is where water sits in the frame channel if the weep holes are blocked.
- Large units fail earlier than small ones — more area, more pressure change per cycle.
- Blocked weep holes are a major cause. Those tiny slots at the bottom of the exterior frame drain water out. Painted over or packed with debris, the glass unit sits in standing water and the seal is attacked from below.
- Altitude changes on units shipped without capillary tubes.
That weep hole point is worth acting on. Take a look at the bottom exterior of your windows this weekend and clear them with a thin piece of wire. Five minutes, and it genuinely extends the life of the glass you have left.
Your four options, priced
| Option | Cost | Honest verdict |
|---|---|---|
| Warranty claim | $0 to a service fee | Always check first |
| Glass-only (IGU) replacement | $150–$400 per unit | The right answer most of the time |
| Defogging service | $75–$200 per unit | Cosmetic only |
| Full window replacement | $400–$900 vinyl, more for other frames | Right when the frame is failing too |
On defogging
The process: a small hole is drilled in the glass, the cavity is cleaned, sometimes a drying agent is applied, and a one-way vent is installed. The haze goes away and the view comes back.
What has not happened: the seal has not been restored, the argon is gone permanently, and the cavity is now deliberately vented to outside air. Thermal performance stays reduced. In humid weather the unit can fog again.
It is not a scam — it is a legitimate cosmetic service that costs less than glass replacement, and for a window you plan to replace in three years anyway it is a reasonable stopgap. Just buy it knowing what it is.
When to replace the whole window instead
Glass-only replacement is the default. Replace the entire unit when:
- The sash or frame has rot, or the sash no longer closes squarely
- The hardware is failing and parts are unavailable
- Four or more units in the house have fogged. Same age, same seal chemistry — you are on a conveyor belt, and paying $300 a pop as they fail one per year is worse than doing it properly
- They are single-glazed or old aluminum-framed and you were going to upgrade anyway
- The glass is obsolete enough that a replacement unit costs close to a new window
Finding the warranty
Worth twenty minutes before you spend anything.
- Look for an etched code in a corner of the glass, or a small metal tag or sticker inside the spacer between the panes. Manufacturers stamp these with a code and often a date.
- Check the frame channel. Open the sash and look at the top or side jamb for a label with a model and serial number.
- Check your closing documents if the house is newer. Window warranties are frequently in the builder packet nobody has opened since move-in.
- Call the manufacturer with the code. Some warranties are transferable to subsequent owners; some cover glass but not labor.
Even a warranty that covers the glass and not the labor turns a $350 job into a $120 job.
While you have someone out anyway
If you are paying for a glass replacement, this is the cheapest moment in the window's life to improve it. Going from clear glass to a proper low-E coating with argon costs modestly more on the unit and changes how the room feels — particularly on a west-facing window. Our energy-efficient windows guide explains which label numbers to specify.
You can also change the glass type at the same time: obscure glass for a bathroom, tempered where code requires it, laminated for a road-facing room where noise is the real complaint.
What you should not bother doing
Do not try to DIY a glass unit swap on a vinyl or clad sash. The glazing beads are unforgiving, the sealant matters, and a mis-set unit fails again in two years. The measurement alone has to be exact — glass shops build to your numbers and will not eat the error.
Do not replace all your windows because one fogged. One failure is one failure. Four is a pattern.
Do not ignore it indefinitely either. A vented, damp cavity degrades the low-E coating and etches the glass, so the longer you wait, the more likely a defog leaves you with a permanent haze.
Ontario Window Replacement handles failed glass units and full replacements across Ontario and the West End of the Inland Empire — vinyl replacement windows, double-hung units, and energy-efficient upgrades — and we will tell you to call your manufacturer first if we think the warranty covers it. For the bigger decision, see our repair versus replace guide.
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Related services in Ontario
Ontario Window Replacement money pages — go straight to the service you need.
- Energy-Efficient Glass for Homes and Suites →
Energy-efficient glass pays back on a schedule, and the two buildings we work on keep opposite schedules. A house in Ontario sits empty at two in the afternoon and full at eight at night.
- Vinyl Window Replacement →
Vinyl is the correct answer for most houses in Ontario and the wrong answer for almost every storefront. That sentence is the whole page, but the reasons behind it decide whether you get a window that lasts thirty years or one that sags in eight.
- Double-Hung and Single-Hung Windows →
The hung window is a transplant in Ontario. It is the eastern and midwestern default, it arrived here with tract builders copying catalogs, and it is chosen for how a house looks far more often than for how the window works.
- Fiberglass Window Replacement →
Fiberglass exists to solve three specific problems: dark colors at large sizes, thin sightlines on a wide opening, and a frame that will accept paint later. If none of those apply to your house, you are about to spend money for nothing.
- Clad-Wood Windows →
Clad-wood is the only window we sell for a reason that is not technical. You buy it because you want wood on the inside of the room, and everything else about the product is engineering built to make that survivable outdoors.
Frequently asked questions
Why is there fog between my window panes?
The perimeter seal on the insulated glass unit has failed. Moist outside air is now getting into the sealed cavity, the desiccant inside the spacer has absorbed all it can, and the moisture condenses on the inner glass surfaces where you cannot reach it.
Can you fix a fogged window without replacing it?
You can replace the glass unit inside the existing sash for $150 to $400 if the sash and frame are sound. What cannot be repaired is the seal itself. Anything sold as a repair of the original unit is either a glass swap or a cosmetic defogging.
Does window defogging work?
It clears the haze. A hole is drilled, the cavity is cleaned and left vented to outside air. What it does not do is restore the argon fill or the sealed cavity, so the thermal performance stays diminished and the unit can re-fog in humid weather. Treat it as cosmetic, not as a repair.
How much does it cost to replace a fogged glass unit?
Usually $150 to $400 per unit for a standard double-pane in a sound sash, including labor. Large units, tempered glass, obscure glass, grids between panes, and low-E coatings push it higher. Replacing the whole window instead runs $400 to $900 for vinyl.
Is a fogged window still insulating?
Partly. The two panes still create an air gap, so it is better than single glazing. But the argon fill is gone, the cavity is now damp, and the low-E coating can degrade where moisture sits against it. Expect meaningfully reduced performance and continued deterioration.
Are fogged windows covered under warranty?
Very often, and this is the step most people skip. Insulated glass units commonly carry manufacturer warranties in the 10 to 20 year range, some transferable to a new owner. Find the etched code in the spacer or a sticker in the frame channel and call the manufacturer before paying anyone.
Local context
How this applies in Ontario, CA
Every article here is written from window work done in Ontario and the West End of the Inland Empire, so the numbers reflect local labour, permit and material costs rather than a national average. Your own job can land either side of them depending on access, the condition of what is already there, and the materials you pick — center-set aluminum storefront framing in 1-3/4 by 4-1/2 inch profiles, dry-glazed EPDM gaskets and wet-glazed structural silicone joints, subsill flashing with end dams under storefront base members.
The way to turn any of this into a real answer is to have someone look at your actual situation. The free estimate is free, itemized and written down, with no deposit and no obligation. Call (844) 511-2320 or start on the Ontario Window Replacement homepage.
Areas Ontario Window Replacement covers
Ontario first, then the surrounding communities of the West End of the Inland Empire. Each has its own page with local detail rather than a copy of this one:
- Chino, CA — San Bernardino County, incorporated in 1910, population 91,403 at the 2020 census, with dairy-preserve land on the south side turning over to distribution buildings
- Chino Hills, CA — San Bernardino County, incorporated in 1991, population 78,411 at the 2020 census, almost entirely built after 1980 and overwhelmingly residential
- Montclair, CA — San Bernardino County, incorporated in 1956, population 37,865 at the 2020 census, in a five and a half square mile footprint dominated by retail frontage
- Upland, CA — San Bernardino County, incorporated in 1906, population 79,040 at the 2020 census, with a pre-war downtown core and hillside tracts above Foothill Boulevard
- Pomona, CA — Los Angeles County, incorporated in 1888, population 151,713 at the 2020 census, the largest city in the Pomona Valley and the oldest housing stock we cover
- Claremont, CA — Los Angeles County, incorporated in 1907, population 37,266 at the 2020 census, with a walkable village core and a large protected residential historic district
Or jump to the full list of Ontario services.
More from the Ontario Window Replacement blog
- Vinyl vs Fiberglass Windows for Ontario Homes
Vinyl vs fiberglass windows in Ontario, CA — the real price gap, thermal expansion, and when the cheaper frame is the right call.
- Double-Hung vs Casement Windows for Ontario Homes
Double-hung vs casement windows in Ontario, CA — why casements seal tighter, where double-hungs win, egress minimums, and which to pick room by room.
- Bay Window Cost in Ontario: Structure and Roofing
Bay window cost in Ontario, CA — unit price versus the real installed number, the support nobody quotes, and when to skip it.
The Ontario Window Replacement Team
Licensed window pros in Ontario, CA
Written by the Ontario Window Replacement team — licensed, insured window pros serving Ontario, CA and the surrounding area.