Hygroscopic Salts

What does hygroscopic mean in terms of building materials?

All traditional building materials are hygroscopic. Hygroscopic means “water seeking”. So brick, plaster, stone and wood will naturally absorb water and the amount of water absorption will vary depending on humidity or vapour pressure. In old buildings with rattly old sash windows and open fireplaces and timber floorboards the internal relative humidity of the building will follow rises in external vapour pressure. If the internal vapour pressure is constant the internal building materials will be at equilibrium. However, if vapour pressure changes then the building materials will naturally take up or release moisture until the new equilibrium is established.

Where do hygroscopic salts in building come from?

Hygroscopic salts in buildings come mainly in the form of carbonates, sulphates, chlorides and nitrates. These salts can come from ground water, salts from the sea contained in driving rain, clean products, the spreading of salts during the gritting of roads, cleaning products, horse manure, fertilizer and most commonly from chimney soot.

What is the affect of hygroscopic salts in moisture related building defects?

As explained above hygroscopic salts will naturally absorb moisture and create damp patches over walls. These damp patches can appear as random localised patches over or nearby chimney breasts, where they have migrated into the plaster through condensation or falling damp penetration from a open chimney pot. The damp could also appear as a band at the base of a wall, where the brickwork has been affected by manure in the past or from the salts used to grit the roads. Below are several photographs taken during a recent inspection which clearly demonstrate how a electrical resistance moisture meter can be affected by soot from a chimney breast. The meter shows the brickwork covered in soot as being damp and the meter shows the brickwork above and below the soot band as being completely dry. All the brickwork is actually dry and the meter is giving a misleading where it comes into contact with the soot. It can be seen how an untrained homeowner or unscrupulous damp contractor could interpret these readings incorrectly due to lack of experience or on purpose. A recent case study where such a problem was misdiagnosed can be found HERE.

Damp reading above the soot band 9.4%

Damp reading at the level of the soot band 54%

Damp reading directly below the soot band

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