We bought two twin-sized mattress protectors to use on bunk bed mattresses. Even though the mattresses are only 6" thick, the mattress protectors fit snugly. They say they are water proof, and I think what makes them that way is their extremely dense weaving of bamboo, cotton, and a small amount of polyester. There is no plastic or crinkly moisture barrier. Another feature is that the slickness of the fabric is making it a lot easier to slip the fitted sheet onto the top bunk when I make the bed.