Exploring dampness

Traditional masonry structures take in and give out moisture – they 'breathe'. Some building elements, for example masonry external walls, are designed to absorb moisture during rain and release it later by evaporation. This cycle of change in moisture content can be harmless, but it can be problematic if high or low levels of moisture are allowed to persist.

A growing problem is occurring with retro-fit cavity wall insulation in cavity wall voids, where rain soaked outer leaf walls transfer moisture into some types of insulation material, resulting in the further transfer of dampness through the inner leaf wall. It is also a growing problem in areas that are prone to flooding where buildings become very difficult to properly dry out as a result of soaked cavity wall insulation. The flooding will also cause excessive sag to a range of fibrous types of insulation within the cavity void.

In the UK, no porous materials will be completely dry because the air always contains some moisture, and the porous material will take in some of it via various physical or chemical processes:

• Vapour pressure diffusion: when air on one side of a wall is at a different vapour pressure (i.e. moisture content) to the air on the other side, moisture moves through the fabric by diffusion until the two sides are balanced. There are seasonal variations in the relative internal and external vapour pressure. In winter internal vapour pressure is likely to be higher, causing moisture to move from the inside to the outside of the envelope. The opposite is likely to be in summer. 
• Capillary attraction (capillarity): put water in a test tube and you will see that the surface of the water curves upwards where it touches the glass. Do the same in a capillary tube, which is very narrow, and the curvature (the meniscus) is so pronounced that the water seems to be dragging itself up the inside of the tube. This process, known as capillary attraction, is related to the surface tension of water. The strength of the 'attraction' depends on the material. Strictly speaking, capillary attraction is movement under the influence of the downward force of gravity. However, some construction specialists refer to capillary attraction as moisture that may be pulled not just upwards but 'in all directions'. For example, penetrating dampness may arise where water is drawn into the building via small gaps between wettable surfaces at window frames.
• Hygroscopicity: hygroscopic substances readily absorb moisture from the air or from other substances. Hygroscopic salts (e.g. calcium chloride) try to achieve equilibrium with their surroundings by taking up or releasing moisture, and are therefore both a potential source of moisture and a way to transport moisture within a material.
• Diffusion – solid/liquid (Fick's law): Fick's law expresses the diffusion of liquids and solids in mathematical form. Moisture moves from a centre of high concentration to low to achieve a balanced concentration. This may be the mechanism whereby moisture moves upwards in masonry against the forces of gravity (rather than by forces of capillarity, which would require connected voids or fissures).
• Deliquescence: this is where a substance absorbs atmospheric moisture until the material dissolves and forms a watery solution.
• Osmotic pressure: this is the diffusion of a solvent through a semi-permeable membrane into a more concentrated solution, equalising the solution on each side. The effects of dampness caused by osmotic pressure are rarely seen, but may occur, for example, in concrete solid floors where salt-rich soil water is transported through a semi-permeable membrane or layer within the old floor. The porosity structure found in older concrete floors can create semi-permeable membranes, while in newer concrete floors, perhaps using larger aggregates, the larger porosity structure enables more evaporation. Also, most modern solid floors have a damp-proof membrane, which prevents groundwater from reaching the upper surface.
• Another example of this effect is where an older solid floor has been overlaid with an epoxy or thermoplastic type of floor covering and small blisters begin to form within the floor covering. They result from dissolved solute molecules reaching an equilibrium (over time) on both sides of the semi-permeable membrane.
• Wind pressure: moisture adhering to a building's surface by surface tension may be driven in any direction by wind, sometimes forcing the moisture to enter the material. Wind may also create pressure differentials to help pull moisture through the external envelope detailing.
• Gravity (downward movement): gravity causes rainwater to fall, helps water run downward on surfaces, and causes air movement where hot air rises. Water may collect on flat or dished surfaces, so gradients are designed to help direct water to a point of discharge. Change in air density can cause air convection movement. This can move moisture in the air from one part of a room or house to another.
• Example: Rainwater streams down the face of a cement-rendered exposed elevation and penetrates the building via a gap between a cement fillet and the main wall, at the abutment of a lean-to roof.
• Gravity – hydrostatic pressure (upwards and laterally): pressure created by a head of water, for example in a basement where the water table is well above basement floor level.