The hydronic thermal barrier (HTB) makes the building envelope gradually regarded as a multifunctional element, which is an opportunity to transform thermal insulation solutions from high to zero-carbon attributes. However, inappropriate design, construction, and operation control may lead to issues like low efficiency and high investment, and even the opposite technical effects. In this paper, a comprehensive uncertainty and variable ranking analysis is numerically conducted to explore the influence mechanism of twelve risk variables on three types and five thermal performance indexes under summer conditions. The uncertainty analysis results showed that the correct application of HTB could significantly reduce the heat gain that needs to be handled by the traditional air-conditioning system and even have the technical effect of auxiliary cooling if the variables are appropriately selected. The comprehensive influences of water temperature, room temperature, charging duration, and thermal conductivity of the HTB layer were in the first 1/3 range. Among them, the first two variables were identified as the two most influential variables, and they had a significant mutual restriction relationship in all other four indexes except for the exterior surface cold loss. The recommended charging duration was not less than eight hours in practical application, and the HTB layer with a higher thermal conductivity value but less than 3.3 W/(m·℃) was suggested. Besides, the climate zone was no longer the most influential variable affecting the mean radiant temperature of the interior surface due to the combined effects of HTB and static thermal insulation measures. In addition, pipe spacing should preferably be selected between 100 and 250 mm to help form a continuous thermal buffer zone inside the building envelope.

The pipe-embedded building envelope is heavyweight thermally activated building systems (TABS) that has its pipe circuits inside building envelopes, but it has been seldom done in existing buildings. In this context, the concept of thermo-activated phase change material composite wall (TAPCW) is proposed to address the retrofitting challenges facing existing buildings, that is to substitute the pipe-embedded interlayer with macro-encapsulated PCM panel and relocate it to the exterior of the load-bearing layer. This work aims to investigate the thermal and energy saving performances of TAPCW under the winter climate conditions of northern China (i.e, Tianjin city) through a validated numerical model. Furthermore, performances of TAPCW are examined for some key factors, including the pipe spacing, PCM thickness, and orientation. The comparative study of three cases (case 1: TAPCW; case 2: normal wall integrated with PCM; and case 3: normal wall) verifies the effectiveness of TAPCW, and the daily heat loss (G_room), primary energy consumption (PE) and operation cost (C) can be reduced by 105.5%, 14.07%, and 56.03%, respectively. The parametric study shows that the pipe spacing has a more obvious influence than the PCM thickness, and the case 100/30 could be used as an optimum value for thermal barrier function, while the case 75/30 could provide a more efficient supplementary heating. Results also show that the TAPCW applied in the north orientation is more effective, which has the highest value of interior temperature increase (1.8 °C), effective PCM utilization ratio (14.14%), reduction size of PE (64.98%) and reduction size of C (34.43%). Overall, the proposed TAPCW presents a satisfactory thermal behavior in the heating season and could contribute to the progress of energy saving retrofit in the vast existing buildings.