Window Defogging: Does It Actually Fix a Foggy Window?

Window defogging can make a foggy double-pane window look clear again, but it doesn't fix the window. The service removes the moisture between the panes. It does not repair the failed seal or replace the insulating gas inside the unit, and it won't restore the window's original energy efficiency.

Whether it's worth paying for depends on what you want. If you only care about the view and the frame is failing, so the window is due to be replaced soon anyway, it can buy time. If you want the window to insulate the way it was built to, you need a new sealed unit. Natural Resources Canada says a broken-seal unit is best corrected by replacing the glazing unit.

How window defogging works#

Defogging services use one of two approaches:

1. Clean and reseal. Small holes are drilled in the glass, a cleaning solution is injected, moisture and residue are removed, a drying agent is added and the holes are sealed.
2. Clean and vent. Two small holes are drilled in the outer pane and a small vent is fitted so moisture can escape over time.

Either way, the edge seal that failed in the first place is left as it was. Defogging does not fix the seal; it only clears the moisture that fogged the glass.

What defogging can't fix#

The broken seal. A sealed unit fails when the desiccant in the spacer is saturated and can't absorb any more moisture. Drying the cavity doesn't give you a new edge seal.

The lost gas. By the time a unit fogs, the argon is already gone. Defogging leaves the cavity full of ordinary air.

Permanent staining. After repeated wet-dry cycles, mineral deposits can leave residue inside the unit. Cleaning solution can't always remove it.

A damaged low-E coating. Once condensation forms inside a unit, the low-E coating can degrade. No cleaning process restores a coating.

Its status as a sealed unit. The Canadian standard for sealed units, CAN/CGSB-12.8, explicitly excludes permanently vented units. A vented window is no longer the product it was certified as.

How much insulating value are you giving up?#

It's worth being precise here, because defogging sellers and glass replacers both tend to exaggerate.

• Going from argon to air costs roughly 10 to 15% of the insulating value of a standard double-pane low-E unit.
• Seal failure leading to gas loss can raise a unit's U-factor, its rate of heat loss, by up to 32%.

So a defogged window still insulates better than single glazing, but noticeably worse than it did new. On one small window that may not matter much. Across a whole south-facing wall of glass, it adds up. Defogging does not restore the lost argon fill or the unit's full insulating value, so it is really a cosmetic fix.

Defogging vs a new sealed unit#

The frame stays either way. That's why the comparison some defogging services make is misleading. You'll see claims of 70% savings compared to window replacement. But the real alternative isn't a new window, it's a new sealed unit in your existing frame, and the price gap there is a few hundred dollars.

Tempered glass and defogging don't mix#

Tempered glass has stress locked in during manufacturing, and any attempt to cut it shatters the whole sheet. The same goes for drilling. Many sealed units in GTA homes contain tempered glass because the Ontario Building Code requires safety glass in sliding doors, storm doors, larger low panes in entrance doors and some sidelights. That covers most patio door panels and many glass entry doors. Those units generally can't be defogged at all.

To check, look in a corner of each pane for a small etched stamp, often called the "bug". If you see one, plan on a replacement unit. Our guide to tempered and laminated safety glass explains where Ontario requires it.

Check your warranty before anyone drills#

Sealed-unit manufacturers typically warrant against seal failure for 10 to 20 years, though labour may be covered for only the first 1 or 2. Drilling or defogging voids whatever warranty remains. If your windows are under about 15 years old, find the date code on the spacer bar and check the paperwork before you do anything else. A free replacement unit beats a paid defog.

When defogging might still make sense#

Defogging isn't useless. It can be a reasonable choice when:

• The frame is already rotted, warped or cracked, so the window is due for replacement within a couple of years anyway (that's a window-replacement job, not a glass repair), and you only want the view back until then.
• The unit is an unusual size or shape that would be very expensive to remake, and energy performance isn't a priority.
• The glass is not tempered, the unit is out of warranty, and you understand you're paying for appearance, not performance.

In each case, ask the company in writing what it guarantees, for how long, and whether the guarantee covers the fog returning.

First, make sure the fog is actually inside the unit#

A lot of "fog removal" searches are really about condensation on the room side of the glass, and that has nothing to do with defogging. Wipe both faces of the glass. If the moisture comes off, it's on a surface: condensation on the inside face in winter comes from indoor humidity, and dew on the outside face is a sign the glass is insulating well. Lowering humidity and running your fans fixes the first one for free. Our winter condensation guide explains how.

Only fog that stays after you wipe both sides is inside the sealed unit. Heat guns, hair dryers and tape won't reach it.

What we'd do#

We don't offer defogging, because it doesn't fix a broken seal. When a customer asks about it, we measure the unit, check for a warranty date code and a tempered stamp, and quote a replacement sealed unit for the existing frame. Where it helps, we'll suggest low-E glass and argon at the same thickness. If the unit is an odd shape or hard to reach, we'll tell you what that does to the price rather than steer you away from a cheaper fix that suits your plans.

If you're still weighing options, read foggy windows: replace the glass, keep the frame and our 2026 glass replacement cost guide.

Next step#

If you want the fog gone and the window working as designed, replace the sealed unit, not the window. See how our foggy window repair works, or contact us with a photo of the window for a free quote.