While several factors determine the best underlay for laminate flooring, About.com recommends 1/16-inch foam padding that comes in approximately 3-foot wide rolls. This material provides some give to the laminate to increase its useful life and is appropriate in most situations.
When installing laminate over concrete, a special underlay with a moisture barrier is required. Concrete wicks moisture from the ground that would otherwise ruin the laminate. When the installation includes sub-grade applications, it also requires a special moisture barrier.
A third important factor to consider is the need for additional insulation with the new floor. Some types of underlay provide better insulation than others. If a cold floor is a problem in the home, increasing the R-value helps to make it more comfortable.
Underlay provides sound insulation, which is very useful in multi-story homes. It gives the floor a more comfortable feel when one stands on it for an extended time, and it prevents minor imperfections in the subfloor from showing through in the new laminate.
Some laminate floor comes with the underlay already attached. With this flooring, it is unnecessary to purchase separate material. Manufacturers recommend that installers not use a second underlay with these materials. It would make the floor feel too soft and decrease its life expectancy.