NUMERICAL CALCULATIONS OF THE INSTALLATION THERMAL BRIDGE FOR THE OPTIMIZATION OF WINDOWS PLACEMENT

Abstract

Avoiding thermal bridges in the building envelope is one of the key elements for the reduction of energy consumption in buildings. Numerical calculations of heat flux through the joints of building structures are becoming very relevant in the past years due to the EU requirements for nearly zero energy buildings. The most problematic typically is windows installation perimeter, where thanks to fastenings of the frame to the loadbearing structure, linear thermal bridge forms. The use of numerical calculation of temperature and heat flux fields allows to visualise the thermal bridge effect, as well as to calculate the additional energy losses is such critical places with following recommendation to minimize it. This work focuses on modelling of top, bottom and side thermal bridges in a new window installation in a thermal insulation layer with the variable depth of the installation. Created models allows also to calculate the minimum surface temperature and the dimensionless temperature factor f, both describes the risks of potential condensation formation and mould growth. Calculations are made using Flixo software and according to ISO 10211 standard. The results of preformed calculations of different installation depths clearly show the optimum of the distance between the main load-bearing wall and window frame. Numerical value of thermal bridge value (or psi-value) at optimum depth may reach even negative values, meaning that the total heat flux for windows and wall connection becomes smaller when they are connected. Overall, the numerical calculations of thermal bridge flux show a wide range of psi-values from 0.025 W/m/K down to -0.005W/m/K for top and side installations. The psi-value for bottom installation with windowsill may be reduced from 0.1 W/m/K down to 0.03 W/m/K. The optimal installation depth is determined as 9�15 cm with essential dependence on the insulation layer thickness. Assuming that windows are the most thermally conductive building elements, carried out calculations provide information for the engineers in the field building energy efficiency regarding the window installation optimization to reduce the thermal bridge effect and to estimate additional heat losses also numerically at the design stage.