Every window in one bedroom fogs between the panes. The correct diagnosis is:

Fogging between the panes is diagnostic because of where the moisture is. An insulated glazing unit is two panes bonded to a spacer with a sealed, dried cavity between them. Nothing in the room can reach that cavity — that is the entire point of the seal. So condensation appearing there means the edge seal has failed and the desiccant in the spacer has become saturated, admitting humid air that condenses when the glass is cold. Indoor humidity produces condensation on the room-side surface, where it can be wiped; a failed unit cannot be wiped because the moisture is inside the sandwich. That every window in one room has failed is unremarkable — units installed at the same time, on the same elevation, with the same solar exposure tend to fail together.

Moisture between the panes can only arrive through a failed edge seal, since the cavity is sealed and desiccated at manufacture. Multiple failures in one room reflect units of the same age and exposure reaching end of life together.

Insulated glazing units consist of two or more panes separated by a spacer containing desiccant, with a perimeter seal. Seal failure admits humid air, saturates the desiccant and produces permanent interpane condensation, often with visible mineral deposits over time.

If You Cannot Wipe It, It Is Inside. Room humidity lands on the surface; a failed seal puts it in the sandwich.

Note that the glass unit can be replaced without replacing the whole window in most cases, which matters to the client's cost expectations.

• •Attributing interpane fogging to indoor humidity
• •Assuming simultaneous failures indicate something other than equal age and exposure
• •Recommending whole-window replacement when the glass unit alone has failed