Nowadays, saving energy and reducing carbon emissions is one of the top priorities in each sector of human activity. The building sector accounts for a significant part of the world's total energy use and greenhouse gas emissions.Windows are inseparable components of the building envelope. In modern designs the trend of increasing the area of fenestration products for residential andcommercial constructions is seen worldwide.Windows contribute to a better standard of living with daylight and useful heat gains, but, on the other hand, cause higher heat losses or non-desirable heat gains. Currently, the thermal transmittance of fenestration components is still much higher than for walls. Recent improvements of the walls' and windows' thermal properties contribute to a better building performance and hence cause the thermal bridging effects occurring in the building envelope to be more important.Among the other thermal bridges the one of the window-to-wall connection appears to be especially important in energy context and should not be underestimated.

Window placement in the window opening can be accomplished in several ways. Well insulated walls are relatively thick, so the window can be placed at several locations in the window opening. Moreover, different strategies for sealing and insulating the connection between window and wall can be used. This study focuses on investigating different solutions for window-to-wall connections and their potential of ensuring a better thermal performance (reducing the thermal bridge effect). Several heat transfer simulations have been done for various solutions of the window-to-wall connection, using the THERM6.3 computing software. Based on those calculations, a parametric study is carried out to show the importance of various factors on the linear thermal transmittance.

Results show that different window placements influence the linear thermal transmittance values significantly. The optimal window position, considering the reduction of heat loss, was found to be about 35 mmfromthe surface of the external wind barrier to the edge of the window frame (for timber wall case, regardless of the wall thickness). Furthermore, the lowest surface temperatures of inside window sill were compared for different window locations. The lowest temperature for optimal solution was found to be only 1 °C lower than that for other placements. In addition, alternative designs of inside window framing, which can substantially lower the linear thermal transmittance, are presented.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings XII, December 2013