The traditional sequential design of building elements, where every element performs only one dedicated function, carries significant embodied energy. Thus, modular pre-fabricated load-bearing elements could overcome the disadvantages of the current carbon-intensive construction practice and go beyond; as such, lightweight glass fiber-polymer composite profiles could be more advantageous in performance. Cellular structures of such profiles can be advantageous for adding water channels for active heating and cooling indoors and for fire protection. Therefore, the development of such a modular active building slab referred to as P-TACS (Prefabricated Thermally-Activated Fiber-Polymer Composite Slab) is explored in this work. The structural performance of a proposed P-TACS design is verified in terms of serviceability and ultimate limit states. The addition of local carbon fiber inclusions allows for an increase in the span of the slab to 10 m and a more uniform surface temperature. Thermal performance of the structurally optimized geometrical configuration is analyzed by, first of all, determining water parameters based on the 1D approach partially adopted from the standard radiant systems analysis and, secondly, by a detailed 2D thermal analysis using ANSYS Fluent numerical simulations. The hydraulic and thermal performance comparison of the novel P-TACS design with two standard radiant systems (ESS Type A and RCP) reveals that the P-TACS design outperforms the standard embedded surface system ESS Type A, both for floor heating and cooling case. In addition, the response time of P-TACS is three times faster compared to the ESS response time. The main advantage of the P-TACS is in lower mean water temperature, compared to traditional embedded radiant systems (e.g., EES type), required for conditioning the space, potentially resulting in lower operational energy use. The fire outbreak scenario is considered to complete the analysis, and the measures to switch water flow from nominal to fire scenario are proposed.