Underlay may refer to flooring or roofing materials, bed padding, or a musical notation.
Flooring
Underlay or underlayment generally refers to a layer of cushioning made of materials such as sponge rubber, foam, felt, crumb rubber, or recycled plastic; 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,
Carpet underlays are typically 6-12 mm thick. They primarily provide foot comfort, but they also reduce carpet wear and provide sound and thermal insulation.
Underfloor Heating Underlay
Underlays are also available for underfloor heating in either central heating pipes or electric applications which allow the heat to transmit through the underlay and carpet. Specialist underlays include ThermalStream which have air perforated holes to allow a faster heat transaction.
Wood floor
Underlay for timber floors is typically 3mm closed cell plastic foam. This primarily provides sound insulation and a vapour barrier.<br />
A watertight vapour barrier should only be used though, when neither condensation from humid air coming from beneath nor water spillage coming from above are to be expected.
Especially if there is spillage it will otherwise take a much longer time for the moisture to disperse and evaporate if it is prevented by the barrier from seeping through the wooden flooring.
This might make it necessary to raise the temperature of the floor to facilitate evaporation and prevent rot.
Roofing
Underlay is also the term for the material under roofing tiles;
Music
In music, underlay refers to text intended for vocalization – positioned either directly or indirectly under notes on a musical staff.