This international standard specifies the box method for determining the steady state water vapour flow rate, water vapour permeance and water vapour permeability of building materials using either one box or two boxes.The box methods are especially suitable for the following:1) Materials of relatively higher water vapour permeance, i.e., more than 200g/m2hPa2) Because water vapour surface resistance is requested by Annex B, water vapour resistance of material can be measured.3) It is possible to measure the water vapour transmission properties on various humidity condition in both side.4) Hygrothermal performance in the case of horizontal flow water vapour