TL;DR
Window condensation is not a glass problem, it is a humidity problem showing up on the coldest surface in the room. Wiping it daily protects the sill, but the fix is lowering indoor moisture toward the 30 to 50 percent range and improving ventilation. Water that sits on a wooden sill for a season causes damage that cleaning cannot undo.
What is actually happening on the glass
Warm indoor air holds moisture. Cold glass cools the air touching it, and cooled air cannot hold as much, so the excess condenses on the surface.
That makes the window a measuring instrument rather than a defect. It shows you the first place in the room cold enough to reach the dew point, and in most homes that is single glazing, an older double-pane unit, or the frame and sill edges where the assembly is coldest.
The physics is explained in why windows get condensation in cold weather.
Where the water shows up tells you something too. Condensation on the room side of the glass is an indoor humidity issue. Condensation between the panes means the sealed unit has failed, and no amount of cleaning reaches it -- that is a glazing repair.
How do you clean condensation on windows?
Wipe the glass and the sill dry with an absorbent cloth every morning during cold spells, working the corners and the track where water pools. Dry the frame as well as the glass. Then reduce indoor humidity and increase ventilation, because daily wiping manages the symptom rather than removing the cause.
The daily wipe, and why the sill matters more than the glass
The glass is the visible part. The sill is where the damage happens.
Water runs down the pane and collects along the bottom edge, where the glass meets the frame and where the sill meets the track. That junction stays wet all day if nobody dries it.
Use an absorbent cloth or a small squeegee and finish with a dry pass. A squeegee alone leaves the pooled water in the track.
Get into the corners. Both bottom corners hold water longest and are where paint failure and mildew show up first.
Dry the track. On sliding and casement windows, the track is a channel designed to hold water and drain it outside, and in winter it often just holds it. Track cleaning cadence is covered in how often to clean window tracks and sills.
Check the weep holes on the exterior side of the frame if the window has them. A blocked weep hole keeps water inside the assembly.
What repeated wetting does to a sill
This is the reason the daily two minutes is worth it.
Paint blisters and peels first, because water gets under the film and lifts it. Once it lifts, the wood underneath is exposed.
Wood swells, then dries, then swells again. That cycle opens the joint between the sill and the frame.
Mildew colonizes the caulk line and the paint surface, since the area is damp, still, and organic.
Wood eventually softens. Soft spots at the corners of a sill mean rot, and rot is a repair, not a clean.
Metal frames and hardware corrode where brine or condensate sits, and drywall below the sill can stain and soften if water runs over the edge.
Cleaning mildew on frames and caulk, and where to stop
Some mildew on a painted frame or a caulk line is a normal winter cleaning task. Some of it is not yours to handle.
Surface mildew on nonporous paint, vinyl, or aluminum frames usually cleans off with a household cleaner and a soft brush, followed by a thorough dry. The method by frame material is in how to clean mildew off window frames.
Black staining set into caulk usually means the caulk itself is colonized. Cleaning fades it; replacing the caulk removes it.
Growth on drywall, on the wall below a window, or spreading beyond the frame is a different situation. Porous building materials that have been wet long enough to grow mold are a remediation question, not a cleaning question. Common guidance draws the do-it-yourself line at roughly ten square feet of visible growth, with larger areas and anything involving hidden cavities going to a professional. Where that boundary sits is covered in when cleaning mold is not enough.
Source: EPA mold cleanup guidance for homes
One safety line applies throughout: never combine chlorine bleach with an ammonia-based cleaner, with vinegar or any acid, or with rubbing alcohol. Those combinations release hazardous gas. Use one product at a time in a ventilated room.
How to stop condensation on windows
Wiping is maintenance. These are the changes that reduce how much there is to wipe.
Lower indoor humidity. Buy a hygrometer and aim for the commonly recommended 30 to 50 percent range, described in what indoor humidity level a home should target. Most winter condensation problems are a house running well above that.
Run exhaust fans properly. The bathroom fan should run during the shower and for a stretch afterward, not just while someone is in the room. The kitchen range hood should run whenever anything is boiling.
Cover pots and vent the dryer outdoors. Uncovered cooking and an improperly vented dryer put large amounts of moisture into the air.
Move air across the glass. Cold, still air at the window makes condensation worse. Opening blinds and curtains during the day and pulling furniture off the wall under a window both help.
Reduce indoor drying. Racks of laundry drying in a closed room raise humidity substantially.
Raise the glass temperature. Storm windows, insulating film, and thermal curtains that are opened during the day all reduce how cold the surface gets. Heavy curtains kept closed around the clock make it worse by isolating the glass.
If you use a humidifier, treat visible condensation as the signal to turn it down regardless of what the target number says.
When condensation is telling you something bigger
A few patterns deserve a closer look rather than a cloth.
Fog between the panes is a failed seal. It cannot be cleaned and it means the insulating gas is gone.
Condensation on interior walls or in closets, not just windows, suggests a whole-house moisture or ventilation problem.
Water appearing on the outside of the glass on a cool morning is normal and harmless, and often a sign the window is insulating well.
Ice forming on the inside of the glass means humidity is high enough to warrant real attention before spring.
Recurring mold in the same corner every winter usually indicates a cold spot in the assembly rather than a cleaning failure.
FAQ
How do you clean condensation off windows?
Wipe the glass with an absorbent cloth or squeegee each morning, then dry the frame, the corners, and the track where water pools. Drying the sill matters more than the glass, because standing water there blisters paint, swells wood, and starts mildew within a few weeks.
Why do my windows get so much condensation in winter?
Because indoor air holds more moisture than the cold glass surface can support, so the excess condenses. Cooking, showering, drying laundry indoors, and humidifiers all add moisture, while closed windows prevent it from leaving. It is a humidity problem, not a window defect.
Is window condensation dangerous?
The water itself is not, but repeated wetting damages sills and frames and can start mildew or, over time, rot. It also signals indoor humidity higher than the commonly recommended 30 to 50 percent range, which raises mold risk elsewhere in the house.
How do you stop condensation on windows overnight?
Lower the indoor humidity before bed by running exhaust fans, avoiding indoor laundry drying, and turning any humidifier down. Leaving blinds partly open and keeping air moving near the glass also helps, since still cold air at the pane is what makes it worse.
Can you clean condensation between double-pane windows?
No. Fog between the panes means the sealed unit has failed and moisture entered a cavity that cannot be accessed. It is a glazing repair or a unit replacement, and no cleaning method or product reaches inside the assembly.
When is window mildew a job for a professional?
When growth appears on drywall, spreads beyond the frame, keeps returning in the same spot, or covers more than roughly ten square feet. Porous materials that have stayed wet are a remediation issue, and cleaning the surface without addressing the moisture only postpones it.
Winter windows quietly hand a house a damage bill that shows up in spring. If you want the sills, tracks, and frames handled on a schedule through the cold months, get a quote or read more about house cleaning services.