Foggy windows in an Edmonton winter: what condensation is telling you
Every Edmonton winter brings the same round of questions to glass shops: why are my windows sweating, why is frost building up in the corners, and why does one window look permanently hazy no matter how often I clean it. Condensation is not one problem with one answer. It is a symptom, and where the moisture sits on the glass tells you a great deal about what is actually going on.
Condensation forms in only three places on a window: the interior surface facing the room, the exterior surface facing outside, or sealed between the panes. Each points to something different, and only one means the glass itself has failed.
Condensation on the inside of the glass
This is the most common complaint in January, and it is usually a humidity and ventilation issue rather than a window defect. Glass is the coldest surface in most rooms. When indoor air holds more moisture than that cold surface can tolerate, the moisture condenses onto the glass, the way it beads on a cold drink in summer.
In a climate where outdoor temperatures sit well below freezing for months, the dew point at the glass surface drops with them. Indoor humidity that feels comfortable in October can start streaming down the windows in January without anything about the window having changed. Common contributors include:
- Showers, cooking, and laundry without exhaust fans running long enough
- Humidifiers left at a setting suited to milder weather
- A tightly sealed newer home with limited air exchange, or an HRV that is not running or is overdue for service
- Heavy curtains, deep blinds, or furniture blocking warm air from washing over the glass
The response is to lower indoor humidity as the outdoor temperature falls, improve air movement across the glass, and ventilate more consistently. Interior condensation is worth taking seriously even when the window is sound, because sustained moisture damages sills, paint, and drywall, and can lead to mould.
Condensation or frost on the outside
Occasionally there is dew or light frost on the exterior face of the glass, usually on a clear, calm morning. This is not a defect. It happens when the outer pane is cold enough relative to the outdoor air for moisture to settle on it, and it generally clears as the day warms or the wind picks up. If anything, it suggests the outer pane is not being warmed much by heat escaping from inside.
Fog between the panes
This is the one that means the glass unit has failed. An insulated glass unit, often called a sealed unit, is two or more panes separated by a spacer around the perimeter and bonded with a sealant that keeps the cavity closed. The spacer typically holds a desiccant that absorbs the small amount of moisture present when the unit is made.
Over time, thermal cycling, ultraviolet exposure, water sitting in the frame, and ordinary aging can break down that perimeter seal. Once it is compromised, humid air moves in and out of the cavity and the desiccant eventually becomes saturated. From that point the unit can no longer keep itself dry. The signs are recognisable:
- Haze or fog between the panes that cleaning does not touch, because it is on an inner surface
- Fogging that comes and goes with sun and temperature, often worst on a bright cold morning
- Streaks, drip marks, or a chalky mineral film etched onto the inner glass over time
- A visible change in how the glass reflects light compared with the window beside it
A failed seal will not repair itself, and the fogging tends to worsen. Beyond the appearance, the insulating cavity no longer performs as designed, and long-standing moisture can permanently etch the glass.
Replacing the glass rather than the whole window
The useful thing to know is that a failed sealed unit is often a glass problem, not a window problem. In many cases the frame, sash, hardware, and exterior finish are sound, and only the insulated glass unit needs to come out and be replaced. That is a smaller job than tearing out and reinstalling the entire window, and it leaves the rest of the assembly untouched.
Whether that applies to your window depends on the details: the type and condition of the frame, how the glass is retained, the size of the unit, and whether there is rot or hardware damage that needs attention regardless. Some frames make glass-only replacement straightforward. Others do not. That judgement is difficult to make from a photo or a description over the phone, which is why it is worth having someone look at the window in person and measure it.
Before that visit, it helps to have noted a few things:
- Which windows are affected, and where the moisture sits.
- Whether the fogging clears on its own or stays put after cleaning.
- Whether the sill or frame shows staining, soft wood, or peeling paint.
Glass Culture supplies sealed glass units in Edmonton, Alberta, with on-site measurement, custom fabrication, delivery, and installation. If a window in your home has gone hazy between the panes, or you are not sure which kind of condensation you are dealing with, you are welcome to request a free quote and have it assessed before the coldest part of winter arrives.