Pipe-embedded building envelopes, as a burgeoning active insulation technique that has its pipe circuits inside the building envelope, are of particular interest to architects and engineers due to their excellent performances and invisibility characteristic. However, performances of this pump-driven technique highly rely on long-term electricity consumption, resulting in degradation of system efficiency in building load reduction and supplementary heating. In the present study, a facade-built-in two-phase thermosyphon loop (TPTL) for passive thermo-activated building system (i.e. TPTL-embedded building envelope) is proposed to turn the pump-driven heat injection process of active insulation technique into non-pump-driven. Experiments on a prototype are conducted to explore operational characteristics of the TPTL whose condenser is operated in the thermal boundary of Dirichlet and dynamic thermal response characteristics of the composite building envelope after coupling TPTL at different heat loads and filling ratios. Results show that the filling ratio has a critical impact on thermal resistances and startup behavior of TPTL, and the optimal filling ratio (around 116%) is not a fixed value at different operating conditions. Besides, condenser thermal resistance occupies the largest proportion among the system thermal resistance due to the heat accumulation phenomenon, therefore, the interlayer material also has a great impact on heat injection and release of TPTL-embedded building envelope. In addition, reverse startup and operation under specific test condition are observed and discussed accordingly. Overall, these results and conclusions are useful for the system design, operation control, and further investigations on the facade-built-in TPTL and the TPTL-embedded building envelope.