While window frames typically represent 20%"30% of the overall window area, their impact on the total window heat transfer rates may be much larger. This effect is even greater in low-conductance (highly insulating) windows that incorporate very low conductance glazings. Developing low-conductance window frames requires accurate simulation tools for product research and development. The Passivhaus Institute in Germany states that windows (glazing and frames, combined) should have U-factors not exceeding 0.80 W/(m² K). This has created a niche market for highly insulating frames, with frame U-factors typically around 0.7-1.0 W/ (m² K). The U-factors reported are often based on numerical simulations according to international simulation standards. It is prudent to check the accuracy of these calculation standards, especially for high-performance products, before more manufacturers begin to use them to improve other product offerings. In this paper, the thermal transmittance of five highly insulating window frames (three wooden frames, one aluminum frame, and one polyvinyl chloride frame), found from numerical simulations and experiments, are compared. Hot box calorimeter results are compared with numerical simulations according to ISO 10077-2 and ISO 15099 (ISO 2003a, 2003b). In addition, computational fluid dynamics simulations were carried out in order to use the most accurate tool available to investigate the convection and radiation effects inside the frame cavities. Our results show that available tools commonly used to evaluate window performance, based on ISO standards, give good overall agreement, but specific areas need improvement.