Insulating against sound as well as energy loss


January 01, 1994|By James Dulley | James Dulley,Contributing Writer

Q: I am planning an efficient room addition. Will insulation in the walls also block noise? Can you give me some tips for blocking noise between other rooms?

A: The thermal insulation in outside walls will block much of the outdoor noise, but it won't quiet the noise transmission between rooms. Energy-efficient, airtight windows also help block outdoor noise.

The sound transmission class (STC) index is used to compare quietness of various wall construction methods. It rates how well each wall blocks noise between rooms. A higher STC rating is better. An STC of 58 is recommended for a family room-to-bedroom wall and 55 for a bedroom-to-bedroom wall.

As a reference, you can easily hear normal speech through a wall with an STC rating of 25. At an STC of 42, loud speech is audible as a murmur. At an STC of 50, loud speech is not audible. A typical interior wall with one layer of drywall on each side has an STC of 34.

The simplest soundproofing wall construction method is to attach 1/2 -inch soundboard under the drywall on each side of the studs. This increases the STC to 45. Several companies make soundproofing underlayment panels, often from recycled newsprint, tires, sea grass, etc.

Instead of using the soundboard, you can mount resilient metal channels between the drywall and the studs in one room and add fiberglass insulation inside the wall for an STC of 50. This method can also reduce energy loss if you shut off the furnace or air conditioner registers to an unused room.

To further reduce noise, run separate heating ducts to each room. This blocks a direct path for noise between rooms and lets you have better control over the amount of heat going to each room so you can often set your thermostat lower and still be comfortable.

Stagger electric outlets in common walls so they are not directly opposite one another. This eliminates another direct noise path.

The quietest wall design uses staggered studs, separate base plates and batt insulation. The insulation is woven through the staggered studs. This totally isolates one side of the wall from the other for an STC of 60.

For existing rooms, you can add a resilient channel to one wall and cover that with another layer of drywall. Or add a layer of 1/2 -inch soundboard covered with drywall.

Write for Utility Bills Update No. 525 showing 18 soundproof wall designs, STC ratings, recommended STCs between various rooms, list of manufacturers of soundproofing products, and tips to make existing walls quieter. Include $2 handling fee -- cash or check to: James Dulley, The Baltimore Sun, 6906 Royalgreen Drive, Cincinnati, Ohio 45244.

Q: I have noticed carpet discoloration along the baseboards of my home. Someone told me that my furnace and air-conditioning ducts can cause this. Will you explain?

A: Leaky or poorly designed ducts may cause the discoloration problem.It also is an indication of an energy inefficient heating system. This is particularly true if you close off unused rooms.

When your central blower comes on, it can create a slight positive pressure in some rooms and a slight negative pressure in others. In rooms with a negative pressure, dirty air from inside the wall cavity is sucked in by the baseboard and the carpet filters out the dirt. Have your duct system checked for leaks and duct dampers adjusted for even flow.

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