Sandwich wall systems - for a sustainable future
Sandwich wall systems of precast concrete are frequently used in many countries for the building of residential housing as well as commercial buildings. With the current growing focus on environmental and energy issues, the advantages of the sandwich façade are further increased. In addition of the many benefits related to cost-effective production and energy efficiency, the modern sandwich wall also offers a great variety of different surface alternatives. Dr. Gösta Lindström is Technical Manager at Strängbetong in Sweden. At the 19th BIBM Congress and Exhibition in Vienna in May this year, Dr. Lindström spoke warmly and very interestingly on the subject of the sandwich wall system as the wall system of the future. "The essential functions required by the construction products directive (CPD) are all well provided by the sandwich wall - stability, fire protection, hygiene, health and environmental care, safety in use and accessibility, sound insulation, energy saving and thermal insulation", he describes.
The sandwich wall structure
The sandwich wall consists of two concrete panels separated by a layer of thermal insulation of a thickness selected as required due to location and climate. The internal panel is normally load-bearing with a thickness in the range of 120 mm - 150 mm, depending on the design situation. In a Nordic climate, an insulation thickness in the range of 100 mm - 200 mm is normally used. The exterior panel of concrete has a thickness of 75 mm - 80 mm. The external panel is connected to the load-bearing panel by specially designed connectors made of high quality stainless steel to ensure a long life span for the façade of the building.
There exists today an EU directive on the energy performance of buildings that will affect nearly all EU citizens. The perhaps most visible changes this has brought to European energy issues is the energy performance certificate. This certificate is required whenever buildings are sold, rented, or new buildings constructed. The EU has allowed member states three years (from January 2006) to implement this part of the directive. In addition to the certificate, many countries have chosen to set maximum figures of energy use for buildings. For good energy performance, the three most important parameters are thermal insulation, air tightness, and thermal mass. All vital characteristics of the sandwich wall panel.
Thermal mass and energy saving
It is well known, that increased thermal mass of a structure reduces both the heating and cooling energy consumption. In addition, a thermal massive structure reduces the temperature variation. During the hot summer period the maximum indoor temperatures are significantly reduced, when massive structures are used. In many areas, the buildings would not be usable without massive structures and/or mechanical cooling. In the northern latitudes, the maximum temperatures are not that critical, but the thermal mass clearly improves the comfort of the building and reduces the need for cooling. The energy savings are both on heating as well as cooling and depend on the type of building, the magnitude of free energy released from persons, electrical equipment inside the building, and solar radiation. The major emissions of CO2 from buildings are from the energy use in service life due to heating and cooling. By utilising the thermal mass the energy use compared to a light weight structure with the same U-value* is significantly reduced. As still a major part of electricity is produced by coal that in many parts of the world results in a large contribution to the CO2 emissions, even a small reduction of electricity use directly advances the improvement of the environment.
In the early development of the sandwich wall systems, aesthetic and architectural demands were not always given enough attention and the façades were optimized only from a production point of view. This resulted in some areas with apartment buildings with too much repetitiveness in the façade expression. However, the modern sandwich wall system of today has been developed under the influence of architects and there are now many alternatives that provide a long service life and low maintenance costs. The concrete can be made with different cements and aggregates. The surface can be treated with special colour, a retarder combined with washing, sand blasting, acid treatment, or many other expressions. Also ceramic tiles, brick tiles, and natural stone can be used.