The invention relates to a thermal insulating element for thermally insulating an interior of a rail vehicle. The insulating element is characterized in that the insulating element has a length, a width, and a thickness, is at least partly made of a closed-cell material, and has at least one expansion joint on the insulating element outer surface. The invention further relates to a method for assembling a thermal insulating element on an interior surface of a rail vehicle, said method having the steps of providing a thermal insulating element which has a length, a width, and a thickness and which is at least partly made of a closed-cell material and attaching the insulating element to an interior-side surface of the rail vehicle.