TL;DR
Condensation on the inside of a window means indoor air holds more moisture than the cold glass can support. It is a humidity signal, not a window defect, and it is the cheapest moisture meter in the house. Water between the panes is a different problem entirely: that is a failed seal, and it needs a glass repair rather than a dehumidifier.
The physics, in one paragraph
Warm air holds more water vapor than cold air. When warm indoor air touches a surface cold enough, the air right at that surface cools past the point where it can hold its moisture, and the excess turns to liquid. That temperature is the dew point.
Window glass is almost always the coldest interior surface in a room in winter, which is why it fogs before anything else does. The glass is not creating the water. It is simply the first place the water shows up.
That makes condensation genuinely useful information. It tells you the indoor humidity is higher than the coldest surfaces in the house can tolerate, and it tells you before mildew starts growing somewhere you cannot see.
Source: published building science guidance on moisture and dew point
Which side of the glass is wet?
This is the diagnostic question, and the three answers mean three different things.
Inside the glass, in the room. Indoor humidity is too high for the current outdoor temperature. This is the common case and the one you can act on.
Outside the glass, on the exterior. Usually a sign that the window is insulating well. The outer pane stays cold enough on a clear night for dew to form on it, exactly as it does on a car windshield. Harmless, and it burns off in the morning.
Between the panes. The seal on the insulated glass unit has failed and moisture has entered the sealed cavity. No amount of humidity control fixes this, because the water is inside the assembly. It needs a glass or window professional.
Where the moisture is coming from
Households generate a surprising amount of water vapor, and in a well-sealed home it has fewer places to go.
- Showers and baths. The single largest short-burst source in most homes.
- Cooking. Boiling, simmering, and steaming release water directly into the kitchen.
- Drying laundry indoors. A rack of wet clothes puts several pints of water into the air of one room, as covered in indoor drying racks and household humidity.
- Breathing and perspiration. Every occupant contributes a few pints a day just by living there.
- Unvented combustion appliances. Any gas appliance without a flue to the outdoors puts water vapor into the room.
- A damp basement or crawl space. Ground moisture rises through the house, which is why a wet crawl space shows up as fogged bedroom windows two floors up.
- Houseplants and aquariums. Minor on their own, real in quantity.
New construction and newly weatherized homes see this more, not less. Tighter building envelopes leak less air, which is good for the heating bill and means moisture that used to escape now stays inside until something removes it deliberately.
What to do about it
The fix is either less moisture in the air, more ventilation, or a warmer glass surface. In practice, the first two do the work.
Run exhaust fans properly. The bathroom fan should run through the shower and keep running afterward, and the range hood should be on whenever you boil or fry. Run times are covered in how long to run a bathroom exhaust fan.
Vent the dryer outdoors, and check that the duct is intact. A disconnected dryer vent is a major hidden humidity source.
Lower the target humidity as it gets colder. The band that works in mild weather is too high in a cold snap. What that number should be is covered in what indoor humidity level a home should target.
Move air across the glass. Open curtains and blinds during the day, and pull furniture a few inches off exterior walls. Trapped air behind a heavy curtain gets colder than the room, which is why the worst condensation is often behind one.
Deal with the basement or crawl space if the whole house fogs rather than one room. That is where the water is entering.
Turn a humidifier down or off in winter. Many homes running one in cold weather are creating the condition they are complaining about.
Do not let the sill stay wet
This is where condensation stops being a signal and becomes damage.
Water runs down the glass and pools in the sill and the frame corner. Painted wood swells, the finish fails, and surface mildew appears in the corners within weeks in a room that repeats this every morning.
The habit that prevents it is small: wipe the sills when you see standing water, and dry them rather than leaving them damp. If growth has already started, the surface version is straightforward, and the method is in how to clean mildew off window frames.
If the frame is soft, spongy, or crumbling, that is rot rather than a cleaning issue, and it needs a carpenter or window professional. Growth that keeps returning after cleaning, spreads beyond the frame, or appears on the wall around the window belongs with a mold remediation specialist, because the moisture source has not stopped.
FAQ
Why do my windows have condensation on the inside?
Because indoor humidity is higher than the cold glass can support. Warm moist room air touches the coldest surface in the room, cools past its dew point, and gives up water. It is a humidity reading rather than a window fault, and the fix is ventilation or lower indoor humidity.
Is window condensation a sign of a bad window?
Not usually. Condensation on the room side means humidity is too high for the glass temperature, which happens with any window in cold weather. Condensation on the outside actually indicates good insulation. Only moisture between the panes indicates a failed window, since the seal has let water into the unit.
Will new windows stop condensation?
Only partly. Better-insulating glass runs warmer on the room side, so it fogs less readily, but the moisture is still in the house and will condense on the next coldest surface -- an exterior wall corner, a closet wall, or behind furniture. Ventilation and humidity control are the real fix.
How do I stop windows sweating in winter?
Run the bathroom fan and range hood through and beyond every moisture-producing activity, vent the dryer outdoors, stop drying laundry indoors, turn any humidifier down, and lower the indoor humidity target as outdoor temperatures fall. Open blinds during the day so air moves across the glass.
Does condensation cause mold?
It creates a condition mold needs. Repeated water on a sill keeps paint, wood, and caulk damp, and surface growth follows within weeks in a room that fogs every morning. Drying the sills prevents it. Growth that keeps returning means the moisture source is still running.
What does condensation between window panes mean?
The seal on the insulated glass unit has failed and moisture has entered the sealed cavity. Nothing you do to indoor humidity affects it, because the water is inside the assembly. The glass unit needs repair or replacement by a glass or window professional.
This sits inside the broader whole-home work covered by house cleaning services.
Fogged windows every morning usually means a house that needs a different rhythm through the wet months, especially in bathrooms, laundry rooms, and basements. If you want those rooms kept ahead of it on a regular schedule, get a quote and we will talk through what makes sense.