## 2 Double-Skin Facades for New and Retrofitted Buildings  
The Double-Skin Facade is an architectural trend mainly driven by the aesthetic desire for all-glass facade buildings, the need for a consistent reduction in energy consumption and the parallel increase in indoor comfort conditions, both acoustic and thermic [21]. Described as a pair of glass skins separated by an air corridor ranging in width from 0.20 m to several meters by [PERSON] [22], the DSF concept appeared in the early 20 th century in northern European countries as a way to reduce heating consumption thanks to the air buffer enclosed in the cavity, which acts as a barrier for heat loss and as a filter for exchanges through the external envelope. The extreme variety of possible DSF typologiescan be classified according to different criteria, independent of one another and based on the geometric characteristics of the facade and its operation mode [23]. The criteria mentioned above are the following:  
* The ventilation mode (natural, mechanical and hybrid ventilation).
* The compartmentalisation of the facade (Multi-Storey, Shaft-Box, Corridor and Box-Window DSF).
* The airflow type (Exhaust Air, Supply Air, Static Air Buffer, External Air Curtain, Internal Air Curtain.  
In the second decade of the 21 st century, more articulate configurations and advanced materials started to be combined with multi-layer systems to enhance their energy performance, and more accurate simulation approaches were adopted to overcome modelling limitations and difficulties [24; 25; 26].  
Nowadays, the analysis of solutions for improving the energy behaviour of Double-Skin Facades is a topic frequently explored by researchers all over the world. Detailed investigations are conducted to evaluate DSFs' efficiency when different glazing options and shading systems are used. According to the literature review [27], using single clear glass for the inner pane and double reflective glazing for the outer surface is the best option, with annual cooling savings of up to 26% compared to a traditional external wall and single absorptive window glazing. Another essential and widely analysed aspect involves the shading system and the main parameters responsible for better solar protection. According to [PERSON] et al. [28], in fact, building cooling energy consumption can be decreased by up to 23% by paying attention to three main aspects: the location of the blinds, the colour of the blinds and the opening of the Double-Skin. Other investigations involve possible connections of DSFs with the Heating, Ventilation and Air Conditioning (HVAC) system, its use as a solar chimney or the installation of new elements like Photovoltaic (PV) cells, vegetation, or Phase-Changing Materials (PCMs) [29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39].  
DSF innovation does not involve only energy efficiency aspects. A novel area of focus is the installation of structural elements able to confer an extra function, a seismic one, to the so-called Engineered Double-Skin Facade [40; 41; 42; 43; 44; 45]. Such a facade becomes a mass damper system or an exoskeleton, which, in the case of restoration, can reduce the seismic vulnerability of existing buildings.  
Based on the considerations mentioned above, multi-layer facades represent interesting options for both new and retrofitted buildings. In fact, several works study the improvements achievable by installing DSF systems in existing buildings, underlining their key role in cutting building energy consumption and CO\({}_{2}\) emissions if properly designed and oriented [46; 47; 48; 49].