Why is a north-facing wall damp even when the room is heated?

Touching the plaster cannot establish where the water came from. Measuring moisture in building materials also requires a suitable method and interpretation. Investigate possible leaking pipework or ground moisture first if either is suspected.

The air can be at 21°C while the wall behind a tall cupboard is considerably colder. Restricted air circulation reduces heat transfer to the surface. North-facing walls receive little direct sun and may dry more slowly after wetting. Orientation is only one possible factor.

A thermal bridge at a ring beam, a poorly insulated reveal or air leakage can lower local temperatures. Thermal imaging under suitable conditions can locate these differences. Interpret the images alongside an inspection and records of room use, as a cold area alone does not establish damp.

Monitor temperature and humidity over several days, especially after bathing, cooking and overnight occupancy. Record indoor clothes drying and check both supply and extract ventilation. Keep grilles clear. Replacement windows may have reduced the air supply even if household routines stayed the same.

Condensation starts when a surface reaches the dew point. High humidity at the surface can encourage mould before droplets form. Assess the corner temperature together with indoor humidity even if no water is visible.

Take readings in comparable positions and record when windows were opened. A reading immediately after airing describes different conditions from one taken after a full night. Regular notes distinguish short-lived changes from persistent problems.

Internal insulation can warm the new room-side surface while reducing heat reaching the masonry. A layer with high vapour resistance also restricts drying towards the room. Aluminium-faced PIR is therefore unsuitable as a one-size-fits-all remedy for damp walls.

Remove the water source and assess the materials before insulating. Examine heat and moisture behaviour across the construction, including rain exposure. Some buildings call for a different insulation method or an external repair. Select the intervention on the basis of the diagnosis.

For damp beside a window, check the frame-to-masonry joint and insulation at lintel and reveals. If marks recur below the ceiling, examine the ring beam and insulation continuity. Insulating only the broad wall face can leave the critical spot cold or shift the cold strip to the end of the new layer.

Detail junctions with the floor, ceiling and intersecting walls. For internal insulation, control the flow of humid air behind the boards. Check the hidden seals before closing up, since an even decorative finish gives no evidence of their airtightness.

After fixing the cause, allow time for drying and restore the surface as required. A dry wall in July cannot establish whether a winter fault has been resolved. During the next cold spell, check the same points, furniture positions and ventilation. Recurrent staining calls for further investigation.

Log the weather, indoor readings and heating pattern. This helps distinguish the effect of repairing a leaking gutter from that of more frequent airing. The record also helps the designer plan further energy improvements.