Two different problems, two different solutions
Mould forms when surface moisture exceeds a certain level and stays there. The source of that moisture is either water entering the structure from outside, or indoor humidity condensing on a cold surface. Work carried out without identifying the source usually goes to waste.
How to tell them apart
The foil test
Transparent plastic sheeting is taped over roughly 50 square centimetres of the suspect wall, sealed at the edges, and left for two days. If the face towards the room has sweated, the problem is condensation. If the face towards the wall is wet, the moisture is coming through the wall.
Seasonal behaviour
Condensation related damp increases in winter and during periods when ventilation drops. Groundwater related damp follows the rainfall pattern and becomes pronounced in spring.
Location
Condensation usually starts on the inner face of external walls, in corners, and along columns and beams, meaning at cold bridges. Structural water ingress concentrates at levels close to the ground and at the floor to wall junction.
If it is condensation
- Regular ventilation, essential after using the bathroom and kitchen
- Eliminating cold bridges, raising surface temperature with external insulation
- Mechanical ventilation or a dehumidifier in basements
- Moving furniture a few centimetres away from external walls
In this situation applying a membrane or injection will not reduce the damp, because the moisture is already coming from inside.
If it is structural water ingress
Where the source is external water the order is clear: first stop the water entering the structure, then repair the internal surface. Doing it the other way round, renewing plaster and paint first, fails again at the same spot within a few months.
Cleaning mould is not permanent
Surface cleaning with bleach makes mould invisible but does not remove the spores or the moisture source. As long as the damp condition continues, mould returns within a few weeks. A lasting result requires cutting off the moisture source first.
The health dimension
Mould spores can trigger respiratory problems. In rooms occupied for long periods, such as bedrooms and children's rooms, delaying the problem is not the right call.