In the aftermath of the energy-crisis of the seventies different systems were developed to enhance the thermal properties of glass walls. One of the strategies to achieve this is by adding a second glass sheet to the normal double glazing. The climate wall is an example of this technique that has gained a lot of ground in recent office building projects. The system was developed in Germany (ŽKlimafassadeŽ or ŽAbluftfassadeŽ ) in the seventies and reached fame among high/tech architects when Richard Rogers used it very elaborately in his design for the Lloyds Headquarters in London, built 1978-1986.
Characteristic of this technique is the air cavity, which is created by adding a single sheet of (mostly toughened) glass behind the double glazing. A solar control device (mainly louvres blinds, but other systems are possible) is added inside the cavity directly behind the outer sheet of double glazing. The air cavity is ventilated mechanically. Lower pressure inside the cavity draws (part of) the exhaust air from the room into this cavity. Here the air warms up, taking most of the heat from the solar control device, and is then drawn off by means of mechanical ventilation. Although it is possible in principle to choose the direction of the air-flow either up or downwards, it is better and more ŽnaturalŽ to create an upwards flow.
The advantages of these ventilated cavity walls are multiple:
- The main advantage, and indeed the main reason for its development, lie in the minimalisation of the temperature differences between the air in the room and the surface of the glass wall. This improves the thermal comfort conditions in the office space nearer to the wall and thus reduces energy costs for heating in winter and cooling in summer. Also it improves the efficiency of floor use, because working close to the wall is more comfortable.
- The heat insulation properties are much better than that of a normal, double glazing facade, because of the extra insulating properties of the ventilated cavity
- The noise reduction is also better than normal for the same reasons
- A heat exchanger can be used to reclaim energy from the exhaust air.
- In architectural terms, the ventilated cavity wall is one of the few feasible possibilities of using a fully glazed wall.
But there are also some disadvantages:
- Because of the airflow, the inner sheet and the blinds pollute quickly. In order to clean the inside of the cavity, the inner single sheet must be made to open. This can be done by sliding or hinging.
- The system works best when the outer sheet is closed. Nevertheless in many cases the facades are equipped with windows that open up onto the outside, thus disturbing the air flow greatly.
- In the case of very high walls the system doesn't work properly because of over-heating in the upper part of the cavity. The over-heating of this upper part can create a heat radiation that completely neutralizes the gain in the lower part. Because of this air should be extracted on each floor separately.
A lot of variations on the concept of ventilated cavity walls have been developed lately. In fact there is a whole family of multiple skin facades. For the most part these are more complex and elaborate versions of the simple principle. One variation which was developed in the Netherlands, and is tested in the Mercator project by Paul de Ruiter and in the Project XX by Post ter Avest, uses a semi-transparent curtain as inner sheet. The main advantages of this variation are that it's cheaper (The Netherlands, remember), that lighting can be controlled better and cleaning is much easier.
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