Before you start troubleshooting, the single most useful thing you can do is identify where the condensation is forming. Each location points to a different cause, and mixing them up can lead you down the wrong path entirely.
Interior Surface Condensation
This is the most common type, and the one most Canadian homeowners notice first. You'll see moisture on the room-facing side of the glass — usually starting at the bottom or edges, sometimes as a light fog, sometimes as visible droplets running down the pane. It tends to be worse in bedrooms overnight, in kitchens during cooking, and in bathrooms after showers. You can wipe it away with your hand or a cloth, and it comes right back when conditions are the same.
What it means: the air inside your home is carrying more moisture than the glass surface can handle at its current temperature. The window itself is usually functioning fine — it's simply the coldest spot in the room, and that's where the moisture lands first.
Exterior Surface Condensation
This one surprises people. You look outside and see dew or frost on the outer surface of your windows, often on calm, clear mornings. It can look alarming, but it's actually a sign that your windows are doing their job well. Modern windows with low-emissivity (Low-E) coatings are so effective at keeping heat inside that the outer pane stays cold enough for outdoor moisture to condense on it, just like dew forming on grass.
High-performance "superwindows" with three or more layers are especially prone to exterior dew, according to the Efficient Windows Collaborative, precisely because they insulate so well.
What it means: your windows are keeping heat where it belongs — inside your home. No action needed. The moisture typically evaporates within a few hours once the sun warms the glass.
Between-the-Panes Condensation
This is the type that does indicate a problem with the window itself. If you notice a hazy, foggy, or milky appearance between the layers of glass — moisture that you cannot wipe away from either side — the hermetic seal around the insulated glass unit (IGU) has failed. The inert gas fill (usually argon) that was sealed between the panes to improve insulation has leaked out, and moist air has made its way in.
As the National Research Council's consumer guide to windows explains, these gas fills are retained by edge seals whose quality directly affects long-term performance. Once that seal breaks, the window's insulation value drops significantly — in some cases performing closer to a single pane. It won't improve on its own, and no amount of humidity control inside your home will clear the fog.
