Política Monetaria y Mercados Financieros Internacionales

The resulting damage from moisture depends on the moisture source, the moisture quantity and the material type. A number of moisture sources can be identified in the building context. Some important examples are wind-driven rain and rising damp.

Wind-driven rain is rain that has a horizontal velocity component, given to it by the wind. It is a very important moisture source especially affecting the performance of building fac¸ades. A good overview of the state-of-the-art research on wind-driven rain is given by Blocken et al. [2].

When rain hits a building fac¸ade and this fac¸ade has a porous nature, part of the rain will be absorbed by the porous materials. The rain that is not absorbed, runs off. Water accumulating in the porous material may lead to various moisture related problems. When the envelope is poorly designed or constructed the rain can even penetrate to the inside.

Another important moisture source often found in constructions is rising damp. Rising damp in buildings occurs when water from the soil is absorbed in the porous wall by capillary action. This results in walls that are wet at the bottom. This can be avoided by installing a damp proof course. If however this damp proof course is poorly installed or in some way bridged, rising damp still occurs. A third important moisture source in buildings is the indoor environment. Water vapour present in the indoor air comes from various sources. People or animals in a room breathe out moist air and transpire water vapour, but also the presence of plants can increase the moisture levels in a room. Human activity also introduces water vapour in the room air. For example during cooking or showering large amounts of water vapour are produced and released to the indoors. Also the burning of fossil fuels for example for heating purposes releases water vapour. Besides carbon dioxide, water vapour is the most important combustion product when burning for example natural gas. Sufficient ventilation is the key aspect to keep the humidity indoors at acceptable levels.

High humidity levels result in condensation phenomena on cold surfaces. If the temperature of a surface is below the dewpoint temperature, water vapour in the surrounding air will condense on that surface. For example in winter poorly insulated windows are often subjected to condensation. Also thermal bridges (local areas that are poorly insulated [3]) are very sensitive to condensation. Interstitial condensation can occur when water vapour diffuses through the porous building envelope to the outside. If the temperature reaches the dewpoint temperature somewhere in the construction, the water vapour will condense inside the construction. This is a dangerous situation since the moisture and the associated damage stays hidden in the construction.

also be introduced during the construction phase (construction moisture) or by accident (leaking pipes). Some of the construction moisture can be avoided by keeping the construction materials dry during construction and reducing their exposure to outside (rainy) conditions to a minimum.

Moisture in buildings has a large impact on the building performance. Moisture in the building envelope can cause increased relative humidity in the indoor environment and the relative humidity is an important parameter for the thermal comfort in a building. If the air is too humid, it is perceived as uncomfortable [4,5]. On the other hand, a high relative humidity increases the risk of mould growth. If the surface relative humidity is above a certain threshold long enough, there is a risk of mould growth. Although there is still no general agreement on which criterion is most appropriate, in most cases an upper limit of 80%RH is prescribed. If the surface relative humidity does not exceed 80%, mould is most likely avoided. An overview of state-of-the-art mould prediction models and mould risk evaluation is found in [6]. Mould in buildings has an impact on the human health. Besides mould, other biological hazards such as insects and dust mites also thrive under high relative humidity (and temperature).

Another aspect that is influenced by moisture is the energy performance of the building envelope. Moist building envelopes, whether they are wetted by wind-driven rain, rising damp or condensation, will have a decreased heat conduction resistance. Damp walls, especially when the insulation is wetted, will act as a thermal bridge and increase heat losses through the envelope.

Moisture also causes aesthetic problems. Although these problems are not necessarily harmful for the building or the building occupant, they are still undesired. An example of aesthetic consequences of moisture is the possibility of soiling patterns on building fac¸ades. This is caused by wind-driven rain and the accompanying runoff.

Another example is salt efflorescence caused by water transport in moist concrete and masonry. These materials contain high salt concentration by nature and this salt can dissolve in the water present in the material. The salt can then transport through the porous material by diffusion and convection. If the salt solution becomes super-saturated, the salt will crystallize, resulting in salt efflorescence. The problems caused by the efflorescence of salt are not limited to aesthetics, but can also be structural. For example in concrete structural problems can arise. The continuous movement of moisture through the porous structure of the material will eventually wash out all the soluble salts and cause the breakdown of the cement matrix, leaving the concrete weak and sandy. Furthermore the alkalinity of the concrete will drop, and with it, the ability of the concrete to protect the embedded reinforced steel from corrosion. The naturally high alkalinity of good quality concrete is the main mechanism that prevents the corrosion of reinforced

steel by passivating the surface. Without protection, steel rapidly corrodes in the presence of moisture, ions and oxygen.

Salt crystallization also results in surface spalling of concrete. This is caused by the crystallization pressures [7]. A good description of salt transport in porous materials and the parameters that are involved, is found in Nicolai et al. [8].

Figure 1.1 shows some examples of moisture related damage. Besides the pure aesthetic consequences (e.g. Figure 1.1(a), salt efflorescence due to rising damp), moisture can also result in decay. For example Figure 1.1(b) shows the result of frost damage. Moisture for example coming from wind-driven rain or rising damp can wet a wall up to saturation. When the outside temperature drops below the freezing point, the water in the micropores freezes and expands, resulting in high pressures in the material. These pressures cause cracking. Alternating thaw and freeze cycle can eventually lead to significant structural damage.

Frost damage is however far from the only structural damage that can occur. Paint layers and wall paper can come lose or finishing layers such as plaster start to crack and crumble (Figure 1.1(c)). If the moisture content in wood is too high (max 20% by volume for safety according to ASHRAE [9]), wood can start to decay or rot. Finally metal elements corrode faster when moisture is present. For a lot of moisture related problems, moisture transport in air (convection) plays a major role. On the one hand insufficient ventilation can lead to increased air humidity and consequently increased moisture loads. On the other hand, drying of wet materials is to a great extend determined by the air condition flowing over the material. For these cases there is a strong coupling between moisture transport in the air and moisture transport in the (porous) material. Figure 1.1(d) for example shows the consequences of a poorly ventilated cavity wall. Here moisture was infiltrated in a cavity of a wood frame wall. The outside surfaces of this wall were finished with a paint layer acting as a vapour barrier, trapping the moisture inside the cavity. As a consequence mould started to grow inside the cavity. Since the damage occurred inside the cavity wall, the problem could stay hidden for a long time. If ventilation of the cavity would have been allowed, moisture could have been evacuated from the cavity as water vapour and the wall would dry out slowly reducing the risk of mould growth.

This short overview clearly shows that problems caused by moisture affect the occupants health on the one hand and the building durability on the other hand. A good moisture management in the building envelope is a prerequisite to avoid these moisture related problems. For a building designer it is however not always easy to deliver an adequate design, since moisture related problems are complex and difficult to asses in advance. Therefore, hygrothermal models can be a very useful tool to asses the impact of moisture on buildings. These tools can be used

to predict and evaluate the moisture performance of buildings and their envelope. However, for a good evaluation of moisture related problems, hygrothermal models would have to be able to:

• capture heat and moisture transport in a variety of porous building material • combine this with heat and moisture transport in the surrounding air • and apply all this modelling on complex geometries and strongly fluctuating

boundary conditions.

Most state-of-the-art (commercially) available hygrothermal models can handle some of these aspect, but only few combine all in a satisfactory manner. The next section will give a short overview of some of the currently available models.