The short answer
Condensation on windows in winter means the air in your house is holding more moisture than the cold glass can handle. When warm, humid room air touches a pane that is below its dew point, the vapor turns back into water, and below freezing it turns into frost. Bringing indoor humidity down to match the outdoor temperature stops most of it. Fog trapped between the two panes is a different problem. That is a failed seal, and no amount of wiping or dehumidifying will clear it.
What Causes Condensation on Windows
Air always carries some water as invisible vapor, and warm air can carry far more of it than cold air. Every surface in your house has a temperature at which the air touching it can no longer hold its moisture. That temperature is the dew point. When a surface drops below it, the vapor condenses into liquid right on that surface.
In a Wisconsin winter the coldest surface in almost any room is the window glass. The wall around it has several inches of insulation. The glass has a pane or two and a thin layer of gas. On a January night in Dane County, with the temperature outside in the single digits or below zero, the inside face of an older window can fall well under the dew point of ordinary room air. The water you find in the morning did not come through the window. It came out of the air in your house.
That is also why condensation starts at the bottom of the glass. Cold air slides down the pane and pools at the base, and the edge of the glass, where the panes are held apart, is usually the coldest strip of all.
Inside, Between or Outside? Where the Water Sits Tells You Everything
Before you change anything, look at which surface the moisture is on. The three cases have completely different causes.
| Where you see it | What it means | What to do |
|---|---|---|
| On the inside of the glass | Indoor humidity is too high for how cold the glass is. By far the most common case. | Lower the humidity and get air moving across the glass. The steps are below. |
| Between the two panes | The seal around the insulated glass has failed and moist air has gotten inside the unit. | It cannot be wiped or dried out. The glass unit or the window needs replacing. |
| On the outside of the glass | The glass is keeping your heat in so well that the outer pane stays as cold as the morning air. | Nothing. It burns off as the sun comes up, and it is common on new, efficient windows. |
Most of this guide is about the first case, because it is the one you can usually fix yourself. We come back to failed seals, and to the point where the window is the real problem, further down.
How Much Humidity Is Too Much in a Wisconsin Winter
The comfortable indoor range most people aim for is 30 to 50 percent relative humidity. In deep cold that number has to come down, because the glass gets colder as the outdoor temperature drops. The Center for Energy and Environment in Minneapolis publishes a simple guide for Upper Midwest homes, and it works as a daily rule of thumb here too.
| Outdoor temperature | Keep indoor humidity below |
|---|---|
| 20°F to 40°F | 40% |
| 10°F to 20°F | 35% |
| 0°F to 10°F | 30% |
| -10°F to 0°F | 25% |
| -20°F to -10°F | 20% |
| Below -20°F | 15% |
Source, Center for Energy and Environment, Managing Humidity Levels.
To know where you stand, get an inexpensive digital hygrometer and set it in the room with the worst windows, away from the kitchen and bathroom. Check it on a cold morning. If you have a whole-house humidifier on the furnace, its setting is the first thing to look at. Many are left where they were set in November and never turned down when January arrives.
Madison’s record low is 37 below zero, set on January 30, 1951, according to the Wisconsin State Climatology Office. You will not see that often, but a run of below-zero nights is a normal Wisconsin winter, and on those nights the chart says your house should be quite dry.
Where the Extra Moisture Comes From
A house makes its own humidity every day. The usual sources are simply ordinary living.
- Showers and baths, especially with a bath fan that is missing, undersized, or switched off the moment you leave the room.
- Cooking, above all boiling pots under a range hood that recirculates instead of venting outside.
- Laundry, from clothes dried on racks indoors, or a dryer vent that has come loose and is venting into the basement.
- A furnace humidifier set higher than the outdoor temperature allows.
- Houseplants, aquariums and firewood, which all release water. A stack of wood stored indoors releases the most.
- A damp basement, which feeds moisture up into the whole house.
- A tighter house, after new insulation, new siding or new windows, because less humid air leaks out and less dry winter air leaks in.
That last one surprises people, so it is worth being plain about it. An old, drafty house condenses less than you would expect because it is constantly swapping humid inside air for dry outside air. Seal the house up and the moisture stays inside unless something moves it out.
How to Stop Condensation on Windows Overnight
Work down this list in order. The first two steps prevent condensation on windows in most Dane County homes.
- Turn the humidifier down.Match it to the chart above and check again in a day or two. This alone fixes most cases.
- Run the bath fan longer.Leave it on for 20 minutes after every shower, or put it on a timer switch so nobody has to remember.
- Vent the cooking.Use a range hood that exhausts outside, and turn it on before the pot boils rather than after the windows fog.
- Open the blinds and curtains.Heavy coverings trap a pocket of cold, still air against the glass. Opening them during the day, or leaving a few inches at the bottom, lets room air warm the pane.
- Keep the air moving.A ceiling fan on low, clear heat registers under the windows, and furniture pulled a few inches off the wall all help warm air reach the glass.
- Find the hidden sources.Move houseplants away from the windows, keep firewood outside, and make sure the dryer vent actually leaves the house.
- Air the house out briefly.On a milder afternoon, a few minutes with two windows cracked on opposite sides of the house swaps humid air for dry outdoor air faster than any appliance.
If the house is new or was recently tightened up and condensation keeps coming back, ask an HVAC contractor about mechanical ventilation, usually an HRV or ERV. These bring in fresh outdoor air without throwing away the heat.
When Condensation Means the Window Is the Problem
If you have brought the humidity down to the chart and the same windows still run with water or frost every cold morning while others in the house stay clear, the window itself is usually the reason. These are the signs we look for.
- Single-pane glass, or a single pane behind a storm window, common in Madison homes built before the 1960s. The inside pane simply gets too cold.
- Aluminum frames, which conduct cold straight into the room. Frost that forms on the frame before the glass is the giveaway.
- Frost in a band around the edge of the glass, a sign of a metal spacer between the panes carrying cold to the edge.
- Fog, haze or droplets between the panes, which means the seal has failed and the insulating gas is gone.
- Soft, dark or stained wood at the sill, where years of water running down the glass have soaked in. Left alone, rot spreads into the frame and sometimes into the wall.
- Mold on the sill or in the corners, which needs cleaning right away and tells you the surface has been wet for a long time.
Rot is the one to take seriously. When we replace a window, rotten sills and exterior trim are replaced at no charge. If the damage has reached the wall framing, we show you before doing anything else.
What a Better Window Changes
New windows do not change your humidity. What they change is how cold the inside of the glass gets, and that moves the point at which condensation starts.
- A Low-E coating reflects heat back into the room, so the inside pane stays warmer.
- Argon gas between the panes slows heat loss through the glass.
- A warm-edge spacer replaces the metal strip at the edge of the glass, which is exactly where frost forms first on older windows.
- An insulated frame keeps the frame from turning into a cold bridge into the room.
Every window we install is a ProVia window with Low-E, argon-filled glass, a warm-edge spacer and a foam-filled frame, rated for the ENERGY STAR Northern climate zone. You can read more about the glass package and the six window styles. If you are comparing brands, look for the optional Condensation Resistance rating on the NFRC label. It runs from 1 to 100, and a higher number means the window resists condensation better.
One honest caveat. New windows seal much tighter than the ones they replace, so the house holds on to more of its moisture. Homeowners sometimes see a little condensation on brand-new windows the first winter and assume something is wrong. Usually it means the humidity needs to come down a notch now that the drafts are gone, and the chart above still applies.
Condensation on the outside of new windows on a cool fall morning is the opposite of a problem. It means the glass is holding your heat in so well that the outer pane is as cold as the air.
Frost and Ice on the Inside of the Glass
Frost is condensation that happens below freezing. When the inside of the glass drops under 32 degrees, the moisture skips the water stage and forms ice crystals, usually thickest in the bottom corners. When the sun hits it or the day warms up, it melts all at once and runs onto the sill.
Frost is the same problem at a higher level of urgency. If you see it regularly, bring the humidity down further on the coldest nights and check those windows for the signs above. Thick ice on the inside of a window in a heated room is rarely something humidity control alone will fully solve.
Common questions
Window Condensation Questions We Hear Most
Is condensation on windows bad?
A little on the coldest mornings is normal in a Wisconsin winter. Water that runs down the glass every day, pools on the sill or leaves stains is a problem, because it leads to peeling paint, mold and eventually rot in the sill and frame.
Why is there condensation on my windows in the morning?
Overnight is when the outside temperature is lowest and the glass is coldest. Many houses also turn the heat down at night, which cools the air next to the glass further, and morning showers and cooking add moisture at the same time.
Can condensation between window panes be fixed?
Not by wiping or drying. Fog between the panes means the seal around the insulated glass has failed. The fix is replacing the glass unit or the whole window, and on older or builder-grade windows a full replacement is usually the better long-term choice.
Should I run a dehumidifier in winter?
Start by turning down the humidifier and venting bathrooms and the kitchen properly. A dehumidifier helps most in a damp basement, which often feeds moisture to the rest of the house.
Do new windows stop condensation?
They raise the temperature of the inside glass, so condensation starts at a much higher humidity than it does on old single-pane or metal-spacer windows. They do not remove moisture from the air, so humidity control still matters, especially the first winter after replacement.
What indoor humidity should I keep in a Madison winter?
Around 35 to 40 percent on mild winter days, coming down as it gets colder, to about 30 percent in the single digits and 25 percent below zero. Watch your worst window. If it starts to fog, the humidity is too high for that night.
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