Actions aimed at improving the energy performance of buildings increase the share of heat loss through thermal bridges and windows in a building's energy balance. This is especially true of buildings currently under construction. In addition, it is known that the correct installation of windows is one of the biggest obstacles that must be overcome in order to achieve higher energy efficiency and reduce the impact of linear thermal bridges. Therefore, the study analysed, numerically and experimentally, the energy properties of PVC window frames with improved metal stiffening profiles, which were introduced to reduce the risk of window frame deformation and reduce leakage caused by faulty installation. The value of the frame thermal transmittance coefficient and the linear heat transfer coefficients were determined numerically. The simulation results showed that filling large air spaces with insulation material allowed for the reduction of the Uf frame’s thermal transmittance coefficient by over 10%. Moreover, where the window connects with the wall, there was a reduction in the linear thermal bridges’ influence on heat losses. The reduction in the linear thermal transmittance coefficient Ψ was 9.6%, 1.0%, and 3.5% for the window sill, jamb, and lintel, respectively, compared to a frame without insulation. Moreover, experimental studies were conducted on the influence of the insulation of the PVC window frame with metal closed stiffening profiles on the linear thermal bridge located at the joint with the glass. It was found that the incidence factor Itb decreased by more than 6%. Thus, there is also a positive effect on the linear thermal bridge at the joint of the glass pane with the window frame.