Is a moisture-variable vapour retarder for use in the air-tight layer. The functional membrane changes its diffusion resistance depending on the current humidity. During the winter months room atmosphere is generally dryer. The sd-value of the vapour retarder increases. This process results in the penetration of only a little water vapour into the construction and/or into the insulation. In summer, when there is sufficient solar insolation, the relative humidity behind the membrane is higher as a result of reverse diffusion. The water molecules are stored in the membrane. The sd-value sinks, causing more water vapour to escape from the construction and into the rooms and the building element becomes dryer.