Condensation between window panes means the insulated glass unit (IGU) seal has failed. The argon or air gap that insulates your window has been compromised, moisture has worked its way in, and the fog you see is the visible proof. This is not a humidity problem you can solve with a dehumidifier. It is a mechanical failure of the window itself, and your options range from a $75 cosmetic fix to a $400 glass replacement to full window replacement starting around $300.
But before you spend money, make sure you’re diagnosing the right problem. Not all window condensation means a failed seal.
Three Types of Window Condensation (Only One Is a Problem)
This is the diagnostic step most guides skip. Condensation can appear in three locations on a window, and each means something completely different.
Fog between the panes is the one that requires action. You cannot wipe it off from either side because the moisture is trapped inside the sealed glass unit. The seal has failed, argon gas has leaked out, and outside air carrying moisture has infiltrated the gap. Your window still opens and closes, but it has lost most of its insulating value. The argon leaked out months or years before the fog became visible.
Moisture on the interior surface is a humidity issue, not a window defect. When warm, moist indoor air hits cold glass, water condenses on the surface; you can wipe it off with a towel. The fix is reducing indoor humidity: run bathroom exhaust fans, vent your dryer outside, and keep winter humidity between 30 and 35 percent, as Andersen recommends .
Condensation on the exterior surface is actually good news — the window is insulating so effectively that indoor heat is not reaching the outer pane. Cold outer glass drops below the dew point on humid mornings, and the moisture evaporates by mid-morning. No action needed.
| Location | What It Means | Can You Wipe It? | Action |
|---|---|---|---|
| Between panes | Failed IGU seal | No | Repair or replace |
| Interior surface | High indoor humidity | Yes (inside) | Reduce humidity |
| Exterior surface | Window insulates well | Yes (outside) | None — it’s working |
Why Seals Fail: Thermal Pumping and Desiccant Saturation
Every double-pane window contains two layers of glass separated by a sealed gap filled with argon gas (or sometimes krypton, or just air in cheaper units). A strip of desiccant material lines the spacer bar between the panes, absorbing any trace moisture that makes it past the seal.
The failure mechanism is gradual. Daily temperature swings cause the gas between the panes to expand and contract — on a 50-degree swing, the volume change is enough to flex the seal microscopically with every cycle. InterNACHI describes this process , called thermal pumping: sunlight heats the airspace and the gas pressurizes, then the space cools and contracts at night, fatiguing the perimeter seal over thousands of cycles until it fractures.
Tiny cracks develop. Each cycle pumps a small amount of outside air in and insulating gas out. Argon leaks at roughly 1% per year under normal conditions, so after a decade even a properly sealed window has lost about 10% of its gas fill.
The window still functions, but performance degrades incrementally. Outside air carrying moisture enters through those same micro-cracks, and the desiccant absorbs it. Once the desiccant saturates, moisture accumulates visibly between the panes — that is the fog you see.
South- and west-facing windows experience larger daily temperature swings and fail 30–40% faster than north-facing windows on the same house. Builder-grade windows with aluminum box spacers typically see seal failure between 5 and 10 years. Premium windows with warm-edge spacers (stainless steel or foam hybrids) push that to 15–20 years. The spacer material matters as much as the glass itself because aluminum conducts heat, creating a cold edge that accelerates condensation cycling right at the seal.
Fix Options and What They Actually Cost
Frame condition and the number of affected windows determine which repair makes sense.
Defogging ($75–$150 per pane)
A technician drills two small holes in the glass, sprays a cleaning solution into the cavity, vacuums out the moisture, and inserts a small vent or desiccant plug. The fog clears, sometimes over several weeks as residual moisture escapes.
Here is what defogging does not do: it does not replace the leaked argon gas, it does not repair the failed seal, and it does not restore the window’s insulating value. The window looked broken and now it looks clear, but thermally, nothing has changed. Expect fog to return within 1–3 years as the open seal continues admitting moisture.
The defogging industry is polarizing. Some glass companies promote it as an affordable bridge solution for homeowners who cannot afford replacement yet. Other contractors call it a waste of money because you pay $75–$150 for a cosmetic result while the window continues losing energy.
Both sides have a point. If you need clear glass for a home sale next spring and the alternative is $400 per pane for IGU replacement, defogging buys time. As a long-term fix, it fails.
One more limitation: if the trapped moisture has been sitting between the panes long enough, mineral deposits etch the interior glass surfaces permanently. Defogging removes the moisture but not the etching. Hold the window up to strong light before hiring a defogging service. If you see a white haze that does not correspond to current moisture patterns, the glass is etched and defogging will leave a cloudy window.
IGU Replacement ($150–$400 per pane)
A glass shop removes the failed insulated glass unit from the existing frame and installs a new one with fresh seals and argon fill. The frame stays. This is the repair that actually restores performance: new glass, new gas, new seal, new desiccant.
IGU replacement makes sense when the frame is structurally sound. Press your thumbnail into the vinyl on a cool morning. Healthy vinyl flexes slightly and springs back. If it feels rigid, dents permanently, or the surface chalks onto your hand, the frame is degrading and a new IGU will outlast the frame it sits in.
Turnaround is typically 1–2 weeks because the new glass unit is custom-cut to your frame dimensions. Expect $150–$400 per unit installed, depending on size and whether you need low-E coatings or tempered glass (required by code in bathrooms, near doors, and below 18 inches from the floor).
Full Window Replacement ($300–$1,300 per window)
When the frame shows warping or rot alongside failed glass seals, replacing just the IGU puts new glass in a dying frame. Full replacement installs a complete new window unit. Costs run $300–$1,300 per window installed , with most standard replacements landing at $450–$900. Our window replacement planning guide covers budgeting and scoping a full-house project.
Full replacement becomes the clear choice when seal failures cluster. If three or more windows on the same wall or elevation show fogging, the rest are on the same timeline. Seal failure is a sign of aging windows overall, not isolated bad luck. Replacing them all at once saves 10–15% over piecemeal work.
The Decision Framework
| Situation | Best Option | Why |
|---|---|---|
| 1 window fogged, frame solid, budget tight | IGU replacement | Restores performance at ~30% of full replacement cost |
| Home sale within 6 months, foggy windows | Defogging | Cheapest path to clear glass; buyer inherits the real problem |
| 3+ windows fogged on the same house | Full replacement | Remaining windows will follow; bulk pricing saves 10–15% |
| Frame warped, chalky, or cracked | Full replacement | New IGU cannot seal properly against a degraded frame |
| Fog appears and clears daily | No repair needed | That is exterior condensation; window is working correctly |
Prevention
You cannot prevent seal failure entirely. Thermal pumping is physics. But you can slow it down:
Choose windows with warm-edge spacers. This is the single highest-impact spec most buyers overlook. Warm-edge spacers (stainless steel, foam, or hybrid designs) reduce heat loss at the glass edge significantly compared to aluminum box spacers, and that directly extends seal life by reducing the temperature differential that drives thermal pumping. If you can ask one question when shopping for windows, ask about the spacer.
Avoid dark-colored frames on south and west walls. Dark frames absorb more solar radiation and run 20–30°F hotter than white frames in the same position. That amplifies thermal cycling and accelerates seal fatigue. Vinyl frames in particular have a heat deformation threshold around 150–160°F, and dark vinyl on a sun-baked south wall can exceed that on summer afternoons.
Winter humidity at 30–35% reduces moisture load on window seals from the interior side. This slows interior-surface condensation cycling, which stresses seals from the warm side. Avoid humidifiers running above 40% — that accelerates the exact condensation cycling that fatigues perimeter seals.
Trim vegetation away from windows. Bushes and vines trap moisture against frames and block the airflow that helps windows dry after rain. This accelerates seal-adjacent degradation.
When Foggy Windows Signal a Bigger Problem
Seal failure on vinyl windows accelerates after year 15, and it typically means the window is entering its decline phase overall. Vinyl windows last 20–30 years , but the IGU seal is what fails first, not the frame. If your windows are 15+ years old and multiple seals are blowing, you are not looking at a glass problem. You are looking at end-of-life windows that happen to announce themselves through the glass first.
At that stage, replacing individual IGUs is chasing a moving target. By the time you replace four units at $150–$400 each, you have spent $600–$1,600 and still have aging frames with degraded weatherstripping and balances that are starting to bind. A whole-house window replacement at $300–$1,300 per window solves all of it for the next 20+ years. When you are ready to move ahead, our window contractor hiring guide walks through screening installers and comparing bids.