Damp on a north-facing wall: identifying the cause before tackling mould

Look at the shape and position of dampness. A stain following rain may indicate leaking flashing or a façade defect. Marks near the floor call for inspection of the base and damp-proofing. A damp corner behind a wardrobe may involve a cold surface and poor air movement.

These are inspection clues, not a finished diagnosis. Several causes may coexist. Take photographs on different dates and record weather and room use. This helps distinguish leakage from conditions occurring mainly during the heating season.

Measurements of room air and wall surfaces are useful. One midday reading cannot represent a full day. Observe conditions after bathing, cooking and overnight heating reduction. Check unobstructed ventilation and the supply of fresh air to rooms.

Mould risk depends on conditions at the surface, not just average room humidity. A cold corner may therefore be troublesome while the rest of the wall remains dry. Simplified calculations help assess this, but cannot replace inspection, leak investigation or information about actual room use.

Thermal imaging helps locate cold areas, but its interpretation depends on measurement conditions. Sun, wind and heating influence the result. A cold area is not proof of missing insulation by itself; moisture or airflow may also be involved.

Inspect wall-to-floor junctions, reveals and lintels, where a simple layer section may be insufficient. More detailed calculations can assess temperatures within the junction. A successful repair should address that detail too: insulating a broad flat area can leave the corner problem unresolved.

If rain or services wet the wall, remove the cause. New insulation cannot replace a working gutter or sound sill flashing. Assess whether and how the masonry can dry. There is no universal drying time for every thickness and material.

Internal boards need a prepared substrate. Before using PIR-plasterboard, assess masonry and room-air moisture. Excess moisture can hinder adhesive drying and curing. Establish conditions for a lasting installation, and decide when to cover the wall from its actual state rather than a fixed timetable.

External and internal insulation alter temperatures differently, so their assemblies are not interchangeable. Beneath render, use a board intended for the façade system. Internally, a plasterboard composite with a vapour-control layer may be considered.

Both options need detailed junctions. Gaps, sockets and ceiling connections cannot be omitted. Select thickness from wall analysis. More insulation does not always reduce moisture problems when its position or continuity is wrong.

No visible condensation does not prove suitable wall conditions. Humidity beside a cold surface can be much higher than in the room centre, allowing dark spots without obvious dripping. Wiping the surface alone changes neither corner temperature nor water vapour in the air.

Check whether the problem intensified after window replacement or a heating change. That timing suggests checking ventilation, but is not conclusive evidence. Altering one building element affects others. Renovation should address the cause, not only the place where it is most visible.

Remedy moisture damage according to the materials’ condition. Severely damaged finishes may need replacement. Repainting cannot stop recurring damp if its cause remains. Extensive damage calls for an on-site assessment.

Keep critical locations accessible for observation, measure conditions and check ventilation. Do not obstruct airflow with large furniture directly against a cold wall. If staining returns, investigate again: it indicates that the original diagnosis or execution needs review.

No. It influences temperature and drying conditions, but does not by itself determine whether mould develops.

No. In a suitable assembly it reduces heat loss; it does not dry a leaking or damp wall.