Why does mould appear on the wood that houses are clad with on the outside

A problem sometimes faced by those who have wood-panelled houses, particularly with panelling, is the appearance of mould. As well as looking unsightly, wood can rot if steps are not taken to protect it and remove mould. Given this risk, I'm sure many of you are wondering why wood continues to be chosen by architects for their projects? And why does it only appear on some houses? I was talking the other day with a friend who builds houses in Norway. He was telling me that there, the vast majority of houses are clad in wood, but he never sees mould on it. By contrast, in Romania there are more where black mould stains are a regular presence than nice clean ones. I was wondering what the cause is, why mould occurs so frequently here? From my point of view there are several causes that lead to this result, the most important being wrong technical solutions that let water stagnate on wood and lack of ventilation.

Wetness - water stagnates on wood

Mould is a fungus and like any fungus, it prefers moisture. In fact, there are many types of fungi that cause mould and they all multiply by spores. The wind carries the spores in the air and they end up on different surfaces - stone, marble, wood. When they find a favourable environment, i.e. persistent moisture, they settle and grow. Green, black or blue spots appear, depending on the type of fungus, first as dots from place to place. In a favourable environment, the spots become blotches and develop until they join together, covering the whole surface. By growing, the mould keeps the environment moist, so the wood begins to decay, allowing the fungus to penetrate deeper. If left unchecked, the wood degrades further and further until it can no longer be saved.

In order for mould not to grow, water must be able to drain from the surface of the wood, not accumulate and not stagnate. Water stagnates on wood veneer due to incorrect installation of the wood/timbers or incorrect processing. If you have the curiosity to examine an old peasant fence you will see that the planks are machined at an angle at the top (they have a point) and the edges, both top and bottom, are slightly angled. This machining prevents water from accumulating and allows it to drain away.

Problems often occur at the base of the construction. That's why solutions must be adopted that do not allow water to accumulate. Wood used for cladding should not come into contact with the ground, as this is the surest way to damage wood. Also, the minimum distance from the ground should be at least 30 cm to prevent rain splashes from splashing on the wood after they hit the ground. However, if the wood reaches very low down, it would be good to have large canopies or eaves to protect it as much as possible. Last but not least, the wood used should be treated (by autoclave impregnation)or very well finished and regularly maintained.

Ventilation - wet wood cannot dry quickly without ventilation

Even if the water runs off, the wood remains wet in its wake. To make the drying time as short as possible, air needs to circulate. Air circulation is more important than temperature. Even at high temperatures, if air is not circulating, the drying rate is slow. If the air does not move at all, even if it is warm, streets will stay wet for longer after rain, compared to days when the temperature is lower but windy.

In the case of wood cladding of houses there must be space between the wall and the wood so that air can circulate. Under no circumstances should the panelling or planking be glued directly to the wall. It is mounted on a wooden or metal frame that clings to the wall of the house. This creates a space between the panelling and the wall through which air can circulate. This is the principle of ventilated facades, a popular way of insulating and finishing the outside of houses these days. Unlike the classic system, called the continuous or wet system, where all the layers (insulation, plaster, finish) are superimposed on top of each other and are attached directly to the wall of the house, in the ventilated facade, there are metal frames between the wall-mounted insulation layer and the facade, creating the ventilation space. On top of them, metal, plastic, glass or other materials are mounted to the facade. On the insulation, on the outside, anti-condensation, anti-wind and anti-insect film is mounted. It is a modern system which, in addition to providing thermal and sound insulation, allows the choice of highly aesthetic architectural solutions.

To avoid incorrect fitting, always consult a specialist. Successful work is the result of well thought-out design and calculations by experts. True, it may be more expensive, but avoid problems that can cost you much more.

Wrong assembly of the panelling

The appearance of dampness that favours the presence and growth of mould can be the result of the wrong installation of the wood panelling. The panelling can be mounted horizontally or vertically. If the panelling and the tongue and groove is processed, the tongue side should always be facing downwards, thus avoiding water penetration and accumulation. When the cladding planks are fitted by overlapping, the lower lamella will always be covered by the upper one. In order to avoid staining of the wood and rust, the metal elements used should be stainless steel or aluminium.

There are systems that make it easier to fix and allow quick replacement of blades if they have been damaged. In this case, rails and flexible clamps are used instead of screws and nails. The rails are mounted on the wall and the clamps on which the panel is fixed are inserted into the rail channel. The paneling, which has millings on the back for clamping, will be fastened one after the other by the clamps that slide onto the rail. The system is safe and precise, allowing good ventilation of the facades and quick replacement of the panelling if necessary.

Lack of protection can lead to mould on wood

To increase moisture protection, wood used outdoors must be protected with special products. Can be used oils or outdoor varnisheswhich in addition to moisture protection also provides UV protection. This changes the natural colour of the wood over time, which turns grey. Wood that is properly dried and does not create conditions for mould growth will, under the action of the sun, take on that light bluish grey colour. If the humidity is high and the wood cannot dry out, mould will appear and the grey colour will darken, sometimes turning greenish.

More resistant than oiled or varnished wood is wood impregnated in an autoclave, under pressure or vacuum. It can be bought as such from building material companies or taken to specialist companies for treatment. With such treatments, the protective solutions penetrate deep into the wood and ensure long-lasting protection.

Even when wood is treated, mould can occur if the conditions of increased humidity are met. Mould can also develop on varnished surfaces, on PVC panelling, as it does on stone or marble. Therefore, in order to avoid the development of mould, proper drainage must be ensured so that water does not accumulate, air must circulate and construction solutions must be carefully chosen.