Common underlayment materials for flooring include hardboard, particleboard, plywood, cement backerboard, and mastic with binders (latex, asphalt, polyvinyl-acetate). From the list you can see that underlayment is either rigid or thick viscous. In addition, floors with moisture problems need a moisture barrier, typically some type of plastic sheeting.
Rigid underlayment, called backerboard, is popular for installing new ceramic tile flooring. That’s because tile needs a firm base so traffic doesn’t crack it. Popular rigid underlayment includes thin sheets of Philippine mahogany and similar products as they are denser than fir and other plywoods. If the subfloor is relatively thin or space between joists allow the subfloor to sag when walked on, install plywood underlayment. The strongest is cement backerboard.
Mastic is an adhesive with a thickener or binder. The thickener gives the mastic bulk so it can be troweled smooth and level. In addition, some flooring mastics can help waterproof the subfloor, keeping moisture away from the flooring materials. Other flooring mastics retain some elasticity and adhesion after installation, making them an excellent flooring adhesive as well.
Many professional hardwood floor installers use builder’s felt, also known as roofing felt or builder’s paper, as a final underlayment before laying flooring. They recommend 15-lb. felt for most jobs and 30-lb. felt for floors that are not as smooth or that require additional moisture protection.
If you are installing underlayment with an adhesive, make sure that the adhesive is right for the job. It should be moisture-proof for most applications. In addition, don’t spread it on too thick as it may not sufficiently dry.