Find where moisture appears before deciding what to change.
Identify the surface
Condensation forms when moist air meets a sufficiently cold surface. Note whether water is on the room-facing secondary pane, the cavity-facing surfaces or the original external window. The location helps identify the conditions involved.
Photograph the affected area and note the weather, room use and time of day. Check separately for rain penetration, defective putty and damp masonry; not every wet patch is condensation.
Moisture inside the room
Cooking, washing and drying clothes release moisture. Existing extraction and background ventilation need to be considered when an additional inner window changes air movement.
A warmer inner surface may reduce condensation on that surface, but secondary glazing does not remove moisture from the home. Do not assume that installing another pane will cure mould or damp.
Moisture between the windows
Warm room air leaking into a colder cavity may condense on the original pane. The installer should explain the intended sealing and moisture-management strategy for the particular window.
Do not drill ventilation holes or seal every existing opening by guesswork. Ask the supplier or a competent building professional to investigate persistent cavity condensation and check the detailing.
A practical response
Keep a record of the problem, use functioning extraction and arrange an inspection if moisture persists. Check seals and maintenance instructions without disabling designed ventilation.
Persistent mould, wet timber or staining needs attention to the underlying cause. Obtain suitable professional advice where the source is unclear or the fabric is deteriorating.