The thermal Transmittance of a curtain wall fenestration is the primary determinant factor driving the overall building envelope performance of a building to balance with its design aesthetical quality. Research has been performed to establish the merit of comparing digital FEM (finite element method) against the results obtain from experimental evaluation analysis. Thermal modeling technic using the program Therm 7.4 (LBNL) has proven to establish reasonable accuracy accepted by the industry test standard as a design tool to optimize the building fenestration performance and evaluate at early design phase the likelihood of achieving the prescriptive stringent U-Value requirements of new energy codes in the US and China driven by climate change. The paper will first describe what differentiate the two-dimensional conduction heat transfer modeling calculation guidelines between the US (NFRC) and China (JGJ/T151) but leading to similar results for a project located in Beijing. It will then be presented how to leverage multiple numerical iterative thermal simulation results of a curtain wall fenestration assembly, using an interactive engaging exploration tool allowing designers and engineers to simultaneously see the impact of design solution on multiple performance criteria.