Why walls become damp

If a droplet of water is placed on the exposed face of a traditional wall, two things will happen: 1) the droplet will be absorbed into the surface of the wall, and 2) it will spread out to form a small patch of moisture which is visible as a dark area on the surface. The original water droplet has now become a damp patch. The water from the original droplet will continue to spread out, making the patch grow larger but also causing it to fade.

Those areas of the moisture that spread upwards are often called 'rising damp' to make it sound special, while those areas of moisture that spread into the wall are often called 'penetrating damp' to make it sound worse. Strangely, the areas of damp that spread sideways along the wall (lateral damp), or that spread downwards (falling damp) are rarely mentioned by name despite being just as important.

A single water droplet will therefore produce all four 'types' of damp, making it unwise to focus on just one 'type' in the hope of effecting a magic cure-all. Although the above refers to a single water droplet, it is equally valid if the droplet is changed to a pool of water next to a wall, waterlogged soil under a wall, a leaking gutter or pipe, etc. – effectively these are really all just different sized water droplets!

Pores are the key

The speed and extent of the moisture spread from any water droplet, is determined by the pore structures of whatever the wall is built of.

At one end of the spectrum, mortars and renders based on clay or pure lime have dense branching networks of variable width coarse pores, running right through the material. These networks allow moisture to be quickly absorbed and dissipated in all directions. Many building components based on natural materials, including handmade bricks and tiles, generally have pore structures at this end of the spectrum – these sorts of materials were historically used to build all houses before their use gradually died out between 1900 and 1940.

At the opposite end of the spectrum, some types of stone such as flint and slate, materials used for damp proof courses, and much modern insulation, have extremely low numbers of pores which are also either closed at one end or too narrow to allow liquid water through. These types of pore simply form a barrier so moisture runs across the surface of the material. Most man-made building materials, including machine made bricks and tiles, generally have pore structures at this end of the spectrum.

Ironically, it is the pores found between these extremes that are particularly damaging. When you have reasonably dense networks of fine pores running through a material, the pores will suck in water and hold tightly onto it, causing rapid and prolonged saturation to occur. These are the pores found in all cement based products such as concrete, mortar, and render, and they also form a significant proportion of the pores found many modern versions of lime based mortars and renders.

Why are the pores so important?

Moisture from the water droplet hitting a wall with a dense branching pore network at the surface, as found in most natural building materials, will be quickly drawn in all directions, stopping it from saturating the coarse pores. As the water has spread over a large area, it will evaporate very quickly if the wall is warmed or any sort of breeze blows across the surface. A wall will dry out much more quickly if the moisture can turn to water vapour across the face, rather than having to drain down to the bottom and run out as liquid water. To cope with intense prolonged rain, a well designed solid wall will have a further trick up its sleeve. As well as the coarse pore network at the surface, the pores a little deeper into the wall will be partially closed off to restrict water entry. If the surface pores become temporarily saturated, the partially closed off pores hold the water back to stop it reaching the interior. When the rain stops, the saturated surface pores will quickly start to dry, bringing the rest of the water in the wall back to the surface.

If moisture from the water droplet is sucked into the wall by fine pores that hold onto it tightly, as found in cement mortars/renders, it will quickly waterlog the pores and only spread out slowly. In these circumstances, the moisture cannot evaporate so must be drained away, otherwise it will sit in the wall until joined by more moisture to make an ever growing wet patch. To counteract this tendency, houses constructed with cement mortars/renders, usually contain drains within the outside walls to carry away the excess water that accumulates. These drains are commonly called cavities.

When the pores at the surface of a wall exclude water, such as with painted walls, the water droplet will just run over the surface and down the wall unless it encounters a crack/split to flow along – which will act rather like a miniature river running into the wall. Once behind the surface, there is no way out, so depending on the pores in the rest of the wall, the moisture will either just sit there to form an ever expanding wet patch, or will migrate through the wall into the house.

An important difference between water and vapour

Regardless of whether the moisture in a wall from a water droplet is in the form of liquid water or vapour, it will spread in all directions via the available pore structure. However, depending on its form, it will be more prone to travelling in some directions than others.

Liquid water is relatively dense, so is greatly affected by gravity, making it prone to travelling down through walls. It will happily travel horizontally, but its preference is always to travel downwards whenever possible. Liquid water can only travel upwards through a wall when the pores become waterlogged, allowing the water to bridge the numerous wider junctions that normally act as evaporation points between short capillary sections.

Water vapour has a very low density, so it has a slight tendency to travel upwards over time. As the molecules move randomly though, it diffuses slowly in all directions unless assisted by some means, such as an air current.

Water vapour diffusing through a wall can be halted by waterlogged pores, or be cooled by cold weather. If this happens, it will condense within the wall to reform liquid water – increasing the size of the waterlogged area, or forming new areas of dampness. This is most likely to be a problem after old walls have been ‘repaired’ using cement mortars/renders/plasters, or if the outside walls have been insulated internally.

To cure the damp, forget about fancy names for it, just fix the pore structure

From the above, it should be clear a wall will become damp if the moisture from that water droplet is prevented from evaporating from the face of the wall, or if it isn't channelled to a suitable drain point. Any damp investigation must therefore build up a picture of the moisture pattern within the wall, and the nature of the pore structures possessed by the materials used to build and finish the wall. This will highlight the presence of inappropriate materials or missing drain points, and where the corrections must be made to release trapped moisture or stop condensation occurring within the walls. Only then can an appropriate plan to rectify the faults causing the damp be drawn up, to enable the walls to dry out again.

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