Condensation on Windows Inside: Why It Happens and How to Stop It

Condensation on the inside of your windows means the glass is colder than the dew point of the air in the room. It happens most in winter, because the glass is usually the coldest surface in a heated home. A thin line of water along the bottom edge on a very cold morning is normal. Water that runs onto the sill, stays all day or shows up on walls means the indoor humidity is too high for how cold your windows get, and the fix is less moisture, more ventilation or warmer glass.

Why windows sweat: the dew point

Air holds water vapor, and the warmer it is, the more it can hold. The dew point is the temperature at which the air in your room would be saturated. National Research Council Canada’s building digest on window condensation puts the rule simply: water condenses on an inside window surface whenever that surface is colder than the dew point of the room air.

In a 70 °F room, the dew point climbs quickly with humidity:

Room humidity at 70 °F	Dew point
30%	37 °F
40%	44.6 °F
50%	50.5 °F
60%	55.5 °F

Any glass colder than the dew point gets wet. These dew points come from the Magnus formula that our dew point calculator uses. Enter your own room temperature and humidity to see the number for your home. To check a specific window, measure the glass with an infrared thermometer and compare.

How cold does the glass get?

That depends on the weather and your glazing. NRC Canada calculated the inside surface temperature at the center of the glass, with the room at 70 °F and a 15 mph wind outside, and the highest room humidity each window could take before fogging:

Outdoor temperature	Single glazing	Double	Triple
40 °F	47 °F (44%)	59 °F (68%)	63 °F (78%)
20 °F	32 °F (24%)	52 °F (53%)	59 °F (68%)
0 °F	17 °F (12%)	45 °F (41%)	54 °F (57%)
−20 °F	2 °F (6%)	38 °F (32%)	50 °F (49%)

Three caveats. “Double” in that 1960 digest means double windows of that era, not a modern low-e unit. Frames and edges can be colder than the center of the glass, which is why condensation usually starts at the bottom corners. And with no wind the glass is warmer: at 0 °F, NRC puts double windows at 50 °F, good for 49% humidity.

The pattern is what matters. Each drop in outdoor temperature cools the glass and lowers the humidity it can handle. NDSU Extension puts it in one line: the colder the weather outside, the greater the chance of window condensation inside.

When is it a sign of too much humidity?

EPA’s mold course notes that condensation forms when warm, humid air meets a cold surface, and that it can be a sign of high humidity. Whether your humidity is actually too high depends on the season.

In winter, some condensation in a cold snap is expected. A 1974 University of Minnesota Extension fact sheet gives allowable indoor humidity to minimize window condensation in homes with double windows:

Outdoor temperature	Allowable indoor humidity
20 °F	35–40%
10 °F	30–35%
0 °F	25–30%
−10 °F	20–25%
−20 °F	15–20%

The authors called the best winter level a compromise: just high enough that condensation starts along the lower edge or corners of the glass. They also wrote that “window condensation must be your measure of a tolerable moisture level,” whatever your humidity gauge says. In our view, these “allowable” figures are a safer guide than NRC’s theoretical maximums, because they leave room for colder frames and edges. NDSU Extension recommends 30–40% in winter and notes that above about 40%, the chance of condensation on windows and other cool surfaces rises.

It’s a problem when any of these apply:

• Water runs down onto the sill.
• The glass stays wet all day.
• Condensation also forms on walls or around a humidifier.
• Your hygrometer reads above 50% while you’re heating.

EPA’s general range is 30–50%, with 60% as the ceiling. Our guide to ideal indoor humidity explains those numbers by season.

Where the extra moisture comes from

Every household adds water to the air. The University of Minnesota fact sheet estimates that:

• A family of four breathes out 8 to 12 pounds of water a day.
• Cooking for four on a gas stove adds about 5 pounds.
• Each shower adds about half a pound.

EPA’s mold course also lists humidifiers, steam radiators, clothes dryers and combustion appliances such as stoves.

How fast that moisture leaves depends on how leaky the house is. In the same fact sheet’s example house, at 30% humidity on a 0 °F day, a very tight house loses about 18 pounds of water a day through air leakage, while a loose one loses about 156. That’s why a well-sealed home, or a room with the door closed and a humidifier running, can fog its windows even when nothing seems wrong.

How to stop condensation on the inside of windows

1. Measure first. A hygrometer from a hardware store tells you where you are. NDSU warns that mechanical dial hygrometers are often off by more than 20 percent, so treat the reading as a guide and let the windows confirm it.

   Check indoor hygrometers on Amazon

2. Turn the humidifier down. If water condenses on windows, walls or pictures, EPA says to move the humidifier, lower its humidistat or use it less. In cold weather we’d start around 40% and drop it as the temperature outside falls. A closed door makes overshooting easier; see should you close the door when using a humidifier.

3. Vent moisture at the source. EPA says the most effective step is usually to remove the source and exhaust excess water vapor outdoors. Run the bathroom fan during and after showers and the range hood while cooking, and make sure the dryer vents outside.

4. Increase ventilation. EPA’s mold guide and the Minnesota fact sheet both list more ventilation as a fix for condensation. Briefly opening a window or running exhaust fans longer lets drier outdoor air replace humid indoor air.

5. Warm the glass. EPA suggests raising the air temperature as one option. The 1988 Minnesota paper notes that curtains and recessed windows lower the humidity a window can take, because they keep warm room air off the glass. Open heavy curtains and blinds during the day, and don’t block heating vents under windows.

6. Add a layer of glazing. The same paper shows how much storm windows help. At 0 °F outside, single glazing fogs at about 12% humidity, a window plus a tight storm window at about 47%, and sealed double glazing with a storm at 48–52%. We’d try the cheaper steps above before replacing windows.

7. Dehumidify when it stays high. If your humidity sits above 50–60% without a humidifier running, EPA recommends air conditioning or a dehumidifier. Our guide on how to reduce humidity in the house covers the options, and air purifier vs dehumidifier explains why a purifier won’t do this job.

Tip: Wipe the glass and sill dry in the morning. EPA says to dry wet surfaces quickly and to dry water-damaged areas within 24 to 48 hours to prevent mold.

Condensation, mold and when to worry

Occasional window condensation that dries during the day is a comfort problem. Condensation that keeps sills, frames or the wall below the window wet is a mold problem waiting to happen. We’d treat a sill that’s wet every morning as a moisture problem to fix that week, not a cleaning chore. The CPSC warns that when humidity goes above 60%, moisture can condense on surfaces where bacteria and fungi settle and grow.

If you already see mold on frames or sills, EPA’s rule is that moisture control is the key. Clean the mold off, fix the moisture, or it will likely come back. EPA says a homeowner can usually handle a moldy area under about 10 square feet. An air purifier can catch airborne spores but won’t stop the growth, as our guide to the best air purifier for mold explains.

If a humidifier is filling the room with visible haze as well as fogging the windows, the output is too high for the space. Our guide to a humidifier making the room foggy walks through the causes.