Why your windows sweat in winter.

Water on the glass is a temperature problem plus a humidity problem

Warm air holds more moisture than cold air. When the warm, damp air inside your house touches a cold surface, it cools, it cannot hold what it was carrying, and the moisture comes out as water on that surface. The window is simply the coldest surface in the room, so it is where you see it first.

That means there are only two things to work with. How cold the glass is, and how much moisture is in your air. Either one can be the culprit, and which one it is changes the answer completely.

Which side of the glass is it on

This is the whole diagnosis, and you can do it in ten seconds.

• Inside surface, where you can wipe it off with your hand. This is condensation, and it is about your indoor humidity and the temperature of the glass. Keep reading.
• Between the two panes, where you cannot reach it. This is a failed seal on the insulated glass unit. The window is done as insulation and no amount of humidity control will change it. That is a different question, and the FAQ answers it.
• Outside surface, in the morning. That is dew on a cold exterior pane. It is normal and it often means the glass is doing its job.

If it is on the inside, start with the air

Winter air in Utah County is dry, so people run humidifiers, and then they run them too hard. A house that is comfortable through the winter is usually somewhere around 30 to 40 percent relative humidity, lower when it is very cold outside. A cheap hygrometer will tell you where you actually are, and it costs almost nothing.

Then look for the moisture you are not accounting for. Showers without the exhaust fan running, and running for a while after. A gas range with no hood used. Laundry drying indoors. Firewood stacked inside. A crawlspace with no vapor barrier, quietly feeding the whole house. Houseplants in quantity.

And check that the air is actually moving across the glass. Long curtains closed tight, or furniture pushed against the window, keep room air off the glass and let that spot run colder than the rest of the room. That is why one window in the house sweats and the others do not.

If your windows are only a few years old and the glass is sweating, the humidity is where the answer is. Start there before anyone measures your house for a quote.

When the window really is the cause

There is a real exception, and it is common in the older housing stock around here.

Single-pane glass, or a window with an aluminum frame and no thermal break, runs genuinely cold. The metal conducts the outdoor temperature straight through to the inside surface. On those windows you can hold the humidity at a sensible level and still get water, because the surface is cold enough to pull moisture out of ordinary indoor air. Aluminum frames sweat at the frame itself, not just the glass, which is a useful tell.

That water is not just an annoyance. It runs down onto the sill and sits there. Over enough winters it stains paint, swells the wood, and rots the sill and the trim from the bottom up. If you are already seeing soft wood or peeling paint under a window, the condensation has been working on it for a while.

Replacing that kind of window does fix the problem, because a warm edge and an insulated frame keep the interior surface above the temperature where water forms. That is the case where new windows are the answer rather than a humidifier setting.

What we would tell you

If the glass is wet on the inside and the window is modern, buy a hygrometer, not windows.

If the frame is aluminum or the glass is single-pane, and the sill is showing water damage, then it is worth pricing. Utah's utility rebate programs can bring the cost of qualifying windows down, with conditions, and the rebates page has the current figures and what has to be true to get them. A licensed Utah dealer installs every job.

Either way we will tell you which one it is. Plenty of the windows we look at are fine, and when that is the answer, we say so and leave.