What are the white crystals on my damp wall?

All masonry contains soluble salts. Water moving through a wall dissolves them, carries them to the surface, and leaves them behind when it evaporates. That is efflorescence, and on a new build or a drying wall it is normal. The problem is a different family of salts — nitrates and chlorides, from ground water, from old fireplaces, from urine in a former stable or outbuilding — which are hygroscopic: they pull moisture out of the air on their own account, so the wall stays damp for ever even after the original water source is gone.

What you'll need

• Stiff dry brush
• Dust mask
• Hygrometer (optional)

Step by step

1. Brush a sample off dry.Dry brushing only, and wear a mask. If it comes away as a dry powder and the wall behind is sound, you are probably looking at efflorescence.
2. Note whether it returns, and how fast.Efflorescence recurs while the wall is drying and then stops. Hygroscopic contamination reappears within days indefinitely, because the salts are pulling water from the room air rather than from the wall.
3. Watch the weather.This is the giveaway. Hygroscopic salts absorb moisture when the room humidity rises, so a salted wall darkens in a muggy August and dries in a cold dry snap — the opposite of condensation and the opposite of penetrating damp.
4. Feel it.Efflorescence is dry and powdery. Hygroscopic deposits feel slightly damp or greasy and may look glassy rather than fluffy.
5. Check where it is.A fringe just above a tide mark points at ground water. A bloom on a chimney breast points at combustion salts from decades of fires. A patch under a window or beside a downpipe points at penetrating damp that is now drying.
6. Do not wash it off.Water redissolves the salts and drives them back into the wall, where they come out again next week — usually over a wider area. Dry brushing only.
7. Fix the water first if there is any.Efflorescence is a message that water has been moving through the wall. Find and stop the source, then let the wall dry, then deal with the surface.
8. Replace hygroscopically contaminated plaster.It cannot be sealed, painted over or washed clean. It is hacked off, usually to at least 300mm above the highest sign of contamination, and replaced with a specification chosen for the wall — which in an older solid wall should be lime rather than dense cement.

Tips

• A wall that reads soaking on a damp meter but is warm, has no mould and gets worse in summer is very likely salt-contaminated rather than wet.
• New brickwork and new render commonly bloom white in their first year. It is cosmetic and it stops.
• Salt-inhibiting additives and salt-retarding renders exist and have their place, but they manage a contaminated substrate rather than remove the salts.

Common mistakes

• Washing efflorescence off with water — It dissolves the salts and takes them back into the wall, from which they re-emerge over a larger area. Dry brushing removes them permanently once the wall stops delivering more.
• Painting over a salted wall — The salts crystallise behind the paint film and push it off, so the decoration blisters and flakes within a season and the underlying problem is now hidden.

Questions people ask