The windows are, in terms of energy efficiency, the most vulnerable component of a building envelope, given that these products have a heat transfer coefficient much higher than the one related to the opaque facade, even when are used the most advanced profiles and insulating glass units (IGU) with enhanced thermal resistance properties. This is the main reason for what these elements are treated with the utmost care in assessing buildings in terms of sustainability. System suppliers, in collaboration with international research teams and prestigious institutions making efforts to improve the performance of products in an attempt to overcome the current limitations and to obtain solutions adapted to the current requirements of the sustainable strategies. The recent studies show that the performance limits were not reached yet and therefore are justified the efforts in this direction. Regarding the frame and sash profiles, the measurements show that, in terms of heat transfer U-PVC materials remain the most appropriate, although the aluminum and composite solutions (wood-aluminum and GRP - glass reinforced polymers) made good progress. Returning to plastics, outstanding innovations cannot involve the material, which is a subject to strict compliance with the chemical recipe. The only area where the experts could involve improvement is the geometry of the profile, speaking of the depth of the profile, number and positioning of the isolation chambers etc. as well as their structure. Tests show that the distribution of isotherms at different temperature levels depends rather of these items. The promotion of systems with a large number of chambers, the increased thickness of external walls of the material and depth of PVC systems don't imply any improvement in the heat transfer plan, but also induce particular problems by aesthetically point of view or regarding the loads to  transfer to the masonry structure. The equilibrium must be kept including in this area, especially having in view that the contribution of the PVC frames to the value of the overall heat transfer doesn't exceed 20%.