Understanding Moisture in Buildings
The theory of moisture transfer in buildings
Moisture can transfer in buildings through vapour diffusion, air movement, liquid diffusion (by capillary action) and gravitational flow. The air we breathe contains a level of water vapour. Inside buildings, this water vapour comes from breathing and from human activity (such as cooking, drying clothes, taking a shower). The physical amount of water within the air is referred to as its Relative Humidity (RH). Healthy levels of RH are between 35-65% and at 100% RH, the air is fully saturated and cannot hold more water. The temperature at which 100% RH is reached is known as the dew point temperature. Warmer air can hold higher quantities of water vapour than cold air, which means that the dew point is higher in a warm space than in a cold space. Considering a room with an internal RH of 70%, condensation is therefore likely to occur on the outside surface of a glass of iced water. Water in the air has not condensed, but at colder surfaces, condensation occurs. This same process can happen at the surface of internal walls, where there is a significant temperature difference between the internal wall surface and the room. This relationship is known as the temperature factor and to avoid surface condensation and mould growth, the temperature factor should be no less than 0.75.
So, moisture can transfer from the air to surfaces, through internal surface condensation. This can be caused by poor ventilation, cold internal surface temperatures and high humidity levels. Air movement through the building fabric is another moisture transfer mechanism. Where there are gaps and penetrations in the wall build-up (for example), air can pass through until it reaches lower temperatures. Where dew point temperature is reached inside the wall build-up, interstitial condensation can occur. Considering liquid water, transfer in buildings can occur through gravitational flow through leaks and gaps in the building fabric. Ground water liquid can also be transferred into a building, through capillary flow (liquid diffusion). This is where water moves from a region of high concentration (the ground) to a region of low concentration (the wall). Where no DPC is in place, rising damp can occur.
How can you control moisture in buildings?
If a building does not have problems associated with moisture, the building is considered to be in ‘moisture balance’. This means that vapour and/or liquid water within the property (including within the building fabric) is able to escape, permeate through the fabric, and/or dry out, before it becomes a problem. Such problems can manifest themselves in the form of mould growth, material degradation and structural defects. These problems can have further knock-on effects on occupant health, comfort levels, aesthetics and safety. It is therefore vital that moisture balance is maintained or delivered through retrofit activities. To do this, the Retrofit Academy recommend that the ‘Three C’s’ are followed. Considering firstly ‘Context’, it is vital that the existing building is understood. This not only means understanding the building in terms of its condition, form, construction and occupancy type, but also in terms of its location and orientation. For example, properties with large and exposed southeast facing elevations will have a higher risk of water ingress issues associated with driving rain.
When considering the moisture strategy, it is important to note that moisture related defects can occur as a result of poor weatherproofing, low levels of air tightness, inadequate ventilation or poor quality building fabric and materials. User profiles are also a significant contributor. A robust moisture strategy should therefore rectify existing ingress and/or moisture issues, by making repairs and enabling targeted areas to dry out (which can take a long time). Where wall insulation is installed, high quality retrofits require high levels of air tightness and robust detailing to avoid thermal bridging. This will significantly reduce the risk of interstitial condensation. The materials specified must consider the existing fabric and vapour permeable materials should be considered, particularly where there are existing levels of moisture. On a scheme delivered under PAS2035, it is the role of the Retrofit Coordinator to understand and challenge the moisture strategies proposed by Retrofit Designers. Internal conditions are also a key contributor to moisture issues.
It is important that internal spaces are evenly heated to adequate temperatures, which will reduce the risk of condensation and mould growth by reducing the Relative Humidity (RH) of the air. Relative Humidity can be controlled in several other ways, including through ventilation – which is a vital element of a successful moisture strategy. Ventilation can be continuous or demand controlled (reacting to internal RH levels). Hygroscopic materials and plants can also be used to passively control RH levels within a property. A successful moisture strategy must be ‘Coherent’ across the property, ensuring that all spaces are treated according to a central approach. The strategy must also ensure sufficient ‘Capacity’ is delivered, to protect against changing internal and external climates, and user profiles.
Condensation occurs when the level of moisture or water vapour contained within the air reaches saturation point. The temperature at which saturation point occurs is referred to as the dew point temperature. Where saturated air comes into contact with surfaces or materials that are colder than the air temperature, condensation occurs. Surface condensation relates to condensation occurring on surfaces where it is visible, for example on internal wall surfaces. If this is sustained over a period of time, without sufficient ventilation or ability to dry, damp related defects can occur. Interstitial condensation occurs where the due-point temperature is reached at points within the building fabric. For example, water visible in between the panes of double glazing is referred to as interstitial condensation. This occurs because air gaps have formed in the glazing seal, which allows air to pass into low temperature areas where heat transfer can occur. Similarly, thermal bridges in the building fabric can cause the same process to occur. Interstitial condensation can occur in wall build ups and cavities where it is not visible. This can cause damp related defects.