What is the recommended underlayment for this product when installed on a 3/4 in. plywood subfloor?

A lot of professional installers use Aquabar "B" as a moisture retarder. It's not a vapor barrier, it just slows down moisture: Tar+Kraft Paper-Smell.

Hi there! Thank you for taking the time to reach out to us with your question. Nothing is as striking and inviting as a home with true hardwood. Laminated rosin paper or #15 builders felt (tar paper) acts as a moisture retarder and may be used to reduce movement caused by changes in subfloor moisture, thereby reducing cupping and warping. (This is especially helpful over crawl spaces and basements.) In addition, the use of these materials can give the flooring a more solid feeling, reduce sound transfer, prevent noise caused by minor irregularities and debris, and make it easier to slide the wood together across the surface of the subfloor. Kraft paper may be used to make the installation easier but DOES NOT serve any other purpose. For your convenience, please visit the following link to view the full installation instructions for a solid 3/4" harwood product. https://bit.ly/2DBwyxW We hope that this information is helpful. Have a great day.