Underlay's a thin layer of cushioning, made of materials such as sponge rubber, foam, felt or crumb rubber, which's laid beneath carpeting to provide comfort underfoot, to reduce wear on the carpet, and to provide insulation against sound, moisture and heat. Underlayment Underlayment's a term used to describe several different materials that can be used for several entirely different applications. We'll attempt to explain several different applications and how the single word that describes five… (More on Underlay)