A small‐scale test series was carried out using the heating system (radiant exposure) of a cone calorimeter to detect any differences in the way different fibres affect the thermal properties of a standard mortar. The fibres were different polypropylene, polyacrylonitrile, aramide, carbon or steel. Fibres affect the release of moisture from the fibre mortar material. Local pressures caused by water vaporization due to rapid heating can be decreased by incorporating fibres. Fibres have a weak insulating effect. However, use of polyacrylonitrile fibres in mortar may increase the risk to spalling under rapid thermal exposure such as fire. The moisture level in specimens is highly significant for their thermal properties and hence their fire behavior.