You have shopped all over town and have finally found the perfect carpet for your home. As a very competent home handyman, you see no reason why you cannot install the carpeting yourself and save money. During the diy contemplation you realize that the plush carpeting you have chosen is so soft that it does not need an under pad, as shown in Figure 1, and therefore you can save even more money and save some valuable the time in completing the project.
Figure 1 - Carpeting over under padding
Although many carpet supply houses or retailers will promote the installation of an under pad as a something that will enhance the overall feel, which it will. There are other reasons, which are equally if not more important.
Figure 2 - Installing under pad
Carpets are created by weaving fibers made of wool, nylon, polyester, or other synthetic material through a fine mesh and allowing them to protrude on one side of that mesh. The mesh is usually made of Jute, but can also be made of synthetic materials. The underside of the carpet is coated with a glue to hold the fibers in place. If you rub your hand over the underside of a carpet, you will find it to be rough and very abrasive.
If you rub the underside of the carpet against concrete or wood, the glue and mesh backing will wear and eventually release the fibers that they are holding in place. The installation of an under pad between the underside of the carpet and the flooring, that it is to be placed on, will extend the life of the carpet dramatically.
As the mesh is generally made of Jute the mesh is susceptible to damage by moisture. Most under pads provide a moisture barrier between the floor and the carpet. Again, this will extend the life of the carpet.
Do not lay new carpet over old under pad. Under pad deteriorates over time, it looses its ability to protect the carpet from both abrasion and moisture. Always replace the under pad when installing new carpeting and do not place new under pad over old under pad. In the same manner, do not install new carpet over old carpet.
Most people hope and expect that their wall-to-wall carpeting will last for many years. As a home improvement, it is usually a sizeable investment. If it is properly installed, using an under pad, stretched tightly and steam cleaned yearly, wall-to-wall quality carpeting can easily last for 10 to 20 years.