## 1 Introduction  
Building envelopes consist of different structural and functional components, such as windows, walls, floors, and roofs; each of them plays an important role on the overall energy efficiency. Different technologies could be considered to improve building energy performance, especially focusing on the transparent elements; highly insulating glazing windows have been under rapid development in the last years and some commercial products have been used in many applications, both for new buildings and refurbishment [1; 2]. A glazing system with aerogel filling is proposed as a technology able to offer natural light, with the advantage of better thermal insulation performance with respect to traditional double and triple glazing solutions [3; 4]. Aerogel is a nanostructured solid material characterized by low density and high porosity (\(>\)90%), with a thermal conductivity lower than 0.020 W/mK. It comes in granular and monolithic form: granular aerogels are easy to handle because they could be poured like a powder [2], but the monolithic panels are transparent and allow the vision through the window, while the granular layers are translucent and do not allow vision. Nowadays, the great disadvantage of the monolithic form is the difficulty in fabrication of samples without defects, cracks, and inhomogeneity, and their high cost [5]. In the last years, a rapid supercritical extraction method was developed at Union College (Schenectady-NY), allowing a significant reduction in fabrication times (6-8 hours) and in solvent waste. Good optical properties can be obtained (\(\tau_{\mathrm{v}}>80\%\)); when assembled with two glasses (total thickness 22 and 12.7 mm of aerogel), a thermal resistance of 0.86 and 1.55 m\({}^{2}\)K/W is obtained at atmospheric pressure and in evacuated conditions, respectively [6; 7]. [PERSON] et al. evaluated the thermal performance of monolithic alumina aerogel systems, prepared by a sol-gel method combined with the ethanol supercritical drying technique [8]. The thermal conductivities at 30 \({}^{\circ}\)C and 400 \({}^{\circ}\)C were 0.028 and 0.065 W/mK, respectively [8]. [PERSON] and [PERSON] [9] presented the main results of a research project founded by the European Commission. They discussed an aerogel pane prototype of 450\(\times\)450x15 mm\({}^{3}\), made under vacuum conditions and sandwiched between two glasses; a conductivity value of about 0.010 W/mK was measured at ambient temperature. Another interesting study presents a detailed investigation of the mechanical microstructural and thermal properties of monolithic aerogels made from polyethoxydisiloxane [10]. The thermal conductivity is highly dependent on the densities of monolithic panes; a minimum value of 0.0135 W/mK was measured for a density of 120 kg/m\({}^{3}\), a maximum value of 0.024 W/mK for about 300 kg/m\({}^{3}\). Similar values of thermal conductivity were found by [PERSON] et al. [11] for cellulose-silica composite aerogels; the thermal conductivity increases from 0.025 W/(mK) to 0.045 W/mK when density increases in the 140-600 kg/m\({}^{3}\) range and when the percentage of the silica content also increases in the 0-70% range. A monolithic aerogel glazing system in evacuated conditions was studied by [PERSON] et al. [5]; a pane of 0.58x0.58x0.015 m\({}^{3}\) was developed and a thermal conductivity of 0.017 W/mK was reached at atmospheric pressure. When assembled in a glazing system in evacuated conditions (10-50 hPa), a thermal conductivity below 0.010 W/mK could be achieved (corresponding U-value 0.66 W/m\({}^{2}\)K). In different studies and measurements carried out a few years ago [5; 9; 12], it was found that when compared to a double low-emission standard window, a double glazing system with monolithic aerogel in the air gap allows a 55% reduction in heat losses and a 25% in light transmission. A window with a granular aerogel shows a 25% reduction of thermal load and 66% of light transmittance.