The invention relates to a vapor retarder membrane, intended to be used for improving the airtightness of a building or of a room, comprising at least one active layer having a water vapor permeability which increases with the surrounding relative humidity, said active layer comprising at least 90% by weight of a blend of ethylene/vinyl alcohol (EVOH) copolymer and of a copolyamide 6-6.6 (PA666), the latter having a melting point below 210° C.