Airs Moisture Content And Condensation Risk

The lower the temperature, the less water vapour air can hold. At 20 °Cair can hold 14.8 g/m3 of water vapour, while at 0 °C it can only hold 3.8 g/m3. If the internal airat 20 °C only holds 3.8 g/m3, it can pass through the wall to outside air at 0 °C without any condensation being formed; but if the air is saturated with 14.8 g/m3 then there can be condensation within the wall of 11 g/m3. In normal circumstances a room contains about 5-10 g/m3 of water vapour, whilst, for short periods, a bathroom can contain almost14.8 g/m3.

Major condensation problems can occur around air leakages in walls and roofs, especially in winter when the outer parts of the walls are colder. At the same time, moisture diffusing through unbroken layers of materials normally takes place without large amounts of condensation being formed inside the wall. A wall completely free of small cracks is unrealistic, so it is necessary to take certain precautions using the following principles:

• Vapour membranes.

• Moisture buffering materials.

Vapour membranes

The humidity strategy and use of vapour membranes must be seen in conjunction with air tightness. They are most often combined in one single layer, mounted continuously and without perforations, immediately behind the interior surface cladding. Whilst the vapour membrane should ensure that humidity cannot penetrate the construction, the air barrier should prevent cold air from entering, which, in turn, encourages condensation.

The vapour barrier is highly vapour-proof and is now a widely used approach. The theory is that it should totally prevent water vapour from entering the wall; all indoor vapour should instead be controlled and extracted by a ventilation system. In practice, however, this method has certain weaknesses. The only usable materials for this purpose are plastic sheeting or metal foil. How long these materials will last is an unknown factor. During the building process, rips, holes and such will inevitably be caused. At these points, small amounts of vapour will creep through, and after a time condensation will occur in the wall.

A more moderate and less vulnerable solution is a vapour retarder that limits vapour diffusion. This is not as absolute as vapour-proofing, but reduces penetration considerably. Materials used for this are high-density fibreboards and different types of robust cellulose and plastic-based sheeting. The choice of material is determined by the type of wind-proofing used on the outside of the wall. A rule of thumb is that the resistance to vapour diffusion on the inside must be four to five times higher than the wind-proofing layer on the outside to give the vapour a direction (Adalberth, 1998). In this type of wall, humidity can travel through without the risk of condensation. It is important to note that the windbreak's resistance to diffusion is often heavily reduced if it is damp-down to 10% of its original value in the case of a porous wood fibreboard. It is therefore often possible to use the same material on both sides of the wall.

The combined vapour membrane/air barrier is a critical element in a building envelope. The material must be robust and long-lasting; and all holes in it must be avoided through careful detailing as well as very conscientious execution on site. Where there is thick wall insulation, this is often aided by not placing it immediately behind the wall surface, but behind the first layer of say 5 cm of insulation. This reduces the risk of later holes being made when users hang pictures or other items. At the same time, many of the pipes and the electrical conduits can be placed in this inner layer so that they too do not have to go through the membrane.

Membranes are often jointed with tape. This may well become brittle after only a few years. It is much better to overlap the membrane sheets and nail them to the underlying structure with a thin wooden batten covering the whole length of the overlap. Taped solutions can fail after as little as 1 to 3 years (Bohlerengen, 2002).

Moisture buffering materials

Many materials used in building interiors are hygroscopic. This means that they have the capacity to absorb some humidity and then release it later when there is less humidity inside the building. In this way they can deal with excess humidity and even out the fluctuations.

A stable humidity level indoors is healthy as well as reducing loads on the vapour barrier. Untreated wooden panelling in a bathroom is an example. When the bath is being used, the panelling absorbs much of the water vapour produced. Afterwards, when the bathroom is left, the air dries out quickly as a result of the background ventilation. The panelling then releases the absorbed moisture back into the air of the room. The whole process takes place without the need of any mechanical ventilation and therefore also has a substantial energy-saving effect. A similar situation is created when the occupants of a house go to bed or leave for work: the moisture content in a living room with hygroscopic walls will be stabilized. Even if the temperature often falls during this period, the process still continues.

The use of hygroscopic materials is also proving to be important for construction and insulation materials. This is because there will always be some uncontrolled moisture leakage, either from outside or inside, during a building's life, both due to defects, ageing and movement of materials that may be caused by differential settlement, wind or even earthquakes. In addition, these will often be invisible. It is, therefore, a great advantage if all of the main materials have some hygroscopic ability. When damp occurs they will be able to regulate it and avoid mould or rot damage. In dry summer weather, they can return the moisture to the environment. Insulation materials with good hygroscopic qualities include most of the plant based types, whilst mineral wool and plastics based types are almost completely without hygroscopic capacity. Structures of timber and earth are very hygroscopic, concretes much less so and metal ones not at all.

shown that this effect takes place naturally whentheventilationductsinthebuildingfail for some reason. The effect is called gas diffusion - not to be confused with lack of air tightness.

The vapour membrane layer, even if very open to moisture, is always air tight. Gas diffusion occurs following the principle that gases always tend to spread themselves evenly in the surroundings. In a wall open to gas diffusion, the different gases travel in both directions - oxygen coming in and arbon dioxid going out - the speed determined by pressure conditions and the molecular weight of the gases involved (see Chapter 4).

Tests have been carried out in Finland on stud wall constructions with a vapour retarder and wood-based insulation materials. They showed that considerable amounts of water vapour molecules (H2O) migrate through the wall when air change rates are reduced to a minimum; and this happens without the risk of moisture damage (Simonson et al., 2005). There is also considerable diffusion of other light molecules such as CO2 whilst movement of heavier molecules, including some pollutants such as organic solvents, is slower but still occurs. At the same time, oxygen is seen to migrate in the opposite direction, from outside inwards. For example, a 20 cm thick brick wall with an area of 10 m2 lets in about 90 litres of oxygen each hour under normal pressure. This is the equivalent of one person's requirements in the same period. The conditions for this calculation are that the oxygen content for outside air is 20% and for inside air it is 15%.

By increasing the porosity of the wall surface the 'breathing' of gases will increase and the heavier molecules will also diffuse more easily. This involves increasing the permeability of the wind-barrier and vapour membrane. This requires good control of the temperature and pressure conditions, which can be facilitated by placing a building within as semi-acclimatized envelope such as a glazed outer zone.

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