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.
In vinyl flooring or "linoleum", the underlay is the thin layer of plywood that is fastened over the structural subfloor 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 then migrating into the flooring; the underlayment may also have noise-dampening properties.
A 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.
Popular carpet underlays are
- foam rubber
- crumb rubber
Underlay for timber floors is typically 3mm closed cell plastic foam. This primarily provides sound insulation and a vapour barrier.
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