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.