Atrial fibrillation is the most commonly encountered clinical arrhythmia. Despite recent advances in treatment by catheter ablation, its origin is still incompletely understood and it may be difficult to treat. Computer modelling offers an attractive complement to experiment. Simulations of fibrillation, however, are computationally demanding since the phenomenon requires long periods of observation. Because the atria are thin walled structures, they are often modelled as surfaces. However, this may not always be appropriate as the crista terminalis and pectinate muscles are discrete fibrous structures lying on the endocardium and cannot be incorporated into the surface. In the left atrium, there are essentially two layers with an abrupt change in fibre orientation between them. We propose a double layer method, using shell elements to incorporate wall thickness, where fibre direction is independent in each layer and layers are electrically linked. Starting from human multi- detector CT (MDCT) images, we extracted surfaces for the atria and manually added a coronary sinus. Propagation of electrical activity was modelled with the monodomain equation. Results indicate that major features are retained while reducing computation cost considerably. Meshes based on the two layer approach will facilitate studies of AF.