To date, most single hair fiber mechanical testing publications in the literature have focused on tensile deformation with torsional measurements receiving far less attention. However, there is much to be gained from the measurement of torsional properties of a single hair fiber such as providing an insight into the shear stiffness changes that are associated with cuticle damage. This study outlines the potential use of torsional measurements to differentiate between cosmetic treatments where other modes of deformation do not. A core/shell modeling approach has also been applied to separate out the potential contributions of the cuticles and the cortex on the fiber torsional modulus and the effect of relative humidity on hair fiber structural components.