Treats drawbacks of current methods used in East Germany of calculating vapour barriers used to protect building elements from condensation. Notes building materials for which moisture absorbed in winter is sufficient to cause condensation damage before it has diffused in summer. Develops method based on calculation model of determining direct relation between any period of condensation and requisite vapour barriers. In addition amount of wateraccumulated during condensation period can also be determined. Notes part which ventilation plays even for apparently purely structural solutions.