Window Bevel Shape Optimization for Sustainable Daylighting and Thermal Performance in Buildings

Thick insulation of external walls is the standard method for passive reduction in heating costs in residential buildings in the northern climate zone. However, increasing the insulation thickness worsens the lighting conditions inside the rooms. This work demonstrates that diagonal cuts in the insulation around windows (bevels) significantly increase the light entering the building without compromising its heat resistance. The optimized window bevel shape is a cost-effective method for improving daylighting in residential buildings. The research employs traditional finite-element modeling (FEM) alongside a novel method that allows for the simultaneous calculation of heat transfer and daylight distribution within the same simulation environment and geometry. The study analyzes the impact of various incision depths and angles on both daylighting and the thermal performance of the building envelope. The results show that the optimal bevel geometry dependent on the insulation thickness without a negative impact on thermal properties may be found. In addition, a traditional daylight analysis shows that for thick insulation, the introduction of bevels makes the difference between satisfactory and inadequate lighting conditions in the room. Moreover, reduced use of insulating material and resulting solar gains may significantly increase the overall sustainability of modern buildings.