To reduce CO2 emissions and save grey energy, natural materials like wood and wooden materials are becoming more and more important. However, these products are particularly sensitive to moisture, as they can rot or be attacked by decay fungi. In the past years, new and detailed knowledge on the decay process of wood was gained. This knowledge allows the definition of more sophisticated limit conditions depending on coinciding heat and moisture and their duration, which can replace simple steady state limits like the widely used 20 % by mass. For wooden materials and natural fibre insulations, in most regulations the moisture limits are even lower - although there are certainly materials that are more resistant than solid wood itself. Therefore, within a project dealing with wood fibre as interior insulation, which is exposed to increased moisture levels during the winter months, the durability of such materials against decay is compared at different critical temperature and humidity conditions to the one of solid wood. Based on these results, the more resistant wood fibre materials can be evaluated like solid wood. Thus, their performance can be predicted more accurately by hygrothermal simulations and their application fields accordingly be extended.