## 4 Conclusions  
The monolithic aerogel is going to become an interesting solution for building transparent envelopes, thanks to a recent Rapid Super-Critical Extraction technique (Union RSCE) developed at Union College, Schenectady (New York, NY, USA). It is possible to fabricate panes by means of a process lasting hours rather than days. A sample of a glazing system incorporating a monolithic aerogel between two glass panes was thermally characterized and results, together with the optical properties investigated by the authors in a previous work [17], were used to simulate the energy performance of a commercial building when comparing the proposed solution with conventional ones. The thermal resistance of the sample, consisting of a 15-mm-thick aerogel pane sandwiched between two clear float glasses with a thickness of 4.7 mm each, varies in the 0.72-0.74 m\({}^{2}\)K/W range, with a corresponding thermal transmittance of about 1.1 W/m\({}^{2}\)K. The thermal conductivity estimated for the monolithic aerogel pane is 0.020-0.021 W/MK at about 38---41 \({}^{\circ}\)C. When reported at 10 \({}^{\circ}\)C, following the procedure suggested by the Standard ISO 10456 [20], the thermal conductivity of the only monolithic pane is in the 0.018-0.019 W/mK range. A corresponding U-value of the glazing system of 0.99-1.00 W/m\({}^{2}\)K is found. Due to the size limitation of the sample, the g-value could not be measured directly and a simplified method suggested by EN 410 and ISO 9050 was used. The window package was modelled as a three-layer system composed of two glazing panes and an inside aerogel layer, separated by two very thin air gaps; this is another limitation of the modelling phase. An estimated value of g equal to 0.70 was found. Moreover, a dynamic simulation model was used for calculating the annual energy demand for heating, cooling, and lighting of an office building with large strip windows in the east and west wall. Window-to-wall ratios of 50% and 100% were considered in different climate conditions. Results were compared with conventional solutions, such as double and triple glazing units. Considering a non-residential building with the features described in this manuscript, it was observed that aerogel glazing outperforms conventional solutions in all the climate conditions for  
Figure 4: Percentage variation of heating and cooling demand of aerogel glazing system with respect to low-e double (LOW-E DGU) (**a**) and triple (LOW-E TGU) (**b**) glazing units, in different climate conditions (negative values are savings, positive ones are penalties for the aerogel solution). space heating, especially in moderate ones, with a maximum energy demand reduction of about 10% with respect to double glazing units. The benefits increase to 15% when increasing the window-to-wall ratio. On the other hand, the high value of the solar factor increases the energy demand for cooling (about 11% in moderate climates), becoming worse when increasing the window-to-wall ratio. The energy demand for artificial lighting is quite similar for all the investigated solutions, with the aerogel window characterized by a good light transmission. In conclusion, the annual energy demand with a monolithic aerogel solution is quite lower when compared to conventional double glazing systems in cold and moderate climates. The performance is comparable with the one of a triple glazing unit with argon, but it is characterized by a higher thickness (36 mm instead of about 24 mm) and weight and the choice of the best solution would take into account all aspects, including the geometry of the building, the location, the internal loads, and the costs. Future works will be carried out in order to characterize this new material; accelerated aging tests will allow to evaluate the performance of the monolithic aerogel pane over time. Other accurate dynamic simulations will be carried out in order to examine more feasible cases, by considering the influence of the frame in real applications.