First: is the fog inside the glass, or on the surface?
These are two different problems and people mix them up constantly, so let's settle it in ten seconds. Run a finger across the haze. If you can wipe it off — from the inside or the outside — it's surface condensation, and it's about humidity in the room or weather on the exterior, not a broken window. If you wipe both sides and the cloudiness is still there, trapped in the middle, the seal is gone. That's a failed IGU, and that's what this page is about.
A quick tell on a Calgary morning: surface condensation shows up at the bottom edge and corners of the glass and changes through the day as the room warms. A failed seal looks like a permanent milky film, sometimes with visible droplets or a tide-line of mineral residue, and it doesn't come and go — it just slowly gets worse.
What's actually happening inside a sealed unit
A modern window isn't one piece of glass. It's an insulated glazing unit: two (or, in European triple-pane, three) panes bonded around a spacer bar at the perimeter, sealed with a primary and secondary sealant, with the cavity filled with argon gas and kept bone-dry by a desiccant hidden inside the spacer. That dry, gas-filled gap is what does the insulating. It's also what keeps the inner glass surfaces clear.
When the seal fails, three things happen in sequence: the argon leaks out (so the unit insulates worse), humid outside air leaks in, and the desiccant saturates until it can't absorb any more. After that, every time the glass gets cold, the moisture trapped in the cavity condenses on the inner surfaces — exactly where no cloth can reach. The milky film is mineral residue left behind as that moisture evaporates and re-condenses, day after day.
Why Calgary is hard on sealed units
Sealed units fail everywhere eventually — UV and ordinary heat fatigue the sealant over 10-20 years. But Calgary stacks two extra loads on top.
The first is the chinook swing. A 25-30°C jump in a single afternoon expands and contracts the glass, the spacer, and the sealant at different rates. Glass and aluminium and polymer don't move the same amount, so every chinook works the seal like bending a paperclip back and forth. Do that a few hundred times over a decade and a marginal seal cracks.
The second is exposure. South and west elevations take the most sun-driven heat cycling and the most UV. On nearly every Calgary house I look at, the first units to fog are on the west or south wall — a picture window over a sink, a big living-room unit facing the afternoon sun. North-facing units on the same house are often still clear. If you're trying to guess which of your windows will go next, follow the sun.
Can a foggy window be "defogged"? The honest answer
Yes, defogging services exist. They drill a small hole in the glass, flush the cavity to clear the haze, and fit a tiny vent or valve. The cloudiness goes away and the glass looks clear again. For a few hundred dollars a unit, it can be worth it on a window you only need to look good for a year or two.
But be clear about what you're buying. Once the unit is drilled and vented, it is no longer a sealed insulated unit. The argon is gone for good, the gap is now open to outside air, and the insulating value drops toward a single-glazed window. Many defogged units fog again seasonally once humidity cycles back through the vent. It's a cosmetic patch, not a repair of the insulating function. I'll tell a homeowner that to my own cost, because the alternative — quietly selling a "fix" that doesn't restore performance — isn't how I work.
Honest call
One fogged unit, a sound frame, and you're selling the house inside two years? Replace that single glass unit, or even defog it, and move on. Don't let anyone — including me — talk you into a full-house project the situation doesn't call for.
Replace the glass, or the whole window?
This is the real decision, and it's not the same answer for every house. Here's how I sort it on site.
| Situation | Usually the right move |
|---|---|
| Frame solid, hardware fine, one or two fogged units | Replace the sealed glass unit only — cheaper, keeps the existing frame |
| Frame drafty/racked, sashes don't seal, multiple fogged units | Replace the window — new glass in a failing frame still leaks air |
| House 12+ years old, units failing one after another | Plan a phased replacement by elevation — start with the sun-loaded walls |
| Upgrading for comfort/energy, not just the fog | Whole-window replacement so you get the frame, seal, and glass as a system |
The trap to avoid: dropping a brand-new sealed unit into a frame that's already bowed or out of square. The new glass will be clear for a while, but the same frame movement that helped kill the first seal goes to work on the new one. If the frame is the problem, glass-only is a short-term spend.
If one has fogged, look at the rest
Sealed units installed together age together. When one fogs on a 15-year-old Calgary home, the others on the same sun-loaded walls are usually not far behind. That's not a reason to panic-replace everything — it's a reason to know where you stand. Walk the house, note which units are clear, which are starting to haze at the edges, and which have gone fully milky. That map tells you whether this is a one-unit fix or the first sign of an envelope that's reaching the end of its service life.
So what does this mean for your project?
A foggy pane is a small, specific symptom with a big range of correct answers — from a few-hundred-dollar glass swap to a phased whole-house plan. The right call depends on the frame, the age, the exposure, and how long you're staying. None of that can be guessed from a photo.
That's what the on-site visit is for. I come out, measure the openings for a free written quote, and while I'm there I read the actual condition — which seals have gone, whether the frames are still sound, which walls are failing first. If the honest answer is "replace two units and keep the rest for now," that's what I'll tell you. If it's "this envelope is done and you'll feel it every winter until it's handled," I'll tell you that too.