What is the primary purpose of using an underlayment under flooring?

Using an underlayment under flooring serves several important functions, one of which is to create a moisture barrier. This is particularly critical in areas where moisture can penetrate from the subfloor, such as in basements or in contact with concrete. A moisture barrier helps prevent water damage, mold growth, and deterioration of the flooring material, thereby ensuring the longevity and integrity of the flooring system.

While the other options also represent beneficial aspects of underlayment, the primary function as it relates to overall floor protection is its ability to serve as a moisture barrier. Sound insulation and thermal insulation are important too, but they play a secondary role compared to the imperative need to safeguard flooring from moisture exposure. Enhancing comfort is valuable for user experience, yet it does not address the critical protective functions that an underlayment is fundamentally designed to provide.