What underlayment does
Four jobs: a vapor barrier against moisture from below, a cushion that smooths minor subfloor irregularities, a sound layer that reduces impact noise to the room below and hollow sound in the room itself, and a slight thermal break. Products are specified by the flooring manufacturer, and using the wrong one can void the warranty.
Attached pad versus separate underlayment
Most quality vinyl plank and some laminate come with an attached pad. In a single-family house that is usually enough with a vapor barrier beneath. In a unit above another unit, a separate acoustic underlayment is often needed to meet the building's sound requirement, and the manufacturer's instructions say whether a product with an attached pad may be installed over an additional layer.
Ratings
IIC measures impact noise through the assembly: footsteps, dropped objects, dog nails. STC measures airborne noise. Building rules usually state a minimum IIC for hard flooring in upper units, and sometimes require testing in place. Underlayment manufacturers publish IIC and STC values tested over specific assemblies; the right comparison is the one tested over a construction like yours, usually a wood-joist assembly with a drywall ceiling for Tacoma's wood-frame buildings.
What Tacoma buildings require
Condominium associations in Tacoma, University Place and Gig Harbor commonly require a minimum IIC, a specific underlayment class, or board approval of the product and underlayment before installation, and some restrict hard flooring to certain rooms. Apartment leases may require an area-rug percentage over hard floors. Ask for the rule before the measure; the installer specifies to it and provides the documentation.
What installers specify
A rigid-core vinyl plank or laminate with an acoustic underlayment whose tested IIC meets the requirement over a comparable assembly, installed with the perimeter isolated so the floor does not transmit through the walls, and transitions that do not bridge the isolation. The documentation goes in your file and to the board.
Sound in your own unit
A good underlayment also removes the hollow click of a floating floor. If a floor sounds hollow, it is usually the pad or the flatness of the subfloor, not the product.
