Understanding damp – the importance of pores

Whatever your walls are built of, they will contain pores. These pores form complex branching networks within the walls – as a result, when viewed in high magnification most walls resemble a sponge cake.

It is the moisture within these pore networks that determines how damp your walls are.

All pores contain moisture. At best, the only moisture in the pores of your walls will be water vapour – this is termed 'air dry'. At worst, the pores in your walls will be completely full of liquid water – this means they are saturated. A 'damp' wall is normally somewhere between these two extremes.

How does moisture get into the pores?

Most water vapour enters the pores of your walls directly from the air inside your home. It comes from your breath and activities such as cooking, washing and drying clothes. The main source of water vapour in most homes is your shower.

Some water vapour will enter the pores of your walls directly from the outside air and from pores in the soil surrounding the bottoms of the walls. The amounts are small though compared to that entering from inside your home.

If the water vapour inside the pores of your walls builds up, or the temperature inside some part of your walls is much lower than the room temperature, the vapour will condense within the pores to form liquid water. In many houses, this is the main source of water within the pores of the walls.

The only other way liquid water can enter the pores is by direct contact with a wall face. Typical examples are water running down the wall from a leaking gutter, windblown rain hitting the wall, water splashing onto the wall and water pooling against the foot of the wall.

Not all pores are the same…

Ideally, the pores in your walls should help moisture to travel out of your walls as quickly as possible.

For water vapour, it is just a case of the pores not restricting the vapour's movement as it travels through the network. Water vapour will always move from areas of high concentration to areas of low concentration. As the highest concentration of water vapour is inside your home, water vapour will naturally travel through the walls from the inside to the outside.

Unlike water vapour, if liquid water is left to its own devices it will sit in the pores to cause permanent dampness. To avoid this, the water must be actively drawn through the pores by capillary action to the outer face of your walls and released. While some of the water leaves by evaporation from the open pore ends, most of it is sucked from the pores by air moving across their open ends.

The pores in your walls can suffer from several possible defects. Some defects impede the movement of water vapour, causing it to condense within your walls. Some may also fail to draw liquid water to the outer wall face, or refuse to release water that does reach the wall face.

While the pores found in any particular type of material are all broadly similar, the pores in different types of material can vary markedly. This means if any material within or on your walls is changed, it can dramatically affect moisture entering, leaving and travelling through the pore network.

The following tables give examples of both desirable and defective pores and the types of materials they are found in.

Desirable pores

Pore type Comments Where they are typically found
Coarse (typical diameter 0.01mm). Nearly ideal pores, but they draw water to the wall face relatively slowly. Bricks fired at low temperatures (typically old hand-made bricks), carrstone, sandstone, limestone.
Fine (typical diameter 0.001mm). The optimum – they actively draw water from adjacent coarse pores, quickly transport it to the wall face, then readily release it. Traditional (non-hydraulic) lime based plasters/renders/mortars, clay plasters, earth mortars, clay lump, clay daub, clay renders, limewash.

Undesirable (defective) pores

Pore type Comments Where they are typically found
Ultra-fine (diameter 0.0002mm or less). These impede the movement of water vapour, suck liquid water in and don't release it. Cement based mortars/renders, concrete, hydraulic lime, many 'breathable' paints.
Very large (diameter 0.1mm or more). Water vapour can pass through unimpeded, but liquid water just collects in the pores, saturating them. Mineral wool/glass fibre insulation, aerated concrete products such as insulating concrete blocks.
Pores treated with an anti-wetting agent (any pore type). Provided the pores are large enough, water vapour can still pass through successfully, but the ant-wetting agent stops water reaching the wall face, thus preventing it being released. Any wall subjected to a chemical damp proof course or water sealant, silicate masonry paints, many proprietary insulating renders.
Pores with dead ends ('closed pores'). The dead ends cause water vapour to condense. Liquid water just collects in the pores, saturating them. Plastics, sheet metal, polyisocyanate thermal insulation, many paints, rubberised coatings, slate, flint, bricks fired at very high temperatures.

To sum up…

If your house suffers from damp walls, either more moisture is entering the pores than they can cope with, or something is stopping the moisture from leaving the pores.

It is generally easiest to see if too much moisture is entering the pores so check for this first. Is there any plumbing that could be leaking near the damp area? Do you have large pools of water in the mornings on window sills or extensive mould on walls, both of which indicate your home is overloaded with water vapour that needs venting. Next time it is raining properly, check outside for gutter or downpipe leaks, leaks around windows or doors, and water collecting next to the walls or splashing onto them. All of these would need fixing even if they are not the cause of your damp. However, the chances are, if you find and fix any of these problems, your damp will gradually fade away.

If you failed to find any obvious problems, you will need to try and identify what has happened to stop moisture leaving the pores. This is likely to be due to a change of material when some work was carried out on the walls. Sometimes the work was done within the last 5 years so is easy to identify (e.g. installing cavity wall insulation), more usually you have to go back around 20 years to find the problem (e.g. cement render or repointing), but sometimes the problem occurred 50 or more years ago (e.g. painting the outside walls). Unless you can readily identify just a single source of defective pores, which should then be replaced with something more compatible with the original wall, you will probably need professional assistance to work out what has gone wrong with the pores and the best way to remedy the problem.

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