Carpet underlay, also known as padding or cushion, is a material installed under a carpet or rug. Underlay products extend the life of the carpet by protecting it from excessive wear. Carpet padding also adds comfort and warmth to the floor surface, as well as a bit of insulation. While many homeowners may be tempted to buy the thickest underlay available, it is more important to match the underlay to the carpet.
Read the manufacturer's recommendations that come with your carpet. Narrow your search according to the guidelines for your carpet. Only choose underlay that falls within the recommended size and weight range for your carpet.
Match the thickness of the padding to the type of carpet. According to the Carpet and Rug Institute, most residential carpet requires underlay that's 1/4 inch to 7/16 inch thick. Berber, thin loop and cut-pile carpets require underlay no thicker than 3/8 inch.
Select an underlay product with sufficient strength, or density, to support your carpet. The Carpet and Rug Institute recommends padding with a density of 2.72 Kilogram per cubic foot for most residential carpet, and at least 3.63 Kilogram of density per cubic foot for Berber carpets. Low-profile rugs, which are relatively thin and have shorter piles or loops, require at least 3.63 Kilogram of density per cubic foot. The very short fibres on a low-profile rug means that the only cushioning for the carpet's rubber backing comes from the underlay, so proper density is especially critical in this type of application.
Find the right underlay for each room based on traffic levels. In high-traffic areas such as the living room, choose padding that is relatively thin and firm. In low-traffic areas such as bedrooms and guest rooms, choose thicker and softer padding for comfort.
Compare different underlay materials. According to Anderson's Carpet, rubber underlay is the most expensive, but it provides the most air circulation below the carpet, lowering the risk of moisture build-up or mildew. Foam is an environmentally friendly alternative made from recycled materials, and is moderately priced. Felt is the most affordable type of underlay. It provides a high level of insulation but is rarely used in modern installations.
Ask about the warranty for each underlay product you are considering. According to Utah State University, quality underlay should come with a warranty that covers the life of the carpet. Some products even come with a warranty that covers the life of two carpet installations, which means you can completely replace your rugs and not change the underlay.