Post‐deposition resistance increase in island Al films on glass substrates held at room temperature at a pressure of 3 × 10−3 Pa is quantified with the definition of an agglomeration rate. Repeated ageing followed by deposition is employed until the resistance drift becomes negligible. Mobility coalescence of the Al islands is shown to be responsible for the ageing. A steady fall in the agglomeration rate through successive depositions further strengthens the applicability of the above model. The presence of water vapour on the substrate surface is found to alter the functional dependence of the film resistance on time. Annealing of the stable films reveals an interesting six orders fall in resistance. The average island size is determined for some of the films from activation energy data.