Effect of millimetric perforations on CO 2 diffusion and thermal transmittance of multi-layered breathing walls

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The effect of 2 mm millimetric perforations on the performance of multi-layered breathing exterior wall systems was investigated in relation to both thermal transmittance and carbon dioxide (CO 2 ) diffusion. The study aimed to determine whether such perforations on glass fiber coated gypsum board (GFCGB) layers could enhance gas permeability without significantly compromising thermal insulation. Three wall configurations were prepared: a reference wall with unperforated GFCGB, and two alternatives with perforations of 2 mm spaced at 5 and 1 cm intervals. These were tested in a custom-built cold box setup designed to simulate indoor and outdoor conditions and allow simultaneous measurement of steady-state U values and effective CO 2 diffusion coefficients ( D EFF ). Results showed that the introduction of perforations increased D EFF value by 14% and 54% for the 5 and 1 cm intervals, respectively, compared to the unperforated reference. The measured U values of all testes systems, obtained by the cold-box tests under the steady-state laboratory boundary conditions, are within the range of 0.32–0.36 W/m 2 ·K. The findings suggest that improved gas diffusion may be achieved through carefully designed perforation patterns without substantial thermal penalty. This study represents the first application of the D EFF parameter to multi-layered wall systems composed of industrially available materials, marking a shift from material-scale evaluations to envelope-scale performance assessment. The perforated wall system proposed here offers a low-tech, scalable approach for breathable building envelopes, particularly relevant in settings where mechanical ventilation may be limited or absent. Overall, the study shows that air permeability, when quantified and optimized, can be treated as a functional property of building envelopes, contributing to healthier and more sustainable architectural solutions.