Functional Equivalence as a Basis for the Fair Comparison of Traditional, Prefabricated and Industrialized Façade Systems: A Methodological Protocol

The adoption of industrialized façade systems is usually decided by comparing commercial offers that differ simultaneously in thermal performance, external finish, scope of supply and service life, and that frequently lack environmental-impact indicators. Under such heterogeneity, differences in cost or environmental impact cannot be attributed to the construction system itself, but are confounded with differences in specification. This work introduces a protocol that isolates the construction system as the sole independent variable, enabling a fair comparison across traditional, prefabricated and industrialized façades. A functional equivalence protocol constrains all solutions to deliver the same wall thermal transmittance and an equivalent external finish over a 50-year reference service life. Equivalence is enforced at three levels of thermal ambition per Spanish winter climate zone—regulatory minimum, recommended, and cost-optimal high-efficiency. Seven configurations are characterized: three traditional (cavity wall, external insulation, ventilated façade), two prefabricated (precast concrete and timber panels) and two industrialized framed systems (steel-frame and timber-frame). Cost and embodied-impact data are extracted from the BEDEC database under a frozen, traceable specification; thermal transmittance is computed by the ISO 6946 combined method, explicitly accounting for the repeating thermal bridges of framed systems; and reaction-to-fire compatibility is enforced. The protocol delivers, for each system, the required commercial insulation thickness, resulting transmittance, construction cost and embodied carbon per functional unit across ambition levels, separating fixed execution costs from material-driven marginal costs and exposing how rankings and the thickness–carbon trade-off evolve as thermal ambition rises. Functional equivalence provides a replicable, regulation-anchored basis for comparing heterogeneous façade systems on strictly equal terms, supporting evidence-based selection between traditional and industrialized construction.