For walking support and comfort, a carpet cushion for wall-to wall broadloom and large area rugs should be about 3/4 inch thick. It should be resilient and soft, but of high enough density to give firm support for the carpet's backing, so the carpet won't stretch and wear out as quickly. A thin, resilient cushion is better for small area rugs, and a common rule of thumb is that the rug should be no more than 1/8 inch to 1/4 inch from the floor. To lie flat over a rough surface floor, such as brick, increase the thickness. Even if an area rug will be placed atop broadloom carpet, it will still need a stiff -finished cotton or linen, or sturdy felt underlayment, to keep it from shifting and rippling.
Selecting the right carpet cushion type depends on the quality of the carpet, its location in the home, and how much wear it will get. Most cushions are priced according to weight per square yard, and may not be included in the carpet's purchase price. For most rooms, 40-ounce padding is adequate, but a 48-ounce cushion may be wiser for stairs and heavy-traffic areas. Padding is available in several types and prices, and it's always a good idea to purchase the appropriate padding along with the carpet to ensure proper fit. Here are some common cushion types: a Urethane foam padding, made in a continuous, flat sheet, has excellent durability and strength. It's the most popular padding for residential use because it offers comfort and support. It resists moisture, but may lose its bounce and wear thin over time. Sponge or foam rubber padding comes in sheets with waffled or flat surfaces, and the top surface usually is bonded to a facing material to facilitate even carpet wear. Use the flat padding under area rugs to reduce slippage. These tend to wear out more quickly if exposed to floor polishes and cleaners.
Foam rubber padding is non-allergenic and mildew resistant. Sponge rubber padding is not recommended for outdoor use. Felted padding, made of animal hair, jute, or a felt and fiber mixture, wears well and provides firm support, but tends to shed. For the most part, felted padding has been replaced by more cushiony synthetics and foam rubbers for residential use. It mildews in high-humidity areas and may aggravate allergies.
Self-cushioned carpet has a layer of latex cushion bonded to the underside. It is usually thinner and less dense than a separate cushion, and may be less resilient. This carpet can be installed directly over concrete or other subfloors, however.
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See Also: Carpet Cushion Guide