PSI - Issue 13

Barbara Šubic et al. / Procedia Structural Integrity 13 (2018) 503–510 Subic et al. / Structural Integrity Procedia 00 (2018) 000 – 000

504

2

Therefore, it is important to select appropriate glazing and window frame. Windows nowadays have up to 10-times higher thermal transmittance (U w ) than building walls, built according to the Passive house institute standards (Passive house institute 2017). Thermal transmittance of the wall in low energy building is lower than 0.15 W/m 2 K, whereas windows installed in the same building have U w values higher than 0.7 W/m 2 K. Thermal transmittance of window frame (U f ) ranges between 0.8-2.0 W/m 2 K whereas thermal transmittance of insulated glass units (U g ) ranges between 0.5-0.8 W/m 2 K (PHI Certified Component Database 2017). Window frames are mostly made of wood, aluminium or PVC (Interconnection consulting 2017). Different combinations of these materials as well as new materials such as glass fibre reinforced polymers (GFRP) (Appelfeld et al. 2010) and wood-plastic composites (Window & Door 2017) were introduced to market in the last 10-years. Constant search for better material composition and improved window profiles shapes are in pursuit with an aim to improve thermal efficiency of the window frames. The ultimate goal is to equal the thermal transmittance of the wall and therefore drastically reduce the thermal bridge, which is now presented by windows. The trend of increasing window size is forcing window producers to find solutions for stiffer window frames that would resist external influences such as wind, thermal and high installation loads. Studies showed that there are different reinforcing materials that can be used to improve mechanical characteristics of hybrid wood-based beams. The most common are steel (Jasieńko and Nowak 2014; Winter et al. 2012) , carbon-fibre reinforced polymer profiles (CFRP) (Andor et al. 2015; Premrov and Dobrila 2012) and GFRP (Alhayek and Svecova 2012; Nadir et al. 2016). Stiffer material usually correlates well with higher material density but also with higher thermal conductivi ty (λ), which in window frames leads to even higher U f and consequently higher thermal losses through windows. However, the U w is an essential parameter when comparing different window types on the market. Few detailed scientific studies were made on influence of U g (Buratti and Moretti 2012) as well as the effect of different window size, orientation and glazing characteristics on the whole building’s energy performance (Tsikaloudaki et al. 2015). Even fewer scientific studies are available on U f . Appelfeld and coworkers (2010) thermally evaluated a new window frame type made with GFRP whereas Gustavsen and coworkers (2011) have studied the influence of frame material and spacer conductivities on U f . So far no scientific study of thermal properties of hybrid wood-based window beams was published. This study was made as a part of an extensive study which investigated the possibilities of improving bending stiffness, load bearing capacity and flexural rigidity of wooden window frames, with integration of aluminium, stainless steel, GFRP and CFRP reinforcements in the mullion of double casement wooden window. Therefore, the influence of different frame reinforcement parameters on window’s thermal efficiency was analyzed in this study, to guide the future development of hybrid beams in the direction of window profiles, with the lowest possible U f . 2. Materials and methods 2.1. Reinforced wooden window profiles Thermal analysis was made for the middle profile section composed of fixed mullion (Fig. 1 ) and two wooden casements. Casements are connected to the mullion with window hardware and gaskets.

Fig. 1. Middle vertical profile section composed of mullion (colored in grey) and two casements.