![]() ![]() When the limiting curve is exceeded, wood decay need not necessarily begin - this situation must be assessed with an appropriate assessment model such as the wood rot model of Viitanen for which a post-processor is under development. The limiting curve is a simplified representation of experimental results which showed no decay in spore-infested wood samples (poorly resistant sap wood from spruce/pine) even after twelve months, if conditions remained below the limiting curve. This temperature-dependent assessment takes into account that the growth potential of wood rot fungi is reduced at lower temperatures. Well-considered exceptions are permissible for individual short-term violations of the limit. For solid wood products, the daily mean of the relative humidity of the pore air (averaged over the most critical 10 mm of the solid wood product) must not exceed 95% at 0☌ and 86% at 30°. The WTA Guideline on the assessment of humidity in timber constructions proposes a more differentiated criterion. WUFI's Quick Graphs for the moisture contents in the individual layers allow to add to the graph a red reference line at a user-defined mass-percent level which makes it easy to spot moisture contents exceeding that level. ![]() Low temperatures reduce or even prevent the growth of the fungi, so that under these conditions exceeding the limits even for longer time may be admissible. Moisture exceeding the limits for a short time can be safely tolerated. The wood rot fungi need moisture contents exceeding the fiber saturation of about 25 to 30 mass-percent to be able to extract enough moisture from the wood to start the decomposition process. Similar criteria apply to organic fiber insulations, such as cellulose fibers or textile fibers. In addition, water contents of individual layers can increase if built-in moisture from a layer is redistributed and permanently accumulates in other layers.Īccording to standard DIN 68800:2012-2, the prevention of wood rot requires that the mass-specific water content of wood may not exceed 20 mass-percent over prolonged periods of time ( 18 mass percent for processed wood). If the total water content or the mean water content in one of the layers keeps increasing over a multi-year calculation, this usually indicates a potential problem, since one of the issues discussed below will probably occur sooner or later.Ī long-term increase of water contents will result if more ambient moisture is brought into the component or the layer by diffusion, convection or capillary transport than can dry out. Seasonal variations of the water contents of individual layers may be associated with the variations of the total water content, or they may be caused by periodic redistribution of moisture within the component (and are then often anti-synchronous in two layers exchanging moisture). Seasonal variations of the total water content are also normal, as long as they are confined to a limited range of variation. As a general rule, therefore, the accumulation of too much moisture is to be avoided.Īn initial increase or decrease of the water content is normal if the user-specified initial water content is lower or higher than the water contents of the resulting periodic equilibrium state. However, many damage types are caused or promoted by excessive water contents. Water contents in the component at the level of normal sorption moisture when exposed to ambient air are completely normal. The following sections discuss some aspects which, depending on the specific case, may be relevant for the assessment of calculation results (see also ): If such criteria can been established, they may be implemented in WUFI post-processors for automatic evaluation of the numerical results. Many damage mechanisms are the subject of current research, so that automatically applicable assessment criteria might become available. Often, many details of the possible damage mechanisms are as yet unknown, since it is only now that it has become possible to investigate them in detail, assisted by quantitative simulations. Instead, the interpretation of the results must draw on the technical expertise and experience of the user. These numerical results need to be assessed in order to allow the desired conclusions about the operational properties of the component.ĭue to the large variety of possible damage and failure mechanisms, currently no automatic assessment is possible in general. However, WUFI's calculations only provide results in terms of water contents, temperatures, moisture flows etc. WUFI simulations are usually performed in order to gain insight into the performance of a construction, or into its durability and its resistance to damage. ![]()
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