Elastomer degradation occurs in several critical engineering components such as seals and mounts. Chemical ageing is classified as the irreversible change in material properties caused by microstructural changes to the underlying polymer network. In this work we present a micromechanically motivated model for predicting the response of elastomers during chemical ageing. Splitting the polymer into an active and inactive chain set leads to a system of non-linear coupled differential equations describing the network degradation dynamics and the corresponding effect on the cross-link density and average polymer length. During ageing two main effects occur; stress softening due to chain scission and permanent set caused by stress free secondary network formation. Stress softening is modelled using the relationship between the shear modulus and the cross-link density as obtained from the network degradation dynamics. Stress free secondary network formation is modelled through the use of intermediate configurations where new chains are created. The resulting stresses are pushed forward to the current configuration and integrated over time. This model is then placed into the finite element framework with the aid of the microsphere model. Stress relaxation, stress free secondary network formation and the permanent set effect are observed in numerical experiments of a uniaxial test specimen and an industrially relevant automotive component.