For solid wood flooring, it’s best to choose an underlay that offers sound reduction or thermal insulation, depending on your needs. If you're installing over concrete or an uneven surface, look for an underlay with a built-in moisture barrier. In glue-down installations, a glued underlay can be used for both solid and engineered wood, providing additional stability and adherence. Always ensure the underlay is compatible with your subfloor and installation method.