This study deals with the dependence of glass-fibre bending strength and static fatigue on different factors, mainly the ambient atmosphere. The fracture diameter of a loop (as measured by the loop test), which is proportional to the bending strength, was found to be dependent on the diameter of the fibre, as well as on the water concentration of the ambient atmosphere. Heat treatment (105° C) did not increase the bending strength of the fibre. High-temperature treatments (500 to 700° C) decreased the bending strength of the fibre substantially, presumably because of the relaxation of the stress distribution in the fibre. The results showed this to occur in a wide temperature range, whereas in massive glass relaxation has also been found in a wide temperature range. Under long-term stress, the fibres are affected by static fatigue, which is dependent on the diameter of the fibre, the effective force, and the ambient atmosphere. The static fatigue of fibres allows us to explain the long-term properties found in light glass wool-based composites. Difficulties in determining the actual properties of glass wool composites are caused by the considerable inhomogeneity of the composite fibres, rendering usage of them in testing impossible. Accordingly, the fibres are more susceptible to static fatigue than the fibres in the tests.