Underpad 101: Polyurethane Foam

Rebond or Chipfoam Carpet Cushion Underlay
Photo © Cheryl Simmons

Underpad made of polyurethane foam is the most common form of pad used under carpeting in residential settings. It can be categorized into one of three main types: prime cushion, bonded foam, and frothed foam.

Prime

Prime cushion is a polyurethane foam made from virgin (i.e., non-recycled) materials. It can range from very low density to high density. Prime foam is a denser version of the type of foam used in upholstery cushion.

When considering prime cushion, be sure to carefully study the specifications of the pad. Many prime polyurethane pads are not dense enough to adequately withstand traffic, and some fail to meet carpet manufacturers’ specifications for texture retention warranties.

Prime cushion features many “air pockets” which provide a soft step (at least in the beginning). These air pockets are also the reason the pad will not stand up well to traffic. Densified prime cushion features smaller air pockets, which increases the pad’s density and helps to enhance the performance of the cushion.

Bonded Foam

Commonly known as rebond, or “chipfoam”, bonded polyurethane foam cushion is made from scrap bits of various foams. It is easily distinguishable by its multicolored appearance.

Bonded foam is the most common type of carpet underpad used in residential applications; according to the Carpet Cushion Council it represents 85% of the carpet cushion produced in the United States. It comes in various densities and thicknesses, so there is an appropriately bonded foam cushion for virtually every residential carpeting application. Rebond cushion typically offers the best value for your money.

Rebond uses up the scrap bits from the manufacturing process of other types of foam. These scrap bits would otherwise be thrown out, so using them to make other products saves them from ending up in landfills.

Frothed Foam

Frothed polyurethane foam is made with carpet backing machinery, so it is how an attached cushion is applied to carpets (such as KangaBack). It can also be made into a separate cushion, by following the same process but applying it to non-woven material.

An example of a frothed foam cushion is visco-elastic foam, more commonly known as memory foam. The foam depresses as weight is applied, and then returns to its original form.

Frothed foams are generally quite dense. They are also usually considerably more expensive than prime or rebond cushions.

Minimum Requirements

The Carpet Cushion Council has provided a guideline of the minimum thickness and density requirements for each type of carpet underpad. These guidelines have been adopted by the U.S. Department of Housing and Urban Development (HUD).

Cushion density is calculated as the weight, expressed in pounds, per cubic foot of cushion.

Cushion thickness is expressed in a fraction of an inch.

Residential applications are divided into two categories: Class 1 refers to light and moderate applications in the home, such as living and dining rooms, bedrooms, and family rooms. Class 2 refers to heavier use areas, such as stairs, and multi-family units.

Following are the minimum recommendations for each type of foam pad in residential applications, as per the Carpet Cushion Council:

Prime foam:

  • Class 1: density 2.2 pounds, thickness 0.375 inches (3/8”)
  • Class 2: Not recommended

Densified prime foam:

  • Class 1: density 2.2 pounds, thickness 0.313 inches (roughly 10/32”)
  • Class 2: density 2.7 pounds, thickness 0.25 inches (1/4”)

Bonded foam:

  • Class 1: density 5 pounds, thickness 0.375 inches (3/8”)
  • Class 2: density 6.5 pounds, thickness 0.375 inches (3/8”)

Frothed foam:

  • Class 1: density 10 pounds, thickness 0.25 inches (1/4”)
  • Class 2: density 12 pounds, thickness 0.25 inches (1/4”)

Maximum Thickness

In any application, the thickness of the carpet cushion should not exceed 0.5 inches (1/2”). Looped Berber styles require more stability than cut-pile carpets, and as such should have a thinner, firmer pad. The Carpet and Rug Institute recommends a cushion no thicker than 0.375 inches (3/8”) with a minimum density of 8 pounds for Berber carpets.

Feeling Underfoot

The density and thickness of the cushion will affect how it feels underfoot. Typically, higher densities and lower thicknesses provide a firm feel, while lower densities and higher thicknesses provide a softer step. For example, an 8 pound, 3/8” pad will feel firmer than an 8 pound, 1/2” pad will. The density is the same, but in the case of the 3/8” pad, it is packed into a smaller amount of cushion, resulting in a firmer feel.

Selecting the right cushion involves finding the best balance of density and thickness for performance and feel. To determine the best cushion for your home, consider the amount of traffic the cushion will be subjected to, the style of carpet you are installing, and the feel you would like it to have (soft or firm).

Aim Higher

Keep in mind that the above guidelines are minimum recommendations. When possible, we recommend aiming higher than the minimums. For example, for a rebond cushion under a cut-pile carpet (such as a Saxony or frieze), we recommend a cushion with a minimum density of 7 pounds in low-traffic areas, and 8 pounds or higher in heavy traffic areas of the home.