The ventilated facades
Ventilation types and case studies
1) Introduction: ventilated facades
The UNI 8369 and UNI 7959 standards indicate vertical closures as a “class of technical elements with the main functions of regulating the passage of energy between the internal spaces and external spaces of the building organism” and whose choice must be aimed at “maintaining the temperature of the internal surface as close as possible to that of the internal environment in the various external climate and internal climate situations foreseen, limiting to a minimum the energy contribution of the air conditioning systems (heating, cooling, ventilation) and controlling possible condensation phenomena”.
Studying the building envelope is a fundamental and delicate aspect at the same time, since the thermo-hygrometric comfort and the consequent energy saving depend on its good design. In fact, the external coverings of the envelope have a fundamental role both from an aesthetic and functional point of view. According to the laws of physics, the movement of air between two different points occurs due to the difference in pressure. Renewing and changing the air leads to healthier and cleaner air inside the rooms, since in this way concentrations of pollutants such as carbon dioxide are avoided. Nowadays, some techniques are increasingly developed that try to reconcile the needs of protection and sustainability. An example are ventilated walls, particular coverings whose main characteristic consists in the creation of an air cavity where a chimney effect is generated, allowing the air to flow from the bottom to the top, thus creating natural ventilation on the walls of the building.
2) Types of ventilation
2.1) Cross ventilation
Cross ventilation is achieved by placing two openings on opposite walls. This is because the opposite exposure leads to a temperature difference on the facades, thus generating ventilation.
2.2) Solar fireplace
The stratification of air in rooms depends on the temperature, since hot air tends to rise upwards. In fact, this phenomenon is exploited and therefore upper openings can be created in the wall, overlooking a vertical duct (like a chimney): in this way, hot air comes out and cold air can come in, through openings placed lower than the others. Usually the opening in the upper part is crowned by a metal dome, which during the summer season will overheat and allow the activation of air currents, while in the winter it will be closed and will prevent cold air from coming in.
2.3) Aspiring Atri
This solution includes a central atrium located between the buildings involved, which contributes to cooling. Some openings are placed on the walls that overlook the atrium and this creates a “suction” phenomenon, so that the hot air moves towards the upper part of the atrium.
2.4) Ventilation towers
Ventilation towers work in a similar way to the atrium extraction method: in this method, a central tower is created, onto which the homes overlook, and there is a small external tower, the center of the entire system, which is connected to the central one through an underground connection. The air that enters the external tower passes through the underground channel and is therefore cooled, thanks to the ground temperatures of about 18°; after which the air rises in the ventilation tower and is thus distributed to the various rooms, through openings.
2.5) Wind Towers
Wind towers are an ancient construction solution, and are often used in windy areas, and where large openings cannot be created due to the strong heat.
The tower, which is taller than the building, is positioned in the direction of the winds, which are thus channeled inside. The current that is created is in turn directed into the internal environments through some lower openings in the walls; on the contrary, in the opposite wall there are upper openings, to let the hot air out.
3) Ventilation for insulation
3.1) The composition of ventilated walls
The wall of the building is the base from which the ventilated facade is built. The wall must be flat and without interruptions, and to avoid possible problems of this kind, a plaster layer of about 1-2 cm can be applied.
The next layer is composed of insulation, which is applied externally to the wall itself to avoid the creation of thermal bridges; the insulating panels usually have a variable thickness, from 3 to 8 cm, and have the task of preventing and slowing down the passage of heat from one room to another.
After that we move on to the cavity layer, about 4 cm thick, which must not have interruptions, to allow proper air circulation. In fact, it is thanks to this layer that the chimney effect is triggered; ventilation openings must also be designed both in the base area and at the top of the ventilated wall, protected by grilles, to prevent the entry of foreign bodies that could limit the flow of air; these small openings are closed in winter to further protect the building from the cold. Then we move on to the load-bearing structure, composed of uprights, crosspieces, brackets and anchors, preferably in aluminum, since its weight is reduced and because it resists corrosion better than steel.
The methods of anchoring vertical walls are divided into visible and invisible structures; in the first, the panels are hooked to the load-bearing structure and are visible externally, since the anchors are positioned on the vertical uprights; alternatively, there are non-visible structures, created thanks to mechanical anchors connected to the back of the panels. Finally, we move on to the external cladding, which usually consists of fairly large format panels, in terracotta or porcelain stoneware. The materials that are used in a ventilated facade must have high mechanical resistance, high resistance to thermal shock and low water absorption.
3.2) How does the ventilated wall work?
To install a ventilated wall, it is advisable to study the surroundings of the building itself. Furthermore, the function varies depending on the time of year. During the summer season, the ventilated wall maintains a mild temperature inside the room; the steam in the room escapes through convective motion in the cavity. Furthermore, in summer, the ventilated wall acts as a shield for solar rays, since the heat that accumulates on the surface does not penetrate inside the building, but is dissipated thanks to the chimney effect.
In the winter season, the external insulation system protects against bad weather conditions, and at the same time the steam escapes thanks to the convective motion of the air chamber. The ventilated facade protects against solar radiation and at the same time maintains constant air circulation at room temperature.
The ventilated facade requires external insulation, so that a completely uniform insulation is created, which will lead to the reduction of thermal bridges.
Therefore, the ventilated facade helps to improve insulation, and this allows for better use of the thermal capacity of a wall.
This has two positive effects:
-the'elimination of thermal bridges, that is, those defects that cause condensation and mould inside the rooms, due to cracks in parts of the facade;
-the heat shield: in the hottest periods, thanks to the combined action of the external wall that protects from the sun's rays and the constant circulation of air at room temperature, the ventilated facade creates a sort of "shield", thus protecting the building from the heat.
With the ventilated facade there is also an improvement in the acoustic performance of the building; in fact, thanks to this technology there is usually a halving of the sound level in internal environments; the insulation effect is continuous and without interruptions and thus acoustic bridges are easily eliminated.
4) Case studies
4.1) Intesa San Paolo Skyscraper, Turin – Renzo Piano
The Intesa San Paolo skyscraper by Renzo Piano is located in Turin and houses more than 2.000 employees of the Intesa San Paolo banking group. The building envelope is made up of double glass walls, which allow the passage of air, in order to cool the mass of the floors, thus using natural ventilation. Furthermore, the double-skin facade allows for the waste of heat loss in winter; the system is regulated according to the heat inputs that reach the building through a mechanism of openings and solar screens with motorized slats, which in this way control solar radiation and the light arriving in the work areas.
During the summer season, cool night air is conveyed inside the double concrete floors that absorb the freshness, which is then returned during the day to the offices thanks to the radiant panels. The entire system is managed by probes connected to the BMS (Building Management System), a computerized system that verifies and controls the mechanical and electrical devices of the entire building, such as ventilation, lighting and security.
4.2) Project for the Media Library, Toulouse – Buffi Associati, C. Ramin and F. Egreteau
The project proposes a strange and particular interpretation of the envelope, where the high mobility of the elements in the screening allows for different light effects and different visual and aesthetic effects. The facade can assume various configurations thanks to the movement of the elements that compose it, in fact the individual parts can change in a very flexible way over time, creating multiple envelope solutions, depending on the solar radiation.
The casing can assume all possible configurations between the two extreme poles, therefore from total closure (by arranging the screens parallel to the casing behind them) to absolute transparency (by positioning the screens perpendicular to the building). The screens can vary through their own rotation on the central axis. The panels are made up of a frame composed of two lateral steel plates, a plate along the axis of the screen and two profiles that function as crosspieces; the panels themselves are anchored to the structure by means of two pins, one lower and one upper, which allow them to rotate. The movement of the panels is regulated by a computerized control system; in automatic mode the panels rotate according to the external light conditions, which are detected by a sensor.
4.3) Lodi People's Bank – Renzo Piano
The building envelope consists of a double ventilated skin, with external insulation, covered with brick slabs. The brick skin is anchored to the structure behind it by means of a stainless steel grid; the windows are protected by brise soleil, also in brick. The adoption of this system has allowed us to achieve good performance aspects, such as good winter and summer thermal insulation, excellent durability with respect to climatic and atmospheric agents. However, there are also some critical points, such as, from the functional point of view, the stability and resistance over time of the substructure and cladding system.
The presence of the cavity is fundamental from the point of view of hygrothermal behavior, since it acts as a layer of thermal insulation and summer cooling. The space of about 1,5 cm of the cavity is sufficient to decrease the pressure of the vapor coming from the inside, so as not to create condensation phenomena on the back of the terracotta cladding. As regards the durability of the facade, the cladding works as a protective layer that is itself exposed to risks of degradation.
For terracotta elements there may be risks linked to the very nature of the material, in relation to atmospheric agents, such as frost and rain, but also due to polluting agents, and risks linked to assembly conditions. The substructure is obtained through a first vertical framework (uprights) anchored to the building and a second horizontal framework (brackets). Each module that constitutes the covering is formed by a frame to which the terracotta elements are anchored.
4.4) Social housing in Holzstrasse, Linz, Austria – T. Herzog
The project in Linz consists of two parallelepiped volumes of different lengths, which house 400 apartments. Innovative design strategies were adopted, such as the use of ventilated facades. A modular, openable and adjustable glass window covers the internal gallery of the buildings, thus protecting it from atmospheric agents and ensuring natural ventilation.
The control of thermal insulation leads to an increase in internal temperature; in the winter season there is therefore a great energy saving for heating the apartments of the buildings, while in the summer the hot air rises and escapes through the special openings in the roof, while in the lower area fresh air circulates which cools the internal environment. The fronts of the north and south buildings are characterized by the use of ventilated walls, with a metal and brick structure. The vertical uprights of the substructure are anchored to the building by means of stainless steel brackets, to which a horizontal profile is attached on which shaped springs are placed, which represent the supports of the terracotta slabs.
