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"Soundproof your existing and new walls by several methods"

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Dear Jim: We need to soundproof the walls between some rooms and also in a new bedroom we will be adding. What is the best way to block sound and will the same methods work for the old walls and a new efficient bedroom? - Patrick M.

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A: 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 2x4 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.

Instant Download Update Bulletin No. 691 - buyer's guide of soundproofing product manufacturers, 18 soundproof wall designs describing basic construction (single stud or double stud walls) and absorptive materials, STC (sound transimission class) ratings, fire ratings, recommended room-to-room STC's, and soundproofing tips for homes.

Dear Jim: It might sound crazy, but I am looking at room air conditioners now to beat the summer rush. Is there much difference in the effectiveness of through-the-wall air conditioners as compared to window units? - Becky A.

A: It never hurts to beat the rush before the first hot day hits. There is very little difference in the effectiveness of the two installation methods. The efficiency of the units is not affected.

A through-the-wall installation is more expensive; however, it is easier to seal air leaks than a window installation. This is a plus during the winter. A window installation allows you to easily move it to another room.

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