Cooking, showering, drying clothes and even breathing add water vapour to indoor air. If that moisture is not removed or diluted, relative humidity can remain high and water may condense on colder surfaces. Persistent dampness can then support mould growth.
Why cold corners are often affected
External corners, lintels, areas behind furniture and poorly insulated junctions can be colder than the rest of a room. Restricted air movement makes some of these locations slower to warm and dry, which is why mould patterns are not always evenly distributed.
Ventilation has different jobs
Kitchen and bathroom extract fans remove moisture close to its source. Background vents and suitable whole-home ventilation help replace stale air. The systems need clear air paths, appropriate ducting and normal maintenance to work as intended.
Poor ventilation is not the only possible cause. Leaks, rainwater entry, heating patterns, insulation defects and overcrowding may also contribute. Check the building and the existing system before adding a PIV unit or assuming that opening windows is a complete solution.