underlay

Underlay or underlayment generally refers to a thin layer of cushioning made of materials such as sponge rubber, foam, felt, or crumb rubber; this material 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, so underlay is thus also used to describe many different surface-covering products.