Description
Cost-reducing building services systems with low energy consumption standards are being implemented to an increasing extent in the planning of modern office and administrative buildings. The reciprocal interaction of the façade and the air-conditioning plays a decisive role in this, which is why the adaptation of the building to the climatic conditions and the environment should be one of the most important planning parameters. Planning examples show that, taking into account economic efficiency, the use not only of single-skin façades, but also of double-skin constructions is ecologically sensible and at the same time can meet high design demands. In some cases, a double-skin façade specially developed for a particular project actually offers significant additional advantages.
In general, it can be said that double-skin façades make sense when buildings are exposed to high noise levels or wind. This is just as true for tower blocks as it is for lower buildings. If these buildings are to be naturally ventilated by windows for most of the year, the occupants derive significant advantages from the double-skin façades.
Double-skin façades offer:
• The option of nocturnal cooling in the summer under all weather conditions;
• Protection against unauthorised entry in spite of open windows on the inner façade;
• Reduction of wind buffeting and pressure changes in spite of open windows on the inner façade;
• Correct functioning of external solar protection at any wind strength.
Their scope of application can also be extended to renovations: when existing façades cannot or should not be replaced, the second skin provides weather protection and permits a façade design in keeping with the times.
Use of the different structures
In correspondence with the following list of double-skin façade designs, the main areas of use could be defined as follows:
• Double-skin windows (deep box windows with glazing at inner and outer faces of the box) are recommended for perforated façades and stringent discretion requirements (prevention of noise transmission through open windows)
• Double-skin window façades are suitable for particularly stringent sound insulation requirements due to their smaller external openings.
• Corridor façades are particularly effective for ventilation; however, they do have limitations – sound does carry from room to room.
• Multi-storey façades are usually used when mechanical room ventilation cannot be dispensed with for other reasons, or when a glass façade without openings is desired.
Acoustic insulation against external noise
An additional, external façade can appreciably improve sound insulation. Like an acoustically insulated wall, it provides partial protection from the noise from outside. It could be described as a noise-insulating screen placed in front of a window. Other openings for background ventilation do not counteract the noise insulating effect, although they do have some influence on it.
The acoustic insulation properties of a double-skin façade depend primarily on the size and position of the openings in the external shell. In addition, the acoustic insulation can be influenced to some extent by adding absorbent material within the space between the façades. For an outer skin with open areas totalling 10 percent, the sound insulation of the interior façade can be improved by circa 3 to 6 decibels. With only 5 percent open areas, almost 10 decibels’ improvement can be attained; however, this may already be too small for adequate ventilation of the rooms.
As a rule, the requirements for the interior façade can be reduced by one to two acoustic insulation classes. However, it must be observed that for buildings that are not exposed to high external noise levels, too much acoustic insulation can be undesirable. This represents an impediment to contact with the environment, and under certain circumstances can lead to annoying noise inside the building.
Insulation against heat loss in summer, energy savings
In the trade press, there are recurring discussions over the possibility of saving on heating energy and the amount that can be saved thereby. For this, comparisons are often made with the insulation standards of single-skin façades, which from today’s energy-saving perspective are outdated. This is because the worse the original insulation was, the larger the savings that can be achieved by using a second skin. However, for highly insulated buildings in accordance with low energy or passive building standard, the energy savings are very limited.
k-Value
Expected energy savings are often cancelled out by the fact that a double-skin façade shows worse values for the passage of light through the additional outer layer. Although this is often compensated for by large proportions of window surface area, smaller proportions (for example, perforated façades) lead to a noticeable increase in the time the lights have to be switched on. Under certain circumstances, the extra electricity costs can even exceed the savings on heating costs. Significant energy savings only result when window ventilation can be enabled exclusively by a double-skin façade, or when the potential implementation times are significantly extended. Doing without mechanical ventilation saves electricity costs for air conveyance. In addition, dispensing with air-conditioning can save proportionately on heating, cooling and water.
It is true that double-skin façades provide improved insulation in winter. However, this has only a limited effect on the heating energy needs, as the thermal insulation regulations currently in force already ensure a sufficiently high quality interior façade. A more significant effect is to be seen when double-skin façades are used in building renovation, but even here, they do not approach the efficiency of an additional layer of insulation. Therefore the reduction of heating energy needs can at best be regarded as a welcome bonus when such a façade is built for other reasons.
Heat protection in summer
In summer, a double-skin façade can be advantageous when the wind conditions for a tower block or at another particularly exposed site do not permit external solar protection. Sufficiently ventilated solar protection in the space between the two layers of a double façade can achieve almost the same effect as external solar protection and is more variable than internal solar protection behind solar protection glass.
Energy requirements/year [kWh/m²a]
Unlike the solar protection for a single-skin façade construction, solar protection shielded from the wind in the space between the façade layers of a double-skin façade maintains 100 percent effectiveness at any wind speed and thereby reduces the solar radiation in the offices. However, the warming of the air in the space between the façades has an unfavourable effect on the ventilation.
Wind pressure behavior
It is often claimed that a double-skin façade on tower blocks can be used to reduce wind pressure. Unfortunately, this is only partially true. Double-skin façades are quite capable of reducing temporary changes in pressure caused, for example, by gusts of wind, through the buffer effect of the space between the two façade layers.
However, continual (static) pressure on the façade will also extend unchecked into the space between the façade layers and – if windows are open – into the rooms. Static pressures are caused by steady winds or wind proportions flowing around buildings, for example. The seafaring terms “luff” and “lee” (to designate the windward side with overpressure and the leeward side with suction respectively) have therefore been adopted for buildings as well.
Temperature changes at the solar protection in a double-skin façade
Economic efficiency
Of course, a second façade skin has its price. The additional costs that would not arise for a single-skin façade therefore have to find their counterpoint in the lower running costs to be achieved through better functioning and more effective operation. This is shown by an example:
An office building is located on a heavily travelled major road and is exposed to external noise levels of 70 to 75 dB(A). With a single-skin façade, the offices would have achieved class 4 sound insulation and air conditioning. Using a double-skin façade would enable air-conditioning to be dispensed with in favour of window ventilation. Due to the prevailing design specifications the rooms were equipped with a single mechanical ventilation facility with cooling capabilities (air change twice hourly). Because of the external skin in front of it, the requirements for sound insulation for the interior façade could be reduced to class 2, which was easily fulfilled by insulated glazing.
A comparison of the two alternatives shows that when the investment and the follow-up cost are taken into account, the combination of double-skin façade and supported ventilation is, in total, more cost-efficient than the single-skin façade with air-conditioning.
Project examples
Burda Media Park in Offenburg, single-skin façade
Offenburg’s climatic situation in the warmest and least windy part of Germany was taken into account both in the choice of building service facilities and in the planning of the façade. In the comprehensive planning approach and in view of the low project budget, only a 250 mm high area of ventilation louvres (glazed sash flap) was provided directly below the centre-hung window. This additional, finely adjustable natural ventilation option at the lower room level can also be used for nocturnal cooling in the summer.
Façade section: Burda Media Park
The façade planner developed special window fittings, which guarantees that the sash flap is almost rainproof, even in this single-skin façade. The architects made a further contribution with regard to climate adaptation, by integrating a small concrete parapet as thermal mass with background ventilated panelling of untreated wood, thereby taking into account the site’s meadow landscape. The façade’s technical details, strictly adhering to the architects’ planning brief’s requirement for formality, integrate the wood panelling vertically, flush with the building envelope, so that the transparency of the architecture is almost entirely conserved through the use of a concrete parapet.
Business Tower in Nuremberg, double-skin façade
In accordance with the climatic and acoustic emission codes of practice, the round tower, 134 metres tall, was provided with a double-skin, permanently background-ventilated façade construction. This double-shelled façade construction, often considered to be difficult to justify financially, enables natural ventilation of the office space during approximately three quarters of the year. However, by this means, the investment and maintenance costs of the building services could be considerably reduced.
Façade section: Business Tower Nuremberg
Solar radiation in summer is optimally reduced by the cost-efficient solar protection shielded from the wind in the space between the two façade layers, and thereby even in the tower this remains fully functional. To improve the comfort of the natural ventilation of the office space, the façade planner proposed that in the design of the interior façade, a flap sash be integrated between the centre-hung window in every second axis. This louvre in the lower part of the façade is oriented to screen cold outside air flowing in and provide the required ‘dose’ to the office space.
Originally published in: Rainer Hascher, Simone Jeska, Birgit Klauck, Office Buildings: A Design Manual, Birkhäuser, 2002.