Dampness

Dampness is the presence of unwanted water or moisture in or on building elements or materials.

The source of dampness can be from condensation, from leaks, or from anywhere that water can attach itself to building materials. Hygroscopic materials are most susceptible to damage from dampness retention ability or the evaporative process where the building materials try to purge the dampness to achieve humidity equilibrium (the same humidity rate) with its surrounding.

It is therefore important for building element to be designed to be free from dampness or, if dampness does occur, will be inert to it and able to dry itself quickly.

Different building materials react to dampness differently. Materials such as mineral fibre boards (MFB) may expand with dampness. The expansion results in a volumetric increase in size and mechanical force strong enough to cause the MFB particles to break free from its adhesive. In simpler words, it disintegrates.

Bonded timber such as plywood can loose adhesion between the plies caused by the absorption of water into the timber fibres because of the expansion of the cell walls that binds to the adhesive.

Timber elements and furniture may expand and eventually disintegrate due to the cyclic wetting and drying cycle that ultimately damages the natural timber fibres. Timber is considered saturated above 18% water content measured by a protimeter device.

Image: Basic protimeter device is calibrated for timber and works by measuring electrical conductivity between the two prongs.

Bricks and plaster become saturated at 12% and 4% respectively when measured with the same device. Despite that, dampness within these porous materials causes soluble salts to liquify and be transported toward the surface through an evaporative process. Evaporation happens as bricks and plaster surfaces are exposed to the sun, and the water within them migrates out.

The mineral salts will be left on the surface while pure water evaporates, and shows up as white powder or crystals against brick and plaster surfaces. This is known as efflorescence and will normally wash off with water or rain. In certain conditions, when the salt containing large amount of Calcium carbonate (CaOH) migrates to the surface and came into contact with Carbon dioxide from the air, the precipitate forms Calcium Carbonate (CaCO3) that attaches strongly to the surface and can even build up in layers over time.

These are called calthemite (refer to the image below) and have the same composition as stalactites found inside limestone caves.

https://en.wikipedia.org/wiki/Calthemite#/media/File:Calthemite_flowstone_on_concrete_water_tank.jpg