Mode I transverse fracture of an aluminosilicate fiberboard was studied under ambient conditions. The material showed tension softening behavior when loaded perpendicular to the isotropic plane. The measured softening curve was treated as the cohesive bridging stress function in a fictitious crack model. The observed peak loads, fracture energy, apparent fracture toughness, and load–displacement curves of various double cantilever beam specimens were shown to be consistent with model predictions.