This International Standard specifies a method for measuring gas flow through a clay geosynthetic barrier. As clay geosynthetic barriers are used to contain gases in long-term applications, this test especially focuses on the steady state of the phenomenon. The test is conducted with nitrogen, e.g. N2 (ƞ = 1,75 × 10-5 Pa·s, ρ = 1,15 kg/m3 at 20 °C). The test method and described apparatus allow the measurement of gas flows in the range 0,1 ml/min to 5 l/min on specimens with moisture contents in the range of 90 % to 130 %