Urethane foam is the material commonly known as foam rubber, which is produced by blowing gas into a heated, liquid chemical mixture. When it dries, the foam rubber is porous and spongy, and the gas trapped within the urethane foam gives it great insulative properties. This same material is used as cushioning for furniture, mattresses and car seats.
Manufacturers use two types of urethane foam in carpet pads: prime foam and frothed foam. The padding known as prime foam comes in different thicknesses and densities, and it's made from large pieces of foam sliced into sheets. While this type of foam rubber padding makes your carpet soft and comfy, it's not recommended for high traffic areas like stairways or halls. Foam rubber's cushioning comes from its air pockets, but as with waffle rubber, the air doesn't always provide enough support to limit carpet movement and protect your carpet from wear.
The HUD standard for prime foam, when used in light or moderate traffic areas is 0.375 inch (9.525 millimeters) thick and 2.2 pounds per cubic foot (35.2 kilograms per cubic meter). HUD does not recommend it for heavy traffic areas [source: CarpetInspector.com].
Some carpets have built-in pads made from frothed urethane foam. This material is similar to prime foam, but instead of having gas blown into the chemical mixture, it's mechanically frothed while liquid. The result is a denser, firmer type of foam that can be attached directly to the carpet. Carpets with built-in padding don't need a separate pad, but the attached foam is usually too thin to stand up in high traffic areas.
In recent years, manufacturers have extended the use of frothed foam to separate padding. They apply the foam to a nonwoven fabric to make the pad. Frothed foam pads are among the best on the market because they are extremely durable and provide excellent support. Unlike other foam pads, they contain no volatile organic compounds (VOCs), so off-gassing after installation is not a problem. They can be used with all carpets, and they resist indentation from furniture, which decreases carpet wear.
The HUD standards for frothed foam pads are a minimum of 0.25 inch (6.35 millimeters) and 10 pounds per cubic foot (160.2 kilograms per cubic meter) for light and moderate traffic. The same thickness, but 12 pounds per cubic foot (192.2 kilograms per cubic meter) is recommended for heavy traffic areas.
Keep reading to find out about the most common carpet pad.