A double-pane window that has gone cloudy between the glass is not dirty, and no amount of wiping will clear it. That haze is moisture trapped inside a sealed unit whose seal has failed, and it tells you the unit has lost the insulating gas it was built with. The good news is that a fogged window does not always mean replacing the whole window. Here is how a sealed unit works, what the fog actually is, and how to decide between fixing the glass, defogging it, or replacing the window, without the guesswork.
How a sealed double-pane unit works
A double-pane window is an insulated glass unit, or IGU: two panes held apart by a spacer around the edge, with the gap sealed and usually filled with argon, an inert gas that insulates better than air. The spacer holds a desiccant that soaks up the trace of moisture sealed inside, and a Low-E coating on one surface reflects heat back where you want it. The edge seal is the whole game. As long as it holds, the gas stays in, moisture stays out, and the unit insulates as designed. Sealed units do not last forever, though, and most begin to fail somewhere in the range of 10 to 20 years, sooner in harsh conditions and longer with a good warm-edge spacer.
What the fog actually is, and what it is not
It helps to know which of three things you are looking at, because only one is a seal failure:
Between the panes: haze, streaks or droplets you cannot wipe off because they are sealed inside the unit. This is seal failure, and it is the one this article is about.
On the inside surface: condensation on the room-facing glass, which usually means high indoor humidity rather than a broken window, and is eased with ventilation.
On the outside surface: dew on the exterior glass on a cool morning, which is harmless and is often a sign the window is insulating well, since the outer pane is staying cold.
Only the first one, moisture between the panes, points to a failed seal and a unit that has lost performance.
Why seals fail
Seals give out for a few reasons, and Alberta’s climate pushes most of them. The glass heats in the sun and contracts in the cold, and that daily expansion and contraction works the edge seal and spacer over years until they let humid air in. Once that happens, the desiccant fills up with moisture and can hold no more, so water condenses on the cold inner surface as fog. A knock during installation, a poor original seal, or a factory defect all speed this up, and the argon fill slowly diffuses out over time even on a sound unit. When you see fog, the gas is already largely gone and the unit is no longer insulating the way it should.
Repair or replace: the three options
A fogged window gives you three real paths, not one:
Replace the sealed unit only. The glass unit can often be swapped while the existing frame and sash stay in place. When the frame is sound, this is usually the sensible fix, because it restores both the clarity and the insulating performance for far less than a whole new window.
Defog the unit. A technician drills small holes, draws the moisture out under vacuum and reseals. This can clear the haze, but it is worth being honest about its limit: it does not put back the lost argon fill or fully restore the unit’s original insulating value, so it is closer to a cosmetic fix than a full repair.
Replace the whole window. When the frame or sash is rotted or warped, the hardware has failed, or several units have fogged around the same time, replacing the window is usually the better long-term choice.
The deciding factors are the age of the units, whether one or many have failed, the condition of the frame, and what your comfort and heating bills are telling you. One failed unit in a good frame is a glass job; fog showing up across several windows at once points to a window or installation problem worth replacing.
Choosing replacement glass or windows
Whether you replace the glass unit or the whole window, the features that matter are the same: a Low-E coating, an argon fill, and a warm-edge spacer that resists the thermal cycling that kills seals. For a full window, look for the ENERGY STAR label, administered in Canada by Natural Resources Canada, and check the ratings, a lower U-factor means better insulation and a higher Energy Rating (ER) means better overall performance for our climate. In a very cold or noisy spot, a triple-pane unit adds insulation, and a laminated pane adds sound control. Accurate measurement of the opening and a certified install matter as much as the glass, since a unit fitted into a poor opening fails early.
Preventing fog and getting more life from a seal
You cannot stop a seal from ageing, but you can slow it. Keep the weep holes at the bottom of the frame clear so water drains instead of pooling against the seal, keep the exterior caulking sound and refresh it when it cracks, and manage indoor humidity with ventilation so condensation does not sit on the glass through the winter. Check the frames and seals each season for cracked sealant or a spacer pulling away at the corners, and deal with frame rot or loose hardware early, before it lets moisture work into the unit. None of this reverses a failed seal, but it gives a sound one its full life.
FAQs
Ready to get help with your glass issue?
Noticed fog between the panes or a window that feels cold at the glass? Request a free quote, and the team will check whether the sealed unit can be replaced in your existing frame or whether a full window makes more sense, and recommend the right glass for an Alberta winter. Call 310-GLAS (4527), text 780-652-1296, or book online.