What are Hygroscopic salts?
Hygroscopic salts are water attracting in nature, they commonly present as either powder like salt deposits on masonry walls or damp patches on internal wall papered or painted surfaces.

Types of hygroscopic salts and how they find their way into our homes:

Chloride and nitrate – typically found in ground water and drawn up into masonry via capillary action (rising damp) due to some form of building defect, typically high ground levels, a broken drain or water pipe.

Sodium carbonate and ammonium nitrate – which are released by the combustion of wood, coal and other fossil fuels and therefore are the most common form of hygroscopic salts found in chimney breasts.

Calcium sulphated dihydrate (calcium salt) – the primary ingredient in modern gypsum plaster.

Calcium chloride – found in cement and ‘add mixtures’ which can manipulate the characteristics of cement mortar.

Vapour permeability vs. saturation
Salts become mobile when dissolved in water. When this happens within a masonry wall they move towards and concentrate on the evaporative surface. However, if that surface is not vapour permeable the salts block the capillaries, potentially leading damage at the face of the masonry or blistering of plastered or painted surfaces.

Lime plasters and natural paints allow water vapour molecules to travel through your walls without reaching saturation point due to moisture entrapment, which reduces the degree of salt mobilisation. Any salts that are mobilised are now able to pass through the natural materials and crystalise on, rather than below the surface, where they can simply be wiped away.

How to get rid of hygroscopic salts?
Please ignore the damp fraudsters who will want to hide the issue without addressing the cause. All this will do is hide the symptoms of the issue for a period of time.

Firstly, we need to address why the salts are becoming mobilised.

Salts in chimney breasts – is there air flow through the stack? Have the flashings, cowls (should be vented and not capped), crown and pointing been checked on your chimney?

Salts in walls – do you have a drainage or ground height issue presenting as rising damp? Has your render or painting failed and presenting as a penetrating damp issue?

What is lining the interior and exterior of your walls? Are they modern non-vapour permeable plastic or cement-based products that are creating interstitial condensation?

Without elevated levels of moisture, the salts will not mobilise.

Is this issue historic?
By this we mean, have the building defects been rectified and what you are now left with is an internal plaster system that is full of salts? Typically, hygroscopic salts will sit just below or on the surface at the evaporation zone. Even after the building defects have been rectified, the salts will remain and appear as wet patches, staining or efflorescence (crystallised salts).

Removing hygroscopic salts
All salt contaminated plaster and wall coverings should be physically removed. Where possible, the walls should be allowed to dry out before the application of a new lime based plaster system.

But for how long? Well, if you consider that a solid wall dries out at approximately one inch per month and the standard Cornish cottage wall is somewhere between 18 inches or so thick, then the ideal time would be 9-18 months, which is rarely possible.

We recommend a minimum drying period of four to eight weeks, dependant on the severity and level of water ingress caused by the original building defect. This provides a period of 1-2 months for the walls and the new lime plaster to find equilibrium. Once the lime starts to carbonate it immediately begins controlling moisture movement and evaporation.

If salts are presenting on painted masonry we recommend that the paint is removed and over time the salt migration will cease or educe due to the declining moisture levels.

And lastly, for salts presenting on bare masonry.. is the masonry bedded in lime or cement? If cement, then this will be causing the moisture to evaporate through the masonry rather than the pointing, which can lead to spalling of the masonry face due to blocked capillaries. If the salts are migrating through the lime pointing, then this is the perfect route for them to leave the wall.

There will be cases, for instance, where your home is built into a hillside, and due to the topography of the surrounding landmass there will be very little that you can do do cease the movement of moisture. In this case an holistic, broader view of moisture management should be taken, with a primary focus on natural, vapour permeable materials as well as sufficient heating and air exchanges to keep the evaporating moisture in the air until it can be vented externally.

Hygroscopic salts from fossil fuel burning presenting as a damp patch on the chimney breast in Falmouth.

Cement render outside, gypsum plaster inside = saturation point in this Camborne church. Salts now pushing through the gypsum plaster.

Penetrative damp cause by failed pointing – mobilising salts in masonry above a bay window.

Moisture and salt migration due to external cement render, high ground levels and inefficient drainage in Camborne.