To Cut Noise, Block the Conduit for It

    Dear Jim: Our house walls need more insulation, and they should be more soundproof. We are also planning to add on a bedroom. Will insulation make the existing rooms quieter and the new room more soundproof? - Bob J.

    Dear Bob: Adding wall insulation to save energy helps somewhat to soundproof the walls from outdoor noise, but you need additional improvements for a significant improvement.

    If your windows are old, installing new ones makes the greatest improvement in blocking outdoor noise and saving energy.

    Soundproofing a room is a subset of making it quieter. Soundproofing refers to blocking noise from outdoors or adjacent rooms. To have a quiet room, also consider the acoustic properties inside the room. A lot of sound is generated inside a room by plumbing, air from registers, clocks, voices and movement.

    The soundproof quality of a wall is rated by sound transmission class. A higher STC number indicates a more soundproof wall. A typical interior wall with drywall on each side of 2x4 framing has an STC of about 34.

    If there are common heating ducts and holes for electrical outlets and phone jacks, the STC of that interior wall may be only 25. Adding insulation inside the wall increases the STC little. Normal conversation can easily be heard and understood through this wall.

    At the other extreme, walls with an STC of 66, yelling can barely be heard in adjacent rooms.

    Another consideration is the type of noise you want to block. If it is just normal household sounds and voices, there are many soundproofing methods. If it is deep base sound from music or a home theater, a wall with more mass (weight) is most effective.

    Your first step in soundproofing interior and exterior walls is to get out the caulking gun and seal any open joint and gap in the walls. This isolates and blocks the free movement of vibrating air (sound). On exterior walls, this also reduces outdoor air leakage to save energy.

    Decoupling the two surfaces of a wall is critical to blocking sound. This means not allowing the two wall surfaces to be rigidly connected through the wall studs. One simple method is to install a second layer of drywall of the existing one. Don't screw it into the wall studs so it stays decoupled.

    This method also increases the mass of the wall.

    Another method to decouple walls is installing thin resilient metal strips over the studs. The new drywall is attached to these strips and not directly to the studs.

    Consider installing a strong soundproofing board made of recycled materials, such as Homasote 440, over the studs. Screw the drywall or resilient channels to this board.

    Send inquiries to James Dulley, Topeka Capital-Journal, 6906 Royalgreen Drive, Cincinnati, OH 45244 or visit www.dulley.com.

    (c) 2007 Topeka Capital Journal. Provided by ProQuest Information and Learning. All rights Reserved.