Window condensation in winter: why Iowa windows sweat and how to stop it
Water running down the glass on a cold morning. Fog that won’t wipe off. Frost on the inside of the bedroom window. Window condensation in winter is rarely a window problem. The glass is the coldest surface in the room, so it is the first place your house shows you it is holding too much moisture for the weather. Here is how to tell which kind you have, where the moisture comes from, and the order to fix it in.
Where is the water?
- Room side
- Indoor humidity is too high for how cold it is outside. This is the common one, and the one this guide is about.
- Between panes
- The seal on that glass unit has failed. Humidity didn’t cause it and won’t fix it.
- Outside
- Dew on efficient glass on a cool, humid morning. Harmless.
- Fix first
- Measure indoor humidity, then lower it to match the weather. That comes before new windows.
Three kinds of foggy window, three different causes
Before you change anything, find out which side of the glass the water is on. Wipe it with a dry finger from inside the room. If it wipes off, it is on the room side. If you can’t reach it from either side, it is sealed between the panes.
Condensation or frost you can wipe off
Moisture in your indoor air is turning to water on cold glass. It is worst on the coldest mornings, usually starts along the bottom edge, and can freeze into frost or ice.
Fog or streaks you can’t reach
A double-pane window is two sheets of glass sealed around the edge. When that seal fails, outside air and moisture get into the space between them and leave a haze that comes and goes with the weather.
Dew on the outdoor side
On a cool, humid morning, the outside of well-insulated glass can be cooler than the outdoor air and collect dew, the same as your car or the lawn. It clears as the day warms.
The rest of this guide is about the first kind: water or frost on the room side of the glass in winter.
Why the inside of the glass sweats
Air holds water vapor, and warm air can hold much more of it than cold air. When indoor air touches a surface that is cold enough, the air right at that surface can’t hold its moisture anymore and leaves it behind as water. The temperature where that happens is called the dew point. A cold drink sweating on a summer day is the same thing.
In winter the glass in your windows is usually the coldest surface in the room, with the bottom edge and the corners coldest of all. So that is where the water appears first. The colder it gets outside, the colder the glass gets, and the less humidity the house can carry before the windows start to sweat.
That is why the right indoor humidity is not one number. It goes down as the weather gets colder. Match it to last night’s low:
| Overnight low | Keep indoor humidity at or below |
|---|---|
| 40°F and warmer | 60% |
| 20 to 40°F | 40% |
| 10 to 20°F | 35% |
| 0 to 10°F | 30% |
| -10 to 0°F | 25% |
| -20 to -10°F | 20% |
| -20°F or colder | 15% |
The easy rule: watch your windows. If they are sweating, the house is too humid for today’s weather, whatever the number says. Older or leakier windows will sweat at lower humidity than these. Our indoor humidity guide for Iowa homes covers the full year.
Where the moisture behind window condensation comes from
A household puts water into the air all day. Normally it leaves through fans, ventilation and air leaks. Windows sweat when moisture is being added faster than it is leaving. These are the sources we find most often.
| Source | Why it matters | What to check |
|---|---|---|
| Humidifier set too high | A furnace humidifier left at its fall setting keeps adding water when a cold snap arrives. This is the most common cause and the easiest fix. | The humidifier dial or thermostat humidity setting; turn it down as it gets colder |
| Bath fans | A shower puts a lot of water into the air. A fan that is weak, switched off too soon, or ducted into the attic instead of outdoors leaves it in the house. | Run the fan 20 minutes after showers; follow the duct to where it ends |
| Cooking | Boiling and simmering add moisture, and a recirculating hood only filters grease. It doesn’t remove any water. | Whether the range hood is vented outside |
| Ventilation system off | Newer homes rely on a whole-house ventilation system to carry moisture out. Many get switched off or were never set up. | The ventilation control; whether it is actually running |
| A new house drying out | Concrete, lumber and drywall hold a lot of water when a house is built and can release it into the home through the first heating season or two. | Homes one to two years old; expect to ventilate more at first |
| Basement or crawlspace | Damp foundation walls, an open sump pit or a dirt crawlspace floor feed moisture into the house from below. | Sump lid, damp spots on walls or floor, a musty smell |
| Everyday living | People, pets, plants, aquariums, laundry dried indoors and firewood stored inside all add up, especially in a tight house. | Anything that changed about the time the windows started sweating |
Why it happens in new homes and old ones
The house holds on to moisture
A newer home leaks far less air than an older one. That saves energy, and it also means the moisture you make indoors stays indoors unless something carries it out.
- New windows or new air sealing can make condensation appear where there was none, because the old drafts were drying the house all winter
- Construction moisture adds to it for the first year or two
- The fix is ventilation that runs, not a leakier house
The glass is colder
Single-pane glass, older double-pane units and metal frames run much colder on the room side, so they sweat at humidity levels a newer window would handle.
- Air leaking in around a loose sash chills the edge of the glass further
- Fog or frost on the inside of a storm window usually means house air is leaking past the inner window and reaching it
- Many Iowa homes built before the energy code have both problems
Why only some windows get condensation
If one or two windows sweat and the rest stay dry, look at what is different about them. It is usually one of these.
Closed blinds and heavy curtains
Coverings keep room heat away from the glass, so the glass runs colder behind them. Moist air still gets in around the edges. Open them during the day, or leave a gap at the bottom overnight.
Closed doors overnight
Two people breathing in a closed bedroom for eight hours raise the humidity in that one room. It is why bedroom windows are often the wettest in the morning.
Blocked or closed registers
The heat register under a window is there to wash warm air over the glass. Furniture, a closed register or a long curtain over it leaves the glass cold.
Colder glass or a leaky sash
Bay windows, patio doors, older units and any window with air leaking around the sash run colder than their neighbors. An infrared camera shows this in seconds.
Why window condensation is worth fixing
A little fog at the bottom of the glass on the coldest morning of the year is not an emergency. Water that pools on the sill most mornings is. It peels paint, swells and rots wood sashes and sills, stains drywall, and gives mold what it needs to grow on the frame and the wall below.
The bigger reason is what you can’t see. The glass is only the first cold surface the moisture finds. The same humid air is also reaching the attic and the insides of outside walls, where it can freeze onto the roof sheathing as attic frost and drip when it thaws. Your windows are the early warning.
How to stop window condensation, in the right order
Most of these cost nothing. Work down the list and stop when the windows stay dry through a cold snap.
Measure it
Put a digital hygrometer in the room with the wettest window, away from the kitchen and bath. Compare the reading to the table above for last night’s low.
Turn the humidifier down or off
If you have a furnace humidifier or run portable ones, lower them as the weather gets colder. In a cold snap, off is often the right setting.
Send moisture outside where it is made
Run bath fans for 20 minutes after showers. Use a range hood that is vented outdoors when you cook. Make sure the dryer vents outside and the duct is connected.
Let the house ventilate
If the home has a whole-house ventilation system, make sure it is on and running. Cold winter air is dry, so bringing in a controlled amount of it is the cheapest way to dry a house.
Warm the glass
Open blinds and curtains during the day, keep the registers under windows clear and open, and leave bedroom doors open when you can.
Will new windows stop the condensation?
Sometimes, and sometimes they just move it. Better glass stays warmer on the room side, so it can handle more indoor humidity before it fogs. If your windows are old single-pane units, that is a real improvement.
If the cause is too much moisture in the house, new windows don’t remove any of it. The moisture finds the next coldest surface instead, which may be somewhere you can’t see. Find out why the humidity is high first. It is a much cheaper question to answer than a house full of windows.
The exception is fog sealed between the panes. That is a failed glass unit, and replacing that glass is the fix.
What a home energy audit checks
A home energy audit treats wet windows as a symptom and goes looking for the cause. The visit takes two to three hours.
How tight the house is
A blower door test measures how much air the house leaks. That tells us whether the home is tight enough to need mechanical ventilation, and whether it is getting it.
Where the cold surfaces are
With the blower door running, an infrared camera shows the cold spots and air leaks around windows, at the tops of walls and in the ceiling.
Where the moisture is going
We look at where the bath fans, range hood and dryer actually end, whether the ventilation system is running, and what the indoor humidity is doing.
What to do first
You get a written, prioritized list. We don’t sell windows, insulation or equipment, so it isn’t shaped by anything we have to sell. The audit is a flat $699 for any size home.
Window condensation questions
Why do my windows have condensation on the inside in winter?
What should indoor humidity be in winter to stop window condensation?
Is fog between the window panes the same problem?
Why do my new windows or my new house have more condensation?
Will new windows stop the condensation?
Should I run a dehumidifier in winter?
Is frost or ice on the inside of windows a problem?
How does an energy audit help with window condensation?
Where this information comes from
- Winter window condensation as a sign of excess indoor moisture; find and remove the source first, then ventilate: University of Minnesota Extension, Controlling moisture problems in your home
- Window coverings lowering glass temperature and increasing condensation and frost: Pacific Northwest National Laboratory, PNNL-33380
- Winter indoor humidity targets by overnight low: Home Star Iowa, Indoor Humidity Levels for Iowa Homes
Wet windows every winter?
They’re a symptom. One visit with a blower door and an infrared camera shows why your house is holding moisture, and what to fix first.