Definition of Underlayment

Underlayment is a layer of material, lined between the outer face of a structure and its underlying foundation. The word "underlayment" is commonly used, however, in reference to the process of installing floors, carpet and roofing to a building. Underlayment adds cosmetic appeal as well as integrity to the structure.

Uses

One function of underlayment is to serve as a medium that diminishes the transference of sound waves from upper floors to lower units in multi-level buildings. This reduces the intrusion of everyday noises, such as walking, from disturbing those who occupy separate parts of a structure. Also, this buffer adds a layer of protection to a floor or carpet from moisture which may occasionally pass through small openings in the sub-floor. Additionally, underlayment acts as insulation, preventing heat or cold from escaping through the floor or bleeding in from below. In roofing, underlayment is used to act as a secondary defense against leakage, and has recently become an essential part of building practices.

Foam

Underlayment is often made from some type of foam. The basic foam version is 1/8-inch thick and generally provides only minimal insulation and less resistance to sound and moisture than other types. Modified foam, frequently called high-density or closed-cell foam, is packed much tighter, making it less penetrable, providing a more stable walking surface. Modified foam supplies a higher degree of sound renitency and, therefore, is a preferred choice in second floor and higher installation.
While offering little in terms of noise reduction, or cushioning for that matter, combination underlayment is an easy way to get the job done. It entails a ready-made solution of thin foam, connected to a moisture barrier.
Carpet padding comes in a variety of thicknesses and styles. Choosing the right padding is not only instrumental for comfort when walking, but also in affecting the overall life of the carpet.

Other Materials

Rubber underlayment is similar to foam in when it comes to thickness and sound absorption, but excels in moisture resistance. It is also generally made from recycled material, making it more Earth-friendly.
Cork underlayment is considered to be high-grade floor padding, and stands above the rest in all defining areas. Cork underlayment will give a floating floor a more stable feel, and has moved in as a standard requirement in may condominium associations, with 6 mm being the norm. Cork is also a biodegradable product that does not kill the trees from which it is made.
Commonly made from 6 mil polyethylene or visqueen, a moisture barrier is a thin sheet of plastic used to prevent moisture from rising, when floating floors are installed over a concrete sub-floor.

In the Roof

Roofing underlayment adds an extra measure of protection against extreme weather, condensation and excessive temperatures. Synthetic underlayment has become the choice for roofers, minimizing concerns with things like potential shrinkage and expansion. Several manufacturers now list underlayment as a prerequisite to a valid warranty.

Practical Benefits

The pragmatics of using proper underlayment extend far beyond its aesthetic value. Underlayment can support a floor, providing firmness underfoot. When walking on a laminate floor, the right underlayment will minimize movement, and prevent the hollow sound of footsteps from going through. Appropriate padding will leave no high or low areas, and prolong the life of a floor.