Rising damp? Penetrating damp? What's in a name and does the name even matter?

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 is 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 pores, running right through the material. These networks allow moisture to be quickly absorbed and dissipated in all directions. Many building materials 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 very narrow. Very narrow or closed pores either hold onto moisture so it can't spread, or simply form a barrier so moisture runs across the surface of the material. Most man-made building materials, including all cement based materials and machine made bricks and tiles, generally have pore structures at this end of the spectrum.

Why does the ability of moisture to spread within a wall matter?

Moisture from the water droplet that reaches a dense branching pore network, as found in most natural building materials, will be quickly drawn in all directions until it is spread so thinly, it evaporates within the wall and makes its way out through the pores as water vapour. Vapour can travel more quickly than liquid water, especially if the wall is warmed or any sort of breeze blows across the surface, so a wall will dry out much more quickly if the moisture can turn to water vapour rather than having to drain out as liquid water.

If moisture from the water droplet is confined by low numbers of pores that are very narrow or closed, as found in most man-made building materials, it will quickly waterlog the pores and only spread out slowly unless it encounters a crack to flow along – which will act rather like a miniature river. In these circumstances, the moisture cannot evaporate so must be drained away, otherwise it will sit in the wall until joined by moisture from more droplets to make an ever growing wet patch. To counteract this tendency, houses built from man-made materials such as machine made bricks bedded in cement mortar, usually contain drains within the outside walls to carry away the excess water that accumulates. These drains are commonly called cavities.

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 normally only travel upwards through a wall when the variable width pores in natural building materials become waterlogged, allowing the water to bridge the many numerous wide junctions that normally act as evaporation points between short capillary sections.

Water vapour has a very low density, making it very prone to travelling up through walls. While it will still readily travel horizontally, but will travel upwards rather than downwards through a wall at every opportunity.

If water vapour rising through a wall becomes trapped by materials with a poor pore structure such as cement render, or is cooled by cold weather, it will condense within the wall to reform liquid water – which will tend to travel back down through the pores again until it can evaporate once more. It is therefore possible for moisture within a wall to enter a cycle where it evaporates low down, rises up through the wall as vapour, then condenses higher up and trickles back down to start the cycle again. This is most likely to be a problem after old walls have been 'damp proofed'.

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 cannot either evaporate and escape as water vapour, 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 break any vapour-liquid cycles 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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