Facciate Ventilate - Supporting structure
The functional elements that form the ventilated facade are:
Supporting wall structure
It can be monolayer, i.e. made up of a single stratification of material, or it can be formed by a number of layers of material. According to the type of material used, the supporting structure conditions the performance of the whole system, owing to the different coefficients of deformation to which the materials that form it are subject.
Regularisation layer
It generally consists of a layer of mortar distributed uniformly on to the masonry (1-2 cm-thick plastering) with the function of reducing any irregularities on the surface of the underlying layer. In fact, to ensure correct application of teh insulating layer on to the supporting structure, it is essential to accurately analyse its surface from a both geometric and physical aspect.
Insulating layer
The insulating panels commonly used in curtain walls have a thickness that varies from 3 to 8 cm and are applied directly to the wall structure using glues and/or mechanical elements. The choice of fixing materials depends on the state of the building. Mechanical fixing is particularly apt when redesigning façades, since adhesive cannot guarantee perfect adhesion on irregular surfaces, degraded by atmospheric agents. In any case, screw anchors should be in plastic to avoid cold bridges. If the constructions are new, both fixing methods may be applied.
Ventilation layer (cavity)
The ventilation layer is an air space positioned between the insulator and external cladding with a thickness variable from 3 to 5 cm. The dimensions of this layer are important in that they should guarantee good air circulation and achieve the stack effect (upward drawing of hot air).
The essential role of the air space is therefore to permit the upward movement of air and offer thermal comfort. Obviously the functioning of this layer depends on the conditions of air circulation within it, therefore it is important to avoid impediments which may limit such a flow (bottle-necks due to the presence of structural elements or anchorages, irregularities on the surface of the insulating layer and cladding material, etc.). Furthermore, to guarantee natural ventilation within the cavity, there should always be air grating of adequate section, positioned at the bottom and top of the building.
If all these conditions are respected, the presence of the cavity brings about indisputable advantages like:
- evaporation of water deposited in the masonry during construction;
- elimination of water vapour from inside during winter;
- cooling down of the building''s outer skin during summer due to the upward movement of air;
- attenuation of heat flow from outside to inside the ubilding in summer;
- prevention of heat dispersion from inside to outside, due to the absence of cold bridges.
Fixing system
It is made up of an integrated whole of elements with the static function of attaching the outer cladding to the building''s structure, the latter destined to sustain loads and, in particular, wind force.
The fixing system may be made up of:
- anchors located in certain points "Local Fixing"
- a structural frame "Spread Fixing"
There is hence a notable increase in heat loss from the building. Materials currently used for the anchors are inoxidizable or treated with anticorrosive procedures, like stainless steel (AISI 304 and 316) or aluminium alloy.
In the second case, the structural frame is made up of steel section bars arranged vertically (main frame) and connected to the outer face of the floor slabs or beams through means of plates or anchors, with various regulating mechanisms necessary for correcting out-of-plumb walls. To the vertical elements the horizontal ones are connected (secondary frame) which sustain the cladding slabs. Whether using local fixing elements (wall clamps) or continuous vertical or horizontal linear elements, it is necessary to be able to fix the façade structure to the supporting structure lying behind.
Necessary for this purpose are connecting elements that may be local type (mechanical screw anchors, chemical anchors) or linear type (shaped section bars).
In any case, the optimal solution of fixing system is linked to a series of conditions like, for instance, the type of material used for the supporting structrue, thickness and dimension of the slabs, height of the building and its location, and available budget.
Cladding layer
Its function is to protect the building from atmospheric agents as well as be representative from an architectural point of view. The cladding materials should satisfy the following requisites:
- high mechanical and thermal shock resistance (impact strength, thermal endurance, etc.)
- watertightness
- incombustibility
- physical and aesthetic inalterability over time
- wind resistance (pressure and depression)
- versatility of installation
- maintenance
- slab weight
- slab dimension
Not directly adhering to the supporting structure, the cladding slab is free to move according to its own coefficient of expansion, independently of the supporting structure''s movements, and to follow the settling and oscillations of the load-bearing structures thanks to the anchors'' elasticity.
The absorption of elastic movements between supporting structure and cladding is generally resolved through the presence of joints, which permit free expansion without the slabs interfering with each other.
System of joints
The joint is nothing other than the space which separates the perimeters of the slabs, and has the specific task of permitting free movement of the slabs, caused by temperature excursion or settling of the load-bearing structures or fixing mechanisms.
The joints may be open or closed:
Closed joints
Closed joints are those that permit movement of the slabs although the latter are distanced at just 2/3 mm from each other. In the past, most marble facing was achieved with closed joints. The adoption of this kind of joint generally permits the application of small but thick slabs.
The tendency to increasingly reduce slab thickness together with a greater elasticity of current building structures, which are little tolerant to ground settling, do not make application using closed joints an optimal solution, even though the evolution of fixing systems permits the static autonomy of each slab and hence offers greater margins of safety. Such a technique is advisable purely for cladding of limited extensions and modest height. It is impossible to adopt closed joints for the entire façade since inevitable movements of the structure, elasticity of the fixing mechanisms and differentiated thermal strain could cause the slabs to break and an overload on the wall clamps.
The closed joint solution can, however, be used in other cases as long as the joints corresponding to the floor slab are left open (15 or 20 mm). In this case, supporting wall clamps can be provided just for the first slab above the open joint, while for the remaining slabs up to the successive open joint, check-clamps can be used.
Open joints
Open joints are those that permit a greater movement of the slabs since the latter are distanced at 6-7 mm from each other. They permit the application of medium-large slabs. With the open joints, on all sides of the slabs, settling and movements generated by thermal expansion can develop freely without the adjoining slabs interfering with each other.
The advantage of the open joint solution is that less precision is required when applying the anchors and cutting the slab edges, since any imprecision can be eliminated thanks to a larger space between the adjacent slabs (normally 6 mm). The tendency to increasingly reduce slab thickness together with a greater elasticity of current building structures make this open joint type an optimal solution. To summarise, the main characteristics that distinguish open joints from closed ones are:
- dimensional tolerance;
- little working necessary to effect housing for the wall clamps (holes, slots, kerfs);
- use of a larger quantity of sealing materials (neoprene and silicon compounds).
