Underlay

Underlay or underlayment generally refers to a thin layer of cushioning, made of materials such as sponge rubber, foam, felt or crumb rubber, which is laid beneath carpeting to provide comfort underfoot, to reduce wear on the carpet, and to provide insulation against sound, moisture and heat. In general it is a layer which is underneath another layer. Underlay thus is also used to describe many different flooring products.

In vinyl flooring or "linoleum" underlay is the thin layer of plywood that is fastened over the structural sub-floor to create a uniform, smooth, platform for the sheet vinyl. For laminated wood flooring the underlay provides a “vapor barrier” to prevent moisture from coming through the floor of the home and migrating into the flooring, and may have noise deadening capabilities as well. Self-leveling underlay is a cement-like product that can be pumped in liquid form onto the floor in order to create a level floor. Underlay is also the term for the material under roofing tiles. This roofing membrane is often made of rubber and is used to seal the roof and prevent leakage.