There are three major types of carpet cushions that most homeowners recognize: fiber, sponge and foam rubber. There are different types and grades within each grouping. Thickness of cushion and weight in ounces per square yard are the yardsticks which allow you to differentiate one from another.
The type and amount of foot traffic in a room determines the type of cushion you should use. Heavy traffic areas, stairwells, and hallways require cushions that are no thicker than 3/8 inch. These cushions should also be dense and heavy. Do not use a thick, light weight cushion in these areas. It will allow the carpet backing to flex too much. This can cause the carpeting to fall apart.
If you want a soft, luxurious feel in a bedroom or other lightly traveled room, choose a 1/2 inch thick pad. To increase the life of the carpeting, choose a high density or weight cushion. Remember, the cushion is the foundation for the carpet. It just doesn't make sense to install a flimsy, inexpensive cushion beneath an expensive carpet.
The following article was taken directly from Ask the Builder (www.askthebuilder.com). Also, please visit my websites at www.americarpetfloors.com and www.stylishrugs.com for all your flooring needs.