This paper shows how water permeability may be calculated from measurements of drying under a vacuum. The results obtained are for water vapour transport at low pressures and gas slippage theory is then used to compare them with liquid water permeability measurements on samples of the same mixes. The experimental work includes the drying procedure and also the initial surface absorption test (ISAT), a standard absorption test using a partially immersed sample and a ’through flow' high-pressure test for direct permeability measurements. The results from these tests are compared and additional gas and liquid permeability data from the literature is included for comparison. It is concluded that, with appropriate analysis, all of the procedures give comparable values for intrinsic permeability.