Soundproofing on Existing Walls

The basic methods to soundproof an existing wall (room-to-room or outdoors) are similar to new wall construction. They are often more effective on an existing wall because the extra layer of drywall provides an additional acoustical barrier and mass to damp the low-frequency sound.

The soundproof characteristics of a wall are rated by sound transmission class (STC). Normal conversation will be heard and understood through a STC-25 wall. When soundproofing is improved to STC-60, which is not difficult to accomplish, loud shouting can be heard, but not understood.

For an efficient outdoor wall, just packing in thermal insulation, such as fiberglass or cellulose, will save energy, but it alone will marginally help to soundproof it. Making the wall airtight for efficiency will likely have a greater impact on blocking outdoor noise from traffic and neighbors.

Interior walls in most homes are not filled with insulation. With a single layer of drywall nailed to 2×4 studs, the STC is about 34. This means loud talking can be understood through the wall. If the home is older with some settling, there will be gaps and openings to make things even worse.

The minimum recommended STC for any bedroom wall is 48 and this for an adjacent bedroom which is usually not noisy. For a bedroom wall adjacent to a kitchen or noisy family room, a minimum STC-52 is recommended and an STC-58 is considered optimal. Noise tolerance level also varies with individuals.

With an existing wall, just adding another layer of 1/2-inch drywall over the existing wall helps substantially with minimal floor space loss. This also will seal many of the gaps and direct air paths where sound waves move easily. Glue the new drywall in place, possibly with some cushioning sheet (cork) under it. If it is nailed up, this will create a direct sound path.

If you can handle losing a little more floor space, first nail resilient channels up to the old wall. The new layer of drywall will be attached to these soundproofing channels. Another better, but more expensive option, is to attach special sound barrier fiberboard made from recycled newspapers.

For new construction, use fiberglass insulation inside all the walls and hang all the drywall on resilient channels. A thicker staggered-stud wall uses a wide wall base plate so no single stud touches both wall surfaces. Vary the locations of heating register and electrical outlets so they are not directly across from one another on the walls in adjacent rooms.