Why You Get Condensation on Windows in Winter

Condensation on windows in winter happens because warm, humid indoor air touches cold glass and can no longer hold its moisture, so the water comes out of the air and onto the pane. It is not a leak, and it has nothing to do with how clean the glass is. It is the temperature of the glass meeting the humidity of the room, and in a Minnesota house it shows up every year on the first truly cold night.

Why it starts with the first cold snap

Around the Twin Cities that is usually late October or the first hard night in November. Nothing changed inside the house. The glass just got cold enough that the air sitting against it dropped below its dew point.

The water almost always shows up along the bottom of the sash and in the bottom corners first. Cold air settles low in a room, and the edge of a sealed glass unit is colder than the middle because of the metal spacer that holds the two panes apart. Those are the coldest square inches in the window, so they fog before anything else.

Which side the water is on tells you what is wrong

On the inside surface. The indoor air is too humid for how cold the glass is. This is the common one, and it is the one you can actually do something about.

Between the two panes. The seal on that unit has failed and the space between the panes is taking on moisture. It looks like haze that comes and goes with the weather, and no cleaning on either side reaches it. That is a glass replacement, and anyone who tells you they can clean it is selling you something.

On the outside surface in the morning. On newer windows this is dew on a cold outer pane, and it is a sign the window is doing its job and keeping the heat inside. It burns off by mid morning.

Older houses with aluminum storm windows get a fourth version: fog between the storm panel and the window behind it. If you are taking the panels down or putting them up this month, that is the moment to get at that glass, because once they are closed for the season you are not reaching it until April. More on that in how to clean storm windows before winter.

Where the moisture is coming from

All of it is made inside the house. Showers, cooking, the dishwasher, laundry, houseplants, firewood stacked in the basement, people just breathing. A whole house humidifier still set where it was in October will do it on its own. A damp basement feeds the whole place.

Older St. Paul houses often leak enough air through the walls and around the windows to dry themselves out. Tighter houses and newer builds hold that moisture in, which is why a well built house can have worse window sweating than a drafty one.

What actually cuts it down

• Run the bathroom fan during every shower and for twenty minutes after, and make sure it vents outside and not into the attic
• Use the range hood when you cook, especially anything boiling
• Turn the humidifier down as it gets colder. Something in the 35 to 40 percent range in fall, closer to 25 or 30 once it is near zero outside
• Let room air move across the glass. Open the blinds during the day and pull furniture back off the windows. Heavy drapes closed around the clock make a cold pocket against the pane
• Wipe the sills when they are wet. Standing water is what actually does damage

When it stops being normal and becomes a problem

A little fog on a cold morning is just physics. What is worth paying attention to is frost on the inside of the glass in January, water that pools and sits on the sill for days, paint lifting or peeling off the bottom of the sash, black spotting where the sash meets the stop, or wood that feels soft when you press it.

One room doing it points at that room, usually a bathroom, a kitchen, or a bedroom with the door shut all night. Every window in the house doing it points at the humidity level for the whole place.

If water has been sitting in the track all winter, that is also where the grit and the old dirt turn into mud. Pulling that out dry is a different job than wiping a wet sill, and it is covered in how to clean window tracks and sills.

Cleaning the glass will not fix this

Clean glass fogs exactly the same as dirty glass, so nobody should be hiring a window cleaner to stop their windows from sweating. That is a humidity and ventilation job, and most of it is free.

What a cleaning is good for this time of year is the inside of the house. Exterior work winds down once the water stops cooperating, but interior glass, frames, and sills are fine in January, and a sill with no dust and no old grime on it gives mildew much less to grow on through the winter. Window cleaning starts at $99. We work in St. Paul, Minneapolis, and the suburbs around them. You can request a quote or book a free 10-minute consultation.