Creating drying reserves! Consideration of convective moisture stresses for the evaluation of wooden constructions

Already in the year 1983, the general basis paper for noting air tightness, which was elaborated by the Swedish construction council for the AIVC, pointed to the particular risk of moisture stresses in consequence of water vapor carriage due to air flow through leakages in buildings. In Germany in 1989, a laboratory examination of the Fraunhofer Institute for Building Physics (IBP), Stuttgart, indicated that joints in roomsided vapor barriers produce a order of magnitude higher risk of humidification than the mere diffusion of vapor. Tests at timber constructions in North America [TenWolde 1999] allow a quantification of the additional humidity input, which is to be expected by reason of air convection - even if the air seal has been executed professionally. Transferred to German circumstances, [Knzel, H.M. 1999] derived from this that an additional amount of condensate of about 250 g/m by air convection is to be expected. Within the scope of this article, a differentiation of the requirements on the required drying reserve subject to the air tightness standard of the building envelope and an amplification with a model approach for hygrothermal construction element simulations, which also takes the size of the in-sided airspace as well as the current climatic constraints into account, is carried out. The effects of these new construction design requirements will be analyzed with the help of hygrothermal simulation according to DIN EN 15026. The focus is especially on exterior-sided steam-tight constructions such as platform roofs in timber construction with sealing on timber formworks or wooden panels. Thereby, the required drying reserves are determined and the appropriate ways of construction are presented for the different climate constraints and tightness requirements. Such a humidity management needs a certificate about adequate choice of material, a problem oriented construction design and quality control at the building.