The ventilated walls are composed of two slabs delimiting a duct into which air flows; the air flow is generally due to chimney effect and the ventilation is natural. The duct is usually 10­15 cm in thickness and the air flow is turbulent with Reynolds numbers about 10000.

The use of ASW for buildings with outdoor external insulation has recently become widespread in building practice: it is walls in which the outer slab, that is to say, the so-called "advanced screen”, instead of adhering to the insulating layer, is outdistanced from this so as to form a narrow ventilated duct. In these cases the duct is normally 4-5 cm in thickness and the air flow is laminar [9-10]. The ASW reduce thermal stresses, allow the possible condensation water to be drained and the inner slab to be covered using various materials in order to satisfy all the formal and aesthetic requirements peculiar to the site and the surrondings.

The outer slab can be realized with "traditional” materials (e. g. brick, stone, ceramics) or "innovative” materials: metals (e. g. aluminium, stainless steel, copper, titanium), plastics (e. g. high-pressure plastic laminated, thermosetting synthetic resins) and concretes (e. g. Portland mixed with tenso-stabilizers, pasted with glass fibers, fibrous concretes).

The fixing of the outer slab to the structure lying behind is very often carried out by using the techniques peculiar to the "dry-walled” stratified buildings, with various kinds of mechanical fastenings, without having resort to the traditional cement mortars ("moist-wall installation”).

Caption:

1 — Outer slab

2 — Ventilation duct

3 — Thermal insulation

4 — Cement mortar

5 — Inner brick wall

6 — Lime mortar plaster

Figure 1 — Ventilated facade of the Banca Popolare di Lodi by R. Piano (Lodi-I, 1999): outer slab made of terracotta tiles, detail near the duct air outlet.

Figure 2 — Schematization of the advanced screen wall (ASW).

The main building of the Banca Popolare di Lodi by R. Piano, in which the outer slab is made of panels composed of four terracotta tiles, grooved on the outer face and about 4.0 cm in thickness, is an example of brick ASW. The panels, preassembled by fixing the tiles to an apt counter-framework, are anchored to the building structure with a suspension system, consisting of stainless steel components. In Figure 1 a facade detail, lying near the duct air outlet is shown. Notice the original eaves element made of stratified and tempered glass.

It is a widespread tendency to use, for the outer slab, metallic materials in plates of peculiar shapes and dimensions (e. g. the Jewish Museum by D. Libeskind, Berlin-D, 1988; the Guggenheim Musem by F. O. Ghery, Bilbao-E, 1997).

In this paper two examples of ASW are going to be investigated, the first one with a brick outer slab (BW) and the second one with a copper outer slab (CW).

Thermophysical and geometrical properties of the layers composing the ASW are shown in Table 1.

The examined ASW show the same overall thermal resistance (Rtnv=1.87 m2KW-1) and then the same energy behaviour when the air duct is closed; thermal resistance distribution among the layers composing the walls is, on the contrary, very different (see Tab. 1).