AbstractThe thermal performance of window frames can easily be calculated using 2D numerical simulations. Several commercial software packages are available, and international standards provide a clear methodology to calculate the thermal transmittance. However, even though these methods are well known in academia and the research community in general, thermal optimization has not reached its full potential in the building industry yet and there is considerable margin for improvement. Specifically for small and medium size enterprises there is a lack of guidelines that are both generic enough to guarantee a wide-spread use, as well as specific enough to allow an easy and straightforward interpretation and implementation. In this research project, generic window sections were developed for vinyl, aluminium and wooden frames in collaboration with the building industry. Based on a market survey, typical approaches for improving the thermal performance for each type of window frame have been identified and described. Subsequently, the impact of separate improvements, as well as combined effects have been studied using both standardized and advanced calculation methods. For this, the heat transfer phenomena and the way these are modelled according to standard calculation procedures are discussed. Next to that, a number of secondary effect originating from standards are discussed, e.g. the thickness of the IGU, the depth of the window rebate, equivalent thermal conductivities and the impact of reduced heat transfer coefficients.