Why does a north-facing corner get damp even when the room is heated?

Inspect adjoining rooms and the outside of the wall. A stain near the ceiling may point to the roof or a pipe; one near the floor may have a different source. A dark corner alone does not identify the defect. Record its extent and have it inspected before painting over it.

A thermometer in the middle of a room cannot tell you how cold the wall is behind a cupboard. Track air temperature and relative humidity alongside the surface temperature at the stain. Use instruments suited to the range and material. A wood moisture meter does not automatically give reliable readings in masonry.

Take a series of readings and record the time, weather and heating pattern. Thermal imaging also requires suitable conditions: reflections from glossy surfaces and solar heating can distort the picture. If the actual moisture content of a material is needed, let the investigator choose the test method.

Inspect gutters, flashings, joints and the junction between window and façade. Look for splashing at the plinth and cracks beneath the sill. North-facing walls usually receive less sun, but condensation is not the only possible explanation for stains. A mark that appears after wind-driven rain calls for an inspection of the weather seal.

Stop the water at its point of entry, then assess drying and the condition of the plaster. A vapour-tight board on the inside of a damp wall may restrict drying towards the room. Let the condition of the wall determine when to insulate, rather than counting days since the gutter repair.

An external corner, reinforced concrete column or uninsulated ring beam may have a colder inside surface than the middle of the wall. High indoor humidity compounds the problem. At 20°C and 60% relative humidity, for example, the dew point is about 12°C. That value applies to these conditions; it is not a universal pass mark for walls.

Visible droplets are only one sign of surface moisture. The duration of high humidity at the surface and conditions immediately around it also matter. Fitted furniture restricts circulation and can leave the wall colder. Check that the ventilation works and that replacement air can enter as intended.

Where a thermal bridge is responsible, the design should cover its full extent. A patch of insulation over the stain may simply move the coldest point to its edge. External and internal insulation affect masonry temperature and drying differently. The choice depends on the wall and the options available.

Internal PIR insulation requires a heat and moisture assessment and airtight junctions. A simplified model does not capture every route taken by air or water. Leaking old masonry may need further investigation. Plan the removal of damaged finishes and preparation for the new work separately.

After the repair, monitor the same points. Compare periods with similar weather and room use; a few warm days prove little. Record changes to heating or ventilation so you can tell which measures affected the corner.

Inspect the insulation junctions at the ceiling, floor and window before covering them. Keep photographs and material details. If the stain returns, investigate that location again before increasing insulation thickness. Watch the corner particularly closely during the first cold spell and the first heavy rain.