Installing a basement bathroom presents its own challenges. One of them being the natural moisture that can enter through the concrete foundation from the outside.
You should always use a moisture barrier to prevent this from happening. I like using Mapelastic HPG flexible waterproofing membrane for concrete walls. When applying Mapelastic HPG it should be the consistency of peanut butter. You can apply the membrane with a trowel or a roller, but I prefer a roller because the application will be both faster and evener. Usually, two coats are recommended.