Impact factors and correction methods of the cup test for determining the vapor permeability of porous building materials

• Improves cup test accuracy via leakage & resistance corrections; • Leakage dominates errors in low-permeability materials (FCB); • Resistance corrections critical for high-permeability materials (GYB); • Corrections reduce variability across test conditions significantly; • Sealing with foil tape + silicone is simple and reliable The cup test is the most widely used method for measuring a material’s vapor permeability, but its reliability is still subject to further investigation. This study aims to improve the test accuracy by examining both experimental procedures and data processing methods, using three porous building materials with varying permeability, namely gypsum board (GYB), expanded polystyrene (EPS) and fiber cement board (FCB). Two correction methods for data processing are first investigated: leakage corrections and air layer resistance & surface resistance corrections. Subsequently, analyses of four impact factors – sample surface area, sample thickness, air layer thickness and sealing method – are conducted. Results show that leakage corrections notably affect the low-permeability materials, while corrections for air layer resistance and surface resistance have more pronounced effects on high-permeability materials. After applying these corrections, the relative standard deviations of vapor permeability under varying test conditions are reduced from 7.11% to 4.96% for GYB, from 8.81% to 5.40% for EPS, and from 9.78% to 9.48% for FCB. Regarding experimental impact factors, sample and air layer thickness has minimal impact after corrections, and surface area is non-dominant neither. Among sealing methods, using aluminum foil tape with silicone sealant is recommended for simplicity.