# Thermal and Energy-Efficiency Assessment of Hybrid CLT-glass Facade Elements
###### Abstract  
Facade elements are a building component that satisfies multiple performance parameters. Among other things, \"advanced facades\" take advantage of hybrid solutions, such as assembling laminated materials. In addition to the enhanced mechanical properties that are typical of optimally composed hybrid structural components, these systems are energy-efficient, durable, and offer lighting comfort and optimal thermal performance, an example of which is the structural solution developed in collaboration with the University of Zagreb and the University of Ljubljana within the Croatian Science Foundation VETROLIGNUM project. The design concept involves the mechanical interaction of timber and glass load-bearing members without sealing or bonding the glass-to-timber surfaces. Following earlier research efforts devoted to the structural analysis and optimization of thus-assembled hybrid Cross-Laminated Timber (CLT)-glass facade elements, in this paper, special focus is given to a thermal and energy performance investigation under ordinary operational conditions. A simplified numerical model representative of a full-size building is first presented by taking advantage of continuous ambient records from a Live-Lab mock-up facility in Zagreb. Afterwards, a more detailed Finite Element (FE) numerical analysis is carried out at the component level to further explore the potential of CLT-glass facade elements. The collected numerical results show that CLT-glass composite panels can offer stable and promising thermal performance for facades similar to national and European standard requirements. Cross-Laminated Timber (CLT); laminated glass; hybrid facade element; thermal performance; energy efficiency; numerical modelling 2020 May 20  
An innovative and promising solution (see Figure 2a and [7, 8, 9]) was found in the form of a hybrid, full-size, multi-purpose building element composed of a \(3.22\times 2.72\) m Cross-Laminated Timber (CLT) frame and a glass infill. The design concept for this hybrid system was developed through the optimal mechanical combination of two relatively new constructional materials, CLT and glass. On the one side, it is recognized worldwide that wooden buildings can benefit from the versatility of timber and its seismic, fire, thermal, and acoustic potential. For CLT, major applications take the form of floor and wall panels for high-rise buildings [10, 11]. On the other hand, glass has been also increasingly used in buildings over the last decade. While glass products are well covered by European standards [12, 13], harmonised European standards for structural designs are still in preparation [14, 15, 16], and some design issues can be solved with the support of technical guidelines (see [17, 18, 19]). One of the most challenging issues for load-bearing glass elements is their thermal performance assessment and potential effect on mechanical parameters [20, 21, 22, 23]. The hybrid panel in Figure 2a was structurally optimized by including a series of b \(\times\) h = \(90\times 160\) mm CLT framing members and two laminated glass plates (Figure 2b). A two-ply semi-tempered section was used for each of the glass panels and was obtained by bonding 10 mm sheets and a middle 1.6 mm thick EVASAFE(r) layer. A continuous flexible rubber edge trim was then used to seal the interposed air cavity (see Figure 2b) with s = 12.8 mm the cavity thickness. The mechanical efficiency of the full-size facade system designed in [7, 8, 9] was found on friction mechanisms only, without the need  
Figure 1: Examples of timber–glass facade solutions: (**a**) a typical cross-section of curtain wall detail (_www.stablux.com_); (**b**) full-size, adhesively bonded framed panels (from [5]). Figure 2: Full-size CLT-glass facade element: (**a**) front view, (**b**) CLT-to-glass frame connection detail (cross-section), and (**c**) timber frame corner joint.