What is actually happening on your glass
Warm air carries water you cannot see. Cool it down far enough and it can no longer hold what it's carrying. The water has to go somewhere, so it lands on the coldest thing nearby. In your house on a January night, that's the glass.
Your house is generating that moisture constantly. Showers. Cooking. The dishwasher. Laundry drying on a rack. Houseplants. Every person breathing in the building. In summer it leaves through open windows and you never notice. In February the house is shut tight, the moisture has nowhere to go, and the glass becomes the place where it shows up.
So the first useful thing I can tell you: fog on the glass is a message about your air. It is not automatically a verdict on your windows.
This is the part that costs me money to write, and I'd rather write it. A lot of people are sold new windows for a problem that a bathroom fan would have fixed.
Read the location, get the diagnosis
Before anyone quotes you anything, go and look at which part of the window is wet. It takes thirty seconds and it decides everything that follows.
| Where the water is | What it means | What to do |
|---|---|---|
| Outside face of the glass, clear mornings | Your window is insulating well. So little heat reaches the outer pane that dew forms on it, the same dew that forms on a car roof. | Nothing. It burns off with the sun. |
| Inside, centre of the glass | The air in the room is carrying more moisture than that glass temperature can tolerate. | Work on the air first. Fans, humidifier setting, a hygrometer. |
| Inside, edges, corners and the frame | The edge of the assembly is running colder than the middle. That is about the spacer, the frame and how the window sits in the wall. | Worth a proper look. This one is a window and installation question. |
| Sealed between the panes | The glass unit has failed. The space between the panes is no longer sealed. | Glass unit replacement. Often without replacing the whole window. |
Notice that only one of those four rows is a defect in the glass, and one of them is your window telling you it works.
The outside one, because it worries people
Every spring somebody phones convinced their new windows have failed, because the outside of the glass is soaked at seven in the morning.
That is a good sign. Dew lands on the outer pane precisely because your heat is not leaking out to warm it. On an older window enough heat escapes to keep that surface above dew point, so it stays dry and you never see it. The dry window was the leaky one.
I will not even go into how strange it feels to reassure someone that the water on their window is a compliment.
The edges, which is the row that matters
Centre-of-glass fog during a hard cold snap is common. Water and frost that concentrate at the perimeter, in the corners and along the frame are a different signal. That's the one I pay attention to.
The edge of a sealed glass unit is the hardest place in a window to keep warm. The spacer that holds the two panes apart runs around that perimeter, the frame surrounds it, and heat takes the shortest path it can find. If those parts conduct well, the edge sits colder than the middle, and the moisture picks the edge every time.
Two things live in that row. What the window is made of, and how it was put into the wall. A gap that was never properly filled and sealed leaves cold air sitting against the frame, and no glass upgrade fixes that. It is the same point I keep making about installation, and here it shows up as water you can wipe with your finger.
The part worth sitting with
Persistent water at the bottom of a window is not just cosmetic. It sits on the sill, it soaks the paint, and given a winter or two it feeds mould in the very corner nobody moves furniture away from. Wipe it if you like, but treat it as a question rather than a chore.
Do this before you spend anything
Buy a hygrometer. They cost almost nothing and they turn an argument into a number you can watch.
Then give the moisture a way out. Run the bathroom fan for the whole shower and leave it going afterwards. Use the range hood every time you cook, not just when something burns. Dry laundry somewhere ventilated. Turn the humidifier down when the forecast drops, because the setting that felt right in November is usually too high when it hits minus thirty.
One more that surprises people: open the blinds. Heavy curtains trap a pocket of cold, still air against the glass. That makes the condensation worse on the very windows you're worried about. Let room air wash over them.
Do that for a couple of weeks in cold weather and watch what happens. If the glass dries up, you just solved it for the price of a hygrometer. You will be very pleasantly surprised.
When it really is the window
If the humidity is under control and the glass still runs wet every cold night, the surface is too cold, and now we are talking about the window itself.
Same conclusion if the pattern is stuck at the edges and the frame while the middle of the glass stays clear. And same again if you find soft or blackened sills, or paint peeling under the window. That is water that has been sitting there far longer than this winter.
What actually changes that surface temperature is the whole assembly, not one headline number on a sticker. The frame and its reinforcement, the spacer at the edge of the glass, how tightly the sash pulls in against its seal, and the quality of the seal between the window and the wall. Get those right and the glass stays warm enough that ordinary indoor air never reaches its dew point in the first place.
And if you want to know how a specific window handles that edge, ask for the test documents rather than the adjectives. Whoever has good ones will send them.
So what should you do about it?
Go and look at the window tonight, and note exactly where the water sits. That single observation puts you in one of four rows, and three of them do not involve buying anything.
If it turns out to be the air, fix the air. If it turns out to be a failed glass unit, ask about replacing the glass instead of the window. And if it's the edges, that is worth a proper conversation before another winter goes by.
I would rather tell you to run your bathroom fan than sell you eight windows you didn't need.