Single fibre tensile testing of thermally conditioned water sized and gamma-aminopropyltriethoxysilane (APS) sized boron-free E-glass has been carried out. The fibres were produced from identical melts following which bare fibre had only water applied to it before winding whereas the sized fibre had a solution containing only APS applied to its surface. Both fibre types experience a loss of room temperature tensile strength after exposure to elevated temperature. By application of a novel method of single fibre thermal conditioning, it was demonstrated that the tensile strength of heat-treated glass fibre can be significantly underestimated. Strength loss was found, in most cases, to be caused by a combination of thermal effect and mechanical handling damage. The latter is found to be influenced by thermal loading of the fibre. The onset of mechanical handling damage in APS-sized fibre was found to be controlled by the thermal degradation of the silane sizing. This suggests that silane-based coatings, even when they are present as only a relatively thin surface layer, can protect fibres from the development or growth of critical surface flaws. The relative contribution to overall fibre strength loss from mechanical handling damage highlights the need to minimise processes which may cause fibre mechanical damage during glass fibre recycling procedures.