Instead of installing ventilation systems in the building, the ventilation can be provided by thermal insulation between the two layers of the façade. There is no need for double façades on all sides of every new building. Under certain circumstances, however – e.g. high levels of street noise, high wind loads or increase in building height – such façades may be the appropriate and economical solution.
From a structural point of view double façades consist of three functional layers. Typically the exterior façade layer is made of single glazing. It is separated from the interior glazing, which in turn forms the room enclosure and usually consists of double glazing. Depending on the specifi c design, the distance between interior and exterior façade layer can vary. In order to utilise the effect of a thermal buffer in the space between the two façades, ventilation openings are installed in either one of the exterior and interior façade or in both. The air in the gap between the façades heats up due to solar radiation and hence serves as a buffer toward the interior space. Due to the thermal difference, the warm air can be used as a generator of natural ventilation of the interior room or the space in between the façades.
Double façades are most suitable for protecting the interior space from noise sources such as highly frequented streets. However, when designing a double façade as sound insulation for a specifi c project, we need to examine the different methods of construction.
The first variant, known as the second-skin façade, is obtained by adding a second layer of glass over the entire outer surface of the building. This has the advantage of technical and structural simplicity and the fact that it does not involve a large number of moving parts since the outer layer of glass is simply mounted on the inner façade structure and ventilation mechanisms only have to be provided at the top and bottom zones of the façade. The disadvantage is that it offers few possibilities of controlling the interior environment of the building; there is thus an attendant risk of overheating.
The exterior façade is ventilated through openings at floor and ceiling level. The vents can be closed during winter to make use of the greenhouse effect and to increase the thermal protection. In summer, the façade flaps can be opened to prevent overheating. The limited number of ventilation openings ensures good sound insulation from the outside but, within the façade, entails the risk of sound propagation from room to room. Fire protection is another critical issue because, in case of fi re, the smoke spreads quickly throughout the space between the façade layers.
The second variant embodies the above-mentioned principle of the box window, by including storey-high façade elements in the system, which individual users can open at the top and the bottom. The advantage of this model is the freedom the system gives individual occupants in controlling their own internal environment. The disadvantage is that the freedom given to one occupant may have an adverse effect on the conditions experienced by another, since e.g. the exhaust air from one floor can influence the quality of the incoming air on the floor below. This problem can be avoided by staggering the ventilation inlets and outlets.
Horizontal as well as vertical separation from adjacent elements ensures optimum sound insulation not only from the outside but from neighbouring offi ces as well. Unpleasant odour and flashover can be prevented rather easily if the compartmentalization is designed correctly. Thermal shorts, meaning exhaust air from a lower element flowing into an element above, can be avoided by offsetting the supply and exhaust openings from storey to storey.
To deal with the problem of interference between the ventilation systems at different levels, the third variant – the corridor façade, with staggered air inlets and outlets – was developed. This used vertical baffles in the space between the two skins to prevent horizontal flow of air that could give rise to noise interference between neighboring rooms. It is, however, not always possible to install these baffles since this type of façade depends on the presence of horizontal connections.
Air inlets are located near the floor and the ceiling. They are arranged at an offset to avoid thermal shorts by exhaust air mixing with fresh air. Separating the individual storeys from each other effectively prevents overheating that can occur at upperlevel storeys when the air is led across several storeys. This corridor is accessible and is typically designed to be wide enough to be used as a service platform. The space between the façades is ventilated through openings at ceiling level. The airfl ow can be regulated by motorised flaps.
The most effective version of the double façade, but that involving the greatest constructional and control-engineering effort, is undoubtedly variant number four, the shaft-box façade. Discrete box windows or other façade elements release their exhaust air into a shaft mounted on the façade and extending over several floors for greater thermal efficiency. The height of the shaft means that a stack effect ensures vertical motion of the air in the shaft, hence enhancing the efficiency of the system.
The exhaust air can be extracted from the façade gap mechanically. However, the required fan performance would be very high, which usually renders this method uneconomical. During winter, low ventilation increases the buffer effect but this can result in condensation forming on the interior side of the outer glass pane when the interior façade is open.
The double façades described above do not offer complete solutions to the problem of variable ventilation requirements. One approach to this problem was the development of alternating façades. These are basically single-skin façade constructions that can be converted locally to double façades by the addition of a second skin. The objective here is to combine the benefits of the simplicity of the single-skin façade with the buffering effect of the double façade.
Because the double and single façade areas alternate, in winter, warm air can be drawn from the façade gap of the double façade to supply adjacent offices with fresh pre-heated air, thus reducing the energy demand for ventilation. In summer, the single façades provide natural ventilation when very warm air from the double façade sections can cause problems. The space in between the layers of a double façade can be ventilated by opening ventilation flaps so that adjacent rooms are not overheated. Alternating façades can be realised as storey-high façades as well as in-line or fenestrated façades.