Most residential carpet is installed over a separate carpet pad (called "cushion" in the industry) that makes the carpet softer and more resilient underfoot, reduces noise, provides a little thermal insulation, and prolongs carpet life. Don't make the mistake of installing a quality carpet over the top of a marginal carpet cushion.

Carpet cushion may be made from any of several materials, including various types of natural and synthetic fibers and urethane foam.

A carpet cushion that is too thick over-stresses the carpet backing and is too soft underfoot. The density of the carpet cushion–not its thickness–is the most important factor to consider when choosing a carpet pad. Ask your carpet retailer for a Class 1 cushion for moderate traffic areas and a Class 2 cushion where there will be heavy traffic, such as stairs, hallways, and family rooms.

The Carpet and Rug Institute recommends a maximum thickness of 7/16 inch for a residential carpet cushion. If the carpet has a lower pile height, such as a Berber or a commercial-like cut pile, the cushion should be even thinner--3/8 inch or less. But it should be firm and resilient.

FHA minimums are 6-pound cushion under standard residential cut pile and 8-pound cushion under a thinner Berber or low pile. Expect to pay about $3 to $3.50 per yard for quality cushion.