Wikipedia indicates that "Aluminium foils thicker than 0.025 mm (0.001 in) are impermeable to oxygen and water. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process." I think most heavy duty is 0.020mm or less. It may be that years ago every day aluminum foil was either thicker or the manufacturers took the time and energy (extra steps) to get more impurities out during the manufacturing process. It would be interesting to know if aluminum foil became more permeable contemporaneously with the implimentation of the continuous casting manufacturing method.

Has anyone tested Metro Extra Heavy Duty Aluminum Foil for leaks?