In all the tiling books I am reading, you are supposed to caulk the expansion joints and leave them out of the graouting process. Some expansion joints are obvious (the joint between the tub and the walls). However, in all my bathrooms one wall is an outside wall of my house and the other two walls of the tub are interior walls. Does the joint where the exterior wall and the interior walls meet constitute an expansion joint due to the difference these walls will see in temperature extremes? I have not been able to find the answer in any of the books I have looked in.