Windows produce two completely different mould questions that look like one. Growth on the frame, the reveal or the sill is condensation on the coldest surface in a room and is the same problem as a bathroom ceiling. Growth between the panes of a sealed unit is not a cleaning problem at all: it means the seal has failed, and nothing can be cleaned in a cavity that cannot be opened.
Mold inside window pane cavities means the seal has failed
A double glazed unit is two panes bonded to a spacer with a desiccant inside, sealed at the factory. Once that seal fails, humid air enters the cavity, the desiccant saturates, and moisture condenses on the inner faces of the glass. What grows there feeds on the dust that came in with the air. It cannot be reached, cleaned or dried, because the cavity was never designed to be opened. The unit is replaced; the frame usually stays.
What failure looks like before the growth
Misting or a haze between the panes that appears in particular weather and clears in others is the first sign, often years before any growth. Then a permanent film, then mineral staining at the bottom edge from repeated condensation, then visible growth. Mold inside window pane assemblies is therefore a late symptom of a seal that failed a while ago, and it is worth checking the other units on the same elevation, because they are the same age and the same exposure.
Mold on windows and frames, which is condensation
Growth on the frame, the reveal, the sill or the beading is the ordinary condensation story: the glass and frame are the coldest surfaces in the room, humid indoor air condenses there, and dust on the surface feeds growth. Black mold in windows of this kind concentrates at the bottom of the glass and along the frame junction because water runs down and sits. It cleans off a sound surface, and it comes back unless the humidity, the ventilation or the surface temperature changes.
What actually fixes each one
For a failed unit, replacement of the glazing unit. For condensation, reduce the moisture load and raise the surface temperature: extract at source in kitchens and bathrooms, ventilate bedrooms, avoid drying laundry indoors without extraction, and consider trickle vents or a heat recovery unit. Where growth on frames and reveals is extensive and returning, it is worth measuring the humidity rather than guessing, because a house that sits persistently damp will keep producing it at every cold surface, not only the windows.
Questions people ask about mold between window panes
Can mold between window panes be cleaned?
No. The cavity of a sealed unit cannot be opened, reached or dried. Growth there means the seal has failed and the glazing unit is replaced rather than cleaned.
Why is there mold on my window frames?
Condensation. Glass and frames are the coldest surfaces in a room, humid air condenses on them, and dust on the surface feeds growth. It cleans off but returns unless conditions change.
Is black mold in windows dangerous?
Guidance is to remove indoor growth and correct the moisture whatever its colour. On a window the growth is usually small and on a hard surface, so the moisture is the part worth acting on.
Does a failed unit mean the whole window is replaced?
Usually not. The sealed glazing unit is replaced within the existing frame, provided the frame itself is sound.
Is mold on window glass the same problem as mold on the frame?
No. Inside a sealed unit, between the panes, it means the seal has failed and the cavity is holding moisture, which is a unit replacement. On the room side of the glass or on the frame it is condensation on the coldest surface in the room, which cleans off and comes back until the humidity or the ventilation changes.