The energy transition requires the reduction of emissions in the building industry, for which new and sustainable technologies are required to overcome the massive environmental footprint of construction. One of the most promising materials for roofing and facades is the semi-crystalline polymer ethylene tetrafluoroethylene (ETFE), typically produced as lightweight structural membranes. Despite its potential, there is still a lack of robust material models for ETFE, which hinders its use among designers. In particular, a reliable time and temperature-dependent plastic constitutive model and yield criterion are still missing. An ongoing experimental campaign performed within the EU H2020 MSCA LIGHTEN program has enabled the mechanical characterization of ETFE's response. It has been found that both the onset of plasticity and the strain-stress relations strongly depend on temperature and strain-rate effects. Furthermore, ETFE's strain-stress diagrams exhibit a double yield point phenomenon related to its semi-crystalline structure. A molecular model was developed to predict ETFE´s onset of plasticity based on the strain-rate temperature superposition principle. The yield law was then integrated into the von Mises criterion, allowing the prediction of different yield loci under varying thermo-mechanical loading conditions. A plane stress viscoplastic model incorporating the yield criterion was formulated and implemented as a user material subroutine. In this model, the yield surface changes not only due to strain hardening, but also with strain-rate/temperature hardening/softening effects. Furthermore, the formulation offers the ability to integrate various yield laws, allowing for the exploration of a wide range of strain-rate and temperature yield dependencies. The validation of the ETFE viscoplastic model and yield criterion demonstrated good agreement with experimental data, providing a solid foundation for the development of safer and more efficient ETFE building designs.