High rainfall regions can experience condensation on the inside surface of the building shell during cold weather. This is commonly visible on glass doors and windows but can also be found on the walls themselves. In addition to causing damage, the continual dampness results in mould growth and many other health issues.
The winter rainfall regions of the southern and western Cape are particularly susceptible to condensation, due to the differential between heated indoor air and the icy cold outdoor temperatures. An important revision to National Building Regulations is expected to address the problem through changes to SABS 10400-XA regarding standards for masonry walling,roofing and ceilings, flooring, shading and windows.
Technical experts from the Clay Brick Association of Southern Africa (CBA) have been closely involved in the drafting of the new standard. Mariana Lamont, Executive Director of the CBA explains. ‘No matter how thick the wall or what it is made from, condensation will always be a problem if there is only a single walling layer or “leaf”. Cavity walls have been used since the 1800s to naturally insulate buildings in cold, wet climates like the UK and Europe.
‘The new SANS 10400-XA thermal performance requirement for masonry walling is based on the location, the type of building and the hours of day the building is used. The option is either a cavity wall (with or without insulation) or a composite wall with insulation. ‘In South African Energy Zones 1,2,4,6 and 7 there are new requirements for concrete and clay brick external masonry walling. A minimum 50mm uninsulated cavity will be mandatory. For Energy Zones 3 and 5, solid external masonry comprising two masonry leaves (without a cavity) will be sufficient. Cities in Zone 5 include Durban, Pretoria, East London and Polokwane. ‘The new regulations ensure thermal comfort inside a building without additional heating and cooling’.
What is a cavity wall?
A cavity wall is composed of two masonry walls separated by an air space or cavity. The result is a watertight, energy-efficient masonry structure appropriate for external load-bearing walls. Between the two masonry leaves, the cavity must be a minimum width of 50mm and consistent from the bottom of the wall to the top. The cavity can be partly filled with insulation which is attached to the inner leaf. Insulation provides additional thermal performance. Weep holes are drainage holes left in the exterior leaf of the cavity wall; traditionally non-mortared gaps left between two adjacent bricks. They provide an exit point for water condensing in the cavity, or if rain enter through gaps in the roof, window frames or masonry.
Purpose of a cavity wall
Cavity walls offer two key benefits:
1. The cavity prevents water condensation from penetrating through to the inner leaf.
2. If walls are made of a high-density masonry material, like clay brick, it provides extra protection from temperature extremes and noise; even without additional insulation within the cavity. By building cavity walls, homeowners reduce energy use and save on heating and air-conditioning bills
Where does indoor condensation come from?
Think of your favourite ice-cold beer! You take it out the fridge and within minutes water is dripping down the outside of the can and pooling on the table. Condensation is a result of cold beer inside the aluminium can or glass bottle, meeting warm air outside the container. Thicker glass slightly delays condensation but won’t prevent it. Similarly, you may have noticed that in cold weather, windows mist up on the inside and form water droplets that run down the window and damage interior paintwork. Any solid, single leaf wall – even one made from thick concrete – will experience condensation. Condensation weeping inside the house causes water damage and encourages mould. With a cavity wall, condensation still forms, but it gathers in the cavity behind the first wall and never reaches the second interior wall – and the house remains warm and dry.
The Clay Brick Association of South Africa
+27 (0) 11 805 4206