A homeowner in Auburn called me last January. She'd noticed a milky haze between the panes of her living room window on a cold morning and assumed the worst — an $800-plus full window replacement. She'd already gotten one quote from a company in Sacramento that confirmed her fears. But when I drove out and looked at the window in person, the frame was rock solid. No rot, no warping, no operational issues. The only problem was a failed insulated glass unit.
That's the story I hear three or four times a month. The window looks terrible, so homeowners assume the whole thing needs to come out. In reality, most foggy windows can be fixed for 50 to 70 percent less than full replacement by swapping just the sealed glass unit — what the industry calls an IGU. This post breaks down what causes the fog, what your repair options actually cost, and how to tell when full replacement genuinely makes sense versus when you're being upsold.
TL;DR: Foggy double pane windows usually mean a failed IGU seal, not a dead window. Replacing just the glass unit costs $250–$700 installed vs $600–$2,000+ for a full window. At Sierra Foothills elevations, seals fail faster due to pressure differentials and 40–50°F daily temperature swings. This guide covers repair options, real costs, and when replacement actually makes sense.
What Causes Fog Between Window Panes?
A failed hermetic seal is the cause in nearly every case. The U.S. Department of Energy estimates windows account for 25 to 30 percent of residential heating and cooling energy use ([U.S. DOE](https://www.energy.gov/energysaver/windows-doors-and-skylights), 2024), and a broken seal eliminates the insulating gas layer that makes dual-pane windows effective. Once the seal breaks, argon escapes and humid air moves in. That moisture condenses on the inner glass surfaces where you can't wipe it away.
Here's how the seal breaks down. Manufacturers bond two panes of glass together with a spacer bar and sealant, then fill the cavity with argon or krypton gas. UV radiation degrades the sealant over time. Temperature cycling — glass expanding in heat, contracting in cold — mechanically stresses the bond at the spacer. Age does the rest. Most seals are designed for 15 to 25 years of service, but that number drops fast in harsh climates.
I got a call last fall from a homeowner in Grass Valley who was convinced his entire window was shot. Both panes were cloudy, the view was ruined, and he figured the glass itself had degraded. It hadn't. The glass was fine. The seal had failed, moisture had moved in, and mineral deposits from repeated condensation cycles left a film on the interior surfaces. We swapped the IGU, and the window looked brand new in the same frame.
Why Do Windows Fog Faster in the Sierra Foothills?
Elevation creates stresses that most window seals weren't engineered for. Atmospheric pressure drops roughly 0.5 PSI for every 1,000 feet of elevation gain ([BuildingGreen](https://www.buildinggreen.com), 2023), which means a window manufactured and sealed at sea level is under constant outward pressure when installed at 2,000 feet. That pressure difference puts approximately 1,150 pounds of additional outward force on each pane ([BuildingGreen](https://www.buildinggreen.com), 2023). The seal has to resist that force every day for decades.
Temperature swings make it worse. The Sierra Foothills routinely see 40 to 50°F swings between morning lows and afternoon highs ([Clearview](https://www.clearviewenergysolutions.com), 2026). Each cycle expands and contracts the glass, spacer, and sealant at different rates. That differential movement works the bond loose over years. And here's a detail most homeowners don't know: sealant permeability to moisture increases 6 to 8 times when surface temperatures hit 140°F versus 68°F ([SILEX](https://www.silex-silicones.com), 2025). A south-facing window in Colfax easily reaches 140°F on a July afternoon.
What does all this mean in practice? Seals that would last 20 to 25 years at sea level in a mild climate often fail in 15 to 20 years in the foothills. Higher elevations shorten that window even further.
I've been installing and replacing windows across Colfax, Auburn, and Grass Valley for over 25 years. The pattern is consistent: south- and west-facing windows fail first, homes above 2,500 feet fail sooner than homes at 1,500 feet, and windows that survived multiple wildfire smoke seasons show accelerated seal degradation. If your home is at elevation with significant sun exposure, budget for IGU replacements starting around year 15.
| Climate Condition | Typical IGU Seal Lifespan |
|---|---|
| Sea level, mild climate | 20–25 years |
| Foothills, 1,500–3,500 ft elevation | 15–20 years |
| High altitude, 4,000+ ft elevation | 10–15 years |
Can Foggy Windows Be Repaired or Do They Need Replacing?
If the frame is structurally sound, IGU replacement is almost always the better value. A 2026 analysis from This Old House puts full window replacement at $600 to $2,000 per window installed ([This Old House](https://www.thisoldhouse.com/windows/reviews/window-replacement-cost), 2026), while IGU-only swaps run $250 to $700 — saving homeowners 50 to 70 percent per opening. The frame is doing its job. Why rip it out?
The decision comes down to five factors. Not every foggy window is an IGU candidate, and not every foggy window needs full replacement. Here's how I think through it when I'm standing in someone's living room looking at the problem.
The frame condition tells most of the story. Press the sill and the corners with your thumb. If the wood is solid with no give, no soft spots, and no visible rot or warping, you've got a good IGU candidate. If you feel softness, see paint bubbling over dark wood, or notice the frame has pulled away from the wall — that's a replacement. The frame's structural integrity matters more than the glass.
- Frame is solid with no rot or warping — IGU replacement candidate
- Frame is damaged, soft, or warped — full replacement needed
- Window is 25+ years old — full replacement (the seal isn't the only worn component)
- 5 or more windows affected — consider whole-house replacement for volume pricing
- Window is single-pane — always full replacement (no IGU to swap)
How Much Does Foggy Window Repair Cost?
Three tiers of repair exist, ranging from a $70 temporary fix to a $2,000-plus full replacement. IGU replacement — the middle tier — is the sweet spot for most homeowners with sound frames. According to pricing data from This Old House, Fixurge, and Budget Glass ([This Old House](https://www.thisoldhouse.com/windows/reviews/window-replacement-cost), 2026; [Fixurge](https://fixurge.com), 2026; [Budget Glass](https://budgetglassllc.com), 2025), the installed cost of an IGU swap runs $250 to $700 depending on window size, glass type, and accessibility.
Let me give you a real example. An Auburn homeowner called me about six windows with failed seals — all on the south side of the house, all installed in 2008. She'd gotten a quote for full replacement: $7,800 for six vinyl retrofit windows. I looked at the frames, and every one was in solid shape. We did IGU replacements on all six for $3,600 total. She saved $4,200 and got factory-sealed units with new argon fills that'll perform for another 15 to 20 years.
The defogging option is worth understanding even though I don't recommend it. Companies drill a small hole in the glass, insert a desiccant to absorb moisture, then reseal the hole. It clears the fog temporarily. But it doesn't restore the argon gas fill, doesn't fix the failed seal chemistry, and the fog typically returns within 2 to 5 years.
| Repair Option | Cost Range | What You Get | How Long It Lasts |
|---|---|---|---|
| Defogging / Resealing | $70–$245 | Drill, dry, reseal existing unit | 3–5 years (temporary) |
| IGU Replacement | $250–$700 installed | New sealed glass unit in existing frame | 15–25 years |
| Full Window Replacement | $600–$2,000+ | Entire new window and frame | 25–30+ years |
IGU Replacement vs. Defogging: Which Actually Works?
Defogging is a bandaid. IGU replacement is the real fix. The National Fenestration Rating Council rates sealed IGUs based on their complete assembly — glass, gas fill, spacer, and sealant working together as a system ([NFRC](https://www.nfrc.org), 2024). Defogging addresses exactly one component (moisture) while leaving the rest of the system compromised. That's why fog returns in 2 to 3 years in most cases.
Here's what defogging actually involves. A technician drills a small hole through the outer pane, inserts a chemical desiccant to absorb trapped moisture, then plugs and seals the hole. The window clears up — sometimes dramatically — and the homeowner thinks the problem is solved. But the argon gas that originally filled the cavity is long gone. The seal that failed is still failed. The window is now performing like a single-pane unit with an air gap, which is better than nothing but nowhere close to its original thermal rating.
IGU replacement takes a different approach entirely. We remove the failed glass unit from the frame, measure and order a new factory-sealed IGU with fresh argon fill and intact sealant, and install it into the existing frame. The window returns to its original thermal and acoustic performance. It's a permanent fix — not a patch.
We don't offer defogging at Colfax Glass. I've seen too many callbacks from homeowners who paid $150 to $245 for defogging and had the fog return within 2 to 3 years. At that point they've spent money twice — once on the defog that didn't hold, and again on the IGU replacement they should have done first. I'd rather give people the right fix the first time.
How Long Do Double Pane Window Seals Last?
Most IGU seals last 10 to 25 years depending on climate, elevation, and glass quality. The NFRC and efficientwindows.org report that standard IGU warranties cover 10 to 20 years on parts with 1 to 2 years on labor ([efficientwindows.org](https://www.efficientwindows.org), 2024). In practice, higher-quality seals from manufacturers like Milgard and JELD-WEN tend to land at the upper end of that range, while budget units can fail in under a decade.
Several factors shorten seal life beyond what the warranty suggests. Direct sun exposure — especially south and west faces — accelerates UV degradation of the sealant. Elevation increases mechanical stress from pressure differentials, as I covered earlier. Wildfire smoke introduces fine particulates that work into weatherstripping and frame joints, creating micro-pathways for moisture to reach the seal. And poor installation — a window that isn't shimmed and sealed correctly from day one — can cause frame stress that prematurely loads the IGU seal.
Can you extend seal life? Somewhat. Keeping exterior paint and caulk maintained reduces moisture pathways to the seal. Awnings or overhangs on south-facing windows reduce UV and heat exposure. And making sure your window frames drain properly — no standing water on sills — prevents moisture from working into the frame-to-glass interface.
Maintenance tips to extend seal life: Keep exterior caulk and paint in good condition around window frames. Clear weep holes on vinyl and aluminum frames so water drains instead of pooling. Add awnings or plant shade trees on south and west exposures to reduce UV and heat load. Inspect seals visually every spring — catching early haze means you can plan an IGU swap instead of scrambling for an emergency fix.
How to Find a Reliable Glass Company for IGU Repair
Not every glass company offers IGU-only replacement, and that's the first question to ask. According to a 2024 consumer survey by the Window and Door Manufacturers Association, 38 percent of homeowners who received window repair quotes were only offered full replacement options ([WDMA](https://www.wdma.com), 2024). Some companies genuinely don't do IGU work. Others push full replacements because the margins are higher. Either way, you want a company that gives you both options with honest pricing.
Here are the questions I'd ask any glass company before hiring them for foggy window repair. Do you offer IGU-only replacement, or only full window replacement? What warranty comes with the new IGU? Will you inspect the frame before recommending a repair path? Can I see the failed unit after removal? A company that won't show you the old IGU or won't inspect the frame before quoting is cutting corners.
Red flags are straightforward. A company that only offers full replacement for a foggy window without checking the frame is likely upselling. No warranty on the replacement IGU means they don't stand behind their work. And any company that quotes over the phone without seeing the window in person is guessing — window sizes, glass types, and frame conditions vary too much for phone quotes to be reliable.
When I do an assessment, I check three things before I recommend anything: frame condition, number of affected units, and the age of the windows. If the frame passes and the windows are under 20 years old, IGU replacement is almost always what I recommend. If the frames are showing wear or the windows are approaching end of life, I'll tell you that too. There's no point saving $400 on an IGU swap if the frame is going to need replacement in three years anyway.