The underlayment for laminate floors is a material that provides a sound barrier or protects the laminate from moisture, depending on the type of underlayment you use. Whether you are laying the laminate below ground level or at ground level or above, you need to have the correct type and number of layers for the underlayment.
The underlayment for laminate floors does not make the floor softer to walk on. It is intended to muffle sounds below the floor or stop moisture from concrete or wood floors from reaching the laminate. Manufacturers do not recommend using two layers of underlayment under laminate flooring because it can create more movement between the plank seams, which could cause the planks to separate or make a squeaking sound when you walk across the floor.
Sound barrier underlayment is used when you want to muffle the sounds between floors. Standard foam underlayment offers some reduction from sound, but modified underlayment made from rubber, fiber or cell foam has a higher level of sound reduction. Standard foam is about 1/8 inch thick, but the modified underlayment is denser and thicker. Sound barrier underlayment is used on floors that don't need a moisture barrier, such as above-ground-level floors.
Combination underlayment uses standard foam and 6 mil polyurethane film to make a moisture barrier that is used on ground floors or below-ground-level floors that are subject to high moisture levels. Concrete floors in basements are especially prone to high moisture content because the concrete wicks moisture from the ground underneath. The moisture barrier protects the underside of the laminate from coming in contact with the moisture to prevent mildew and mold.
Cork underlayment, which is about 1/4 inch thick, must be glued down before the laminate floor is installed. The cork has a high level of sound reduction. Generally, the cork is used in buildings where sound between floors could be a problem, such as a condominium. Glued cork underlayment is also used when the subfloor needs to be raised before installing the laminate. If you are thinking about using two layers of underlayment to raise the subfloor height, you can use one layer of the glued cork plus the laminate.