The distribution is often revealing

Corners, window reveals, ceilings at outside walls and areas behind furniture are common because their surface temperatures are lower. Mould may be widespread across several cold points rather than confined to one construction defect.

Indoor moisture sources matter

Cooking, showering and drying clothes release large amounts of water vapour. Extract fans that recirculate rather than discharge outside, blocked vents and closed trickle vents can allow that moisture to accumulate.

Black mould and condensation damage hidden behind a wardrobe
Furniture tight against a cold external wall can create the still, cool conditions in which condensation and mould develop.

A cold wet wall can have two causes

Penetrating damp makes a wall colder as well as wetter. That can encourage surface condensation and mould on top of the original rain defect. A mixed pattern should not be forced into one diagnosis.

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