The Impact of Controlled-Chamber U-Value Measurements on Building Energy Modelling: Evidence from Sustainable Wall Materials

Building energy models rely on the wall thermal transmittance (U-value), yet, for locally available and novel sustainable materials, this input is usually taken from tabulated conductivities rather than measured, introducing an unquantified bias. The conductivity (k) and transmittance (U) of five locally available sustainable wall materials for social housing in the Mexico City Metropolitan Area—an adobe-type block, volcanic stone, tepetate ashlar, and solid and hollow sargassum-based concrete (SBC)—were estimated from heat-flux and temperature measurements on 1m2 specimens tested in a controlled two-room chamber (17 and 30 °C) using the ISO 9869-1 heat-flow-meter method, and benchmarked against a conventional concrete reference held at its tabulated value. Three reductions were compared—the ISO 9869-1 average method, a direct air-to-air estimate, and a dynamic grey-box (2R1C) model—which agreed within about 5%, with 1–6% repeatability. For four materials, the estimated U was 15–32% below the tabulated value, whereas the hollow SBC block exceeded it (+11%), showing that tabulated inputs can err in either direction. Propagated through a whole-building EnergyPlus model of a representative social-housing archetype, the measurement-derived transmittances changed the predicted annual heating-plus-cooling demand by −28 to −42% for the four better-insulating walls and +19% for the hollow block—a shift large enough to reorder the materials—scaling almost linearly with U (R2=0.95). The values are reported as controlled-chamber, element-level estimates, and an open, simulation-ready material database accompanies the paper.