The introduction of external cavity walls in South Africa
South Africa has now legislated, in terms of the application of the National Building Regulations SANS 10400-XA: 2021 Edition 2 Energy usage in buildings, that cavity wall construction isnow mandatory in certain energy zones.
Background of cavity walls
The construction of buildings with cavity walls was first introduced sometime in the 19th century and became widespread by the 1920s. Initially, they were built without insulation, and cavity wall insulation was first introduced in the United Kingdom during the 1970s. It became compulsory in the 1990s for all new buildings.
So, why external cavity walls?
A reduction in the energy requirements in buildings through heating and cooling results in less demand for power supply on the national grid.
Although the installation of thermal insulation is not mandatory in cavity walls yet, it does not make sense to build a cavity wall without insulation, as it cannot be retrofitted once the wall has been constructed.
Testing cavity walls with and without insulation in the Rotatable Guarded Hot Box in 2011 has shown a substantial increase in the thermal resistance of the external cavity wall when insulated. It is then the conclusion that cavity walls with thermal insulation can make a huge difference to the overall thermal resistance of the building envelope.
The clear benefits of thermal insulation
Thermal insulation usually refers to the use of appropriate insulation materials and design adaptations for buildings to slow the transfer of heat through an enclosure to reduce heat loss and gain. The transfer of heat is caused by the temperature difference between indoors and outdoors. Heat may be transferred either by conduction, convection or radiation. The rate of transmission is closely related to the propagating medium. Heat is lost or gained by transmission through the ceilings, walls, floors, windows and doors.
This heat reduction increases the load on the HVAC system, resulting in more energy waste and reducing the thermal comfort of people in the building. Thermal insulation in buildings reduces unwanted heat loss or gain and can decrease the energy demands of heating and cooling systems, ensuring the comfort of the occupants.
Minimum total thermal resistance requirements for external walls
How cavity walls with insulation work
Cavity wall insulation is used to reduce heat loss or gain through a cavity wall by filling the air space with material that inhibits heat transfer. This immobilises the air within the cavity, preventing convection which can substantially reduce space heating or cooling costs.
Cavity wall insulation increases the thermal mass effect of the bricks and implies that the inner masonry leaf is at an internal, rather than external, temperature. When heating or cooling the building, the insulated walls help to maintain the desired internal temperature for longer and reduce the time and cost of achieving interior comfort.
Good thermal insulation such as expanded polystyrene (EPS), extruded polystyrene (XPS), mineral wool (MW), polyurethane (PU) and polyisocyanurate (PIR) insulation boards can also prevent moisture transferring through the inner leaf. As the walls are warmer, condensation on the internal wall surface is eliminated.