Underlayment is an amazing invention created by the flooring gods out of necessity. Underlay has an impressive list of benefits, including: adding warmth to your floors so your feet aren’t so cold on new tile flooring, adding comfort and a slight ‘give’ underneath wood or vinyl floors, reduced sound transmission (great for houses with noisy teenagers!), as well as reducing in room step noise. This image is a very simplified example of using underlayment versus not using it, but as you can see the benefits greatly outweigh any reason not to.

IIC stands for Impact Insulation Class, which refers to impact sounds such as shoes hitting the ground, dropping a cell phone, or other scenarios where something it dropped or impacts the floor. STC stands for Sound Transmission Class, which refers to any audible noise (not an impact) such as music playing aloud, a conversation, or a loud television, for example. With both of these ratings systems, the higher the number rating is for the underlayment, the better it will suppress sound, and the quieter a room will be. You may want to pay close attention to these ratings when shopping for underlayment if sound reduction is important to you.