Reviewed by Ron McLaughlin, Red Seal Certified Carpenter & Registered Energy Advisor, Shoreline Building Performance. Last updated: 2026-08-31
Condensation between window panes means the airtight seal on your insulated glass unit has failed, letting humid air into the sealed space between the glass. The trapped moisture fogs the glass from the inside, where you can’t wipe it away. It signals lost insulation and rising energy bills, and the glass unit needs replacing, not cleaning.
This guide is for BC and Vancouver Island homeowners who’ve noticed a haze, film, or fog inside their windows that won’t wipe off. Below, we explain what’s happening inside the glass, how to tell it apart from ordinary surface condensation, whether it can be repaired, and what it’s quietly costing you in comfort and energy every season.
What does condensation between your window panes actually mean?
It means the sealed glass unit has lost its airtight seal. Modern windows aren’t a single sheet of glass, they’re an insulated glass unit (IGU) made of two or three panes bonded to a spacer around the edge, with the gap filled by dry, still air or an insulating gas like argon. When the perimeter seal breaks down, humid outside air seeps in. As temperatures swing, that trapped water vapour reaches its dew point and condenses on the inner glass surface, a fog you cannot reach to wipe away.
Insulated glass unit (IGU): A factory-sealed assembly of two or three glass panes separated by a spacer and filled with dry air or insulating gas (argon or krypton). The sealed gap is what gives modern windows their insulating value. Once the seal fails, that value is gone.
How is condensation between the panes different from fog on the inside or outside of the glass?
Not all window condensation is a problem, and not all of it is fixable. There are three distinct types, and they mean very different things. The quick test: if you can wipe the moisture away, it’s on a surface; if you can’t, it’s trapped between the panes and the seal has failed.
| Where the fog is | What causes it | What it means | The fix |
|---|---|---|---|
| Inside surface (room side) | High indoor humidity meeting cold glass | Usually a ventilation or moisture issue, often not a window fault | Improve airflow, run exhaust fans, manage humidity |
| Outside surface (exterior) | Cool night air on efficient, well-insulated glass | A sign your windows are working well, harmless | Nothing needed; it clears as the day warms |
| Between the panes (sealed) | Failed IGU seal letting humid air into the gap | Permanent seal failure and lost insulation | Replace the sealed glass unit or the window |
Only the third type, moisture between the pane, is a window failure. Exterior condensation is often mistaken for a defect, but it actually confirms the glass is insulating properly: the outer pane stays cool enough to attract dew because heat isn’t escaping through it.
Why do window seals fail in the first place?
Seals fail because the edge sealant that keeps the unit airtight degrades over time. Years of daily heating and cooling make the sealed gas expand and contract, a slow process sometimes called thermal pumping, that stresses the perimeter seal. UV exposure, moisture, and age break down the butyl and secondary sealants until humid air can migrate past the warm-edge spacer and into the gap.
On Vancouver Island, damp coastal air and long shoulder seasons accelerate the process, and south- and west-facing windows tend to fail first because they absorb the most sun and the widest daily temperature swings. Homes built in the early- to mid-2000s often show failures across several units at once, since the sealed units age on a similar timeline.
For the full mechanism, how the butyl seal degrades and why the desiccant eventually saturates, see our companion guide on how insulated glass unit seals fail on Vancouver Island.
Can you fix condensation between window panes without replacing the window?
Partially, and only as a stopgap. A defogging service can clear the visible fog, but it does not restore the window’s insulation. Here’s how the process works and why it’s a cosmetic fix, not a performance one:
- A technician drills a small hole into the sealed unit, usually from the edge or side of the glass.
- A cleaning solution and air are used to remove the moisture and clear the internal fog.
- A tiny vent or valve is fitted so the unit can breathe and doesn’t re-fog immediately.
The catch is that once a unit is vented, it is no longer sealed. Any insulating gas is already gone, and the gap now exchanges air with the outdoors, so the U-factor (a measure of heat loss in which lower is better) never returns to spec. Defogging buys time and improves appearance, but replacing the sealed glass unit or the full window is the only way to recover the insulation you paid for.
As a rule of thumb, if one or two units have fogged, a sealed-unit (glass-only) replacement may be enough. If failures are showing up across multiple windows or the frames are also aging, full-window replacement is usually the better long-term value.
How does foggy glass affect your energy bills and home comfort?
A failed seal quietly erodes performance. The insulating gas that once slowed heat transfer has escaped, so the glass conducts more heat out in winter and in during summer. That shows up as higher heating and cooling costs, cold drafts and cool spots near the window, and rooms that never quite feel comfortable.
There are knock-on effects too. Persistent moisture around a failing unit can lead to frame rot, and the extra heat loss puts more load on your heating system. Left long enough, a single foggy pane is a symptom of insulation you’re no longer getting—every month.
Key takeways
- Between-pane fog you can’t wipe away is a failed seal, not a cleaning problem.
- Interior fog is usually a humidity issue; exterior fog is a sign of efficient glass.
- Defogging is cosmetic and it does not restore insulation or U-factor.
- Replacing the sealed unit or window is the only true fix, and bundling upgrades adds value.
When should Vancouver Island homeowners replace foggy windows?
Act when the fog is persistent, spreading, or showing up across multiple windows, especially on south- and west-facing walls, where seal failures usually appear first. If your home was built in the early- to mid-2000s and several units are hazing at once, it’s worth a professional assessment rather than a window-by-window wait-and-see.
Shoreline Building Performance is a CleanBC Better Homes registered contractor serving homeowners and strata corporations across Vancouver Island, in Victoria, Nanaimo, Duncan, Courtenay, Campbell River, and Port Alberni. We assess the whole building envelope, not just the glass, so a replacement actually solves the comfort problem instead of moving it. Government rebates are available for qualifying energy-efficient window upgrades, and most programs require pre-approval before installation, so it pays to start the assessment early.
Explore our window replacement and installation services, or see local options for window replacement.
Seeing fog you can’t wipe away?
Book a free window & door assessment. We’ll confirm whether it’s a failed seal, walk you through your options, and help walk you through the rebate paperwork from our end.
Frequently Asked Questions
Will condensation between window panes go away on its own?
No. Because the moisture is sealed inside a failed glass unit, it can’t evaporate out the way surface condensation does. It will persist and usually worsen as more humid air is drawn in through the broken seal. The unit needs to be replaced.
Can I just replace the glass, or do I need a whole new window?
Often you can replace only the sealed glass unit if the frame is still sound and just one or two units have failed. If the frames are aging or multiple windows are affected, full-window replacement usually delivers better long-term value and comfort. An assessment confirms which makes sense for your home.
Does exterior condensation on my windows mean they’re failing?
No, it’s usually the opposite. Condensation on the outside surface means the outer pane is staying cool because heat isn’t escaping through the glass, which is a sign your windows are insulating well. It clears on its own as the day warms.
Are foggy windows a health or mould risk?
The moisture trapped between the panes is sealed inside the glass, so it isn’t a direct indoor-air problem. The bigger concern is the persistent moisture and heat loss around a failing unit, which over time can contribute to frame rot and dampness at the window. Replacing the unit removes that risk.
Are there rebates for replacing failed windows in BC?
Government rebates are available for qualifying energy-efficient window upgrades in BC, and eligibility depends on factors like the window’s performance rating and your home. Most programs require pre-approval before installation. We help homeowners confirm eligibility and coordinate the paperwork.
