Sandwich a Sound-Deadening Fiberboard Layer Under the Drywall

The Problem

Converting a basement into a rental suite — or any room where two households live on opposite sides of one assembly — makes soundproofing a requirement, not a nicety. Drywall alone transmits footsteps and voices, and once the ceiling is closed up, the chance to fix it is gone.

The Technique

A DIYer building a basement suite used a two-material sandwich before the drywall went on:

1. Sound-deadening fiberboard on the framing first. It's a recycled wood-fiber panel that comes in 4×8 sheets and hangs like drywall, but much lighter. Cover the ceiling and any wall that separates the unit from the rest of the house — in this build, the entire ceiling plus the walls to the garage and main house.
2. Mineral wool batts in the cavity behind it, the dense acoustic type, for absorption inside the assembly.
3. Drywall over the fiberboard as the finished surface, installed normally.

The fiberboard adds a dissimilar, sound-absorbing layer between the framing and the drywall; the mineral wool kills the cavity echo. Neither is exotic — both are stock items at building suppliers — and the labor is the same hanging work you're already doing.