Module 3 · Types of Damp in Detail
3.5 Hygroscopic Damp & Salt Contamination
7 min read
This is the mechanism that makes surveyors look foolish. A wall that has been correctly repaired keeps reading damp, keeps ruining decorations, and keeps generating complaints — and every meter you put on it agrees. The water is coming out of the air, pulled there by salts left behind by a problem that no longer exists.
Learning objectives
By the end of this lesson you will be able to:
- Explain how hygroscopic salts hold moisture from the air.
- Identify the visual and behavioural signs of salt contamination.
- Distinguish hygroscopic damp from an active moisture source.
- Explain why redecoration keeps failing on a contaminated wall.
Key terms
- Hygroscopic
- Able to absorb water vapour from the air. Nitrate and chloride salts strongly so.
- Efflorescence
- Salt crystallising on the surface as moisture evaporates. Usually white and powdery, wipes off.
- Crypto-efflorescence
- Salt crystallising within the material just below the surface, forcing plaster and paint off from behind.
- Equilibrium moisture content
- The moisture level a material settles at for the surrounding humidity. Salts raise it substantially.
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Read the rest of Types of Damp in Detail
Module 1 is open to everyone. The remaining six modules — building science, each damp mechanism in depth, meters and thermal imaging, the full inspection method, and report writing — are free too. We just ask for an email so you can pick up where you left off, and so we can send your certificate when you finish.