Why do my windows have condensation on the inside?

Indoor humidity is meeting glass cold enough to reach dew point. The window is the coldest surface in the room and merely reports the house's moisture level; the fix is humidity and ventilation, not usually the window.

Is condensation on the outside of windows a problem?

No, the opposite: exterior dew on summer mornings means the glass is so well insulated its outer surface stays cool. It is a sign of an efficient window doing its job.

What does fog between the panes mean?

A failed insulated-glass seal: moisture has entered the sealed unit and condenses inside where no cloth can reach. That one is the window's fault, and the cure is glass-unit or window replacement.

Why is condensation worse in the bedrooms at night?

Closed doors, breathing occupants, and nighttime temperature drop concentrate humidity exactly when the glass runs coldest. Cracked doors, moved curtains, and lower whole-house humidity ease it.

Condensation on Windows: What It Tells You

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Window condensation is the cheapest hygrometer a house owns: glass is the coldest surface in most rooms, so it fogs first, and where and when it fogs reads as a report on the building’s moisture. The condensation on windows meaning splits three ways, inside surface, outside surface, and between the panes, and only the last is the window’s own failure. This diagnostic reads all three.

The physics in one paragraph

Air holds moisture in proportion to its temperature, and every parcel of humid air has a dew point, the temperature at which its moisture must condense. A window’s interior glass runs colder than the room in winter, so room air touching it can cross its dew point right there on the surface: fog, droplets, and eventually the puddled sill. The variables are two, how humid the room air is and how cold the glass runs, which is why the same reference governs indoor humidity targets: winter guidance steps humidity down as outdoor cold deepens precisely to keep dew points below glass temperatures.

Interior condensation: the house’s confession

Fog and droplets on the room side of the glass mean indoor humidity is outrunning the glass temperature, and the pattern tells the story. Mornings and bedrooms: overnight breathing plus closed doors plus the night’s coldest glass, the everyday case. Kitchens and baths: point sources outrunning their exhaust ventilation. Whole-house, persistent, with condensation reaching wall corners and closet backs: humidity genuinely high, from humidifiers set optimistically, crawl space or basement moisture migrating up per the musty-basement investigation, or a ventilation-starved tight house. The consequences of ignoring it climb from cosmetic to structural: puddled sills, peeling paint, mold at the glass line and sash corners feeding the general mold economy, and, unseen, the same humid air condensing inside wall and attic assemblies where nobody wipes. The response ladder: measure with a cheap hygrometer rather than argue, drop humidity sources (vent fans used and ducted out, humidifiers turned down, aquariums covered, firewood stored elsewhere), improve air movement at the glass (curtains and blinds cracked open, since draped windows run colder behind their insulation), and, persistent cases, dehumidification or ventilation upgrades. Single-pane and aluminum-framed windows fog earliest and hardest, cold frames earning their own drip lines, and storm windows or replacement raise the glass temperature side of the equation, per the window heat-loss reference.

Exterior condensation: the compliment

Dew on the outside of the glass, summer and shoulder-season mornings, burning off with the sun, is the opposite report: the insulated glass unit is keeping indoor conditions in so well that its outer surface stays cool enough to catch the morning’s dew, exactly like a car windshield. Modern low-e double and triple glazing does this routinely, and the correct response is none.

Between the panes: the window’s own failure

Fog, haze, or mineral ghosting between the panes, unwipeable from either side, means the insulated glass unit’s perimeter seal has failed: the sealed, gas-filled cavity has breathed in humid air, condensing on the cavity surfaces and often leaving permanent deposits. This is the one case where the window itself is the defendant, a known aging mode with its own reference, and the remedy is glass-unit replacement (often possible without replacing the frame) or window replacement, with defogging services occupying a cosmetic middle. Its diagnostic value here is the contrast: between-pane fog says replace the unit; surface fog says manage the house.

The bottom line

Outside dew is a well-insulated window bragging; between-pane fog is a failed seal asking for new glass; and inside condensation is the house reporting its humidity on the coldest surface available. Read the glass daily for free, keep winter humidity stepped down with the cold, vent the point sources, and let persistent fog send you to the hygrometer before it sends the sills to the rot and mold ledger.