Efflorescence follows moisture movement

Water dissolves soluble salts in masonry or mortar and carries them towards an evaporating surface. When the water leaves, salts remain. New brickwork, leaks and ground moisture can all produce this process.

Some salts absorb water from the air

Nitrates and chlorides commonly found in contaminated old plaster can be hygroscopic. In humid conditions they attract moisture, so the surface can feel damp even though no new liquid water is entering at that moment.

Salt crystals and damaged plaster on a damp wall
Some salts remain in old plaster and can continue attracting moisture from the air after the original water source has been corrected.

Meter readings can be distorted

Electrical moisture meters respond to conductivity as well as water. Salt-contaminated plaster can therefore give persistently high readings. This is one reason a diagnosis should not rely on a row of meter readings alone.

The finish may need separate treatment

Stopping the water source and dealing with contaminated plaster are separate jobs. Where salts are severe, simply drying the wall and repainting may not prevent recurrence.

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