Q: Our house walls need more insulation for efficiency and perhaps better soundproofing. We are also planning to add a bedroom. Will insulation make the existing rooms quieter and the new room more soundproof?
A: Adding wall insulation can be an expensive project and above the skill level of most do-it-yourselfers. In many older houses, particularly ones with masonry wall, there is little space inside the walls for additional insulation. In many cases, it’s worthwhile to spend more on insulation with the highest R-value per inch, which is a number showing the ability of insulation to resist the transfer of heat. Higher R-values indicate more effective insulation.
Any type of insulation you add to save energy will help somewhat to soundproof the walls, but you need additional improvements for significant noise-dampening.
For a new room addition, carefully installing fiberglass batt insulation boosts your home’s energy efficiency. The key word is “carefully” because fiberglass batts don’t provide protection from gaps at the wall joists—it’s up to you to make sure you caulk or use spray foam in the spaces before insulating. Every unfilled crack and gap reduces the overall efficiency of the new wall.
Another option is to build a second insulated wall against the inside existing wall. You’ll lose only about four inches of floor space, which you can frame with 2 x 4-foot pieces of lumber, insulate with foil- or kraft paper-faced fiberglass batts, and then cover with drywall. This is particularly effective for older houses with full masonry walls.
Installing new windows makes the greatest improvement in saving energy and blocking outdoor noise. Most new windows also use heavy inert gases in the gap between the panes that further reduces sound transmission.
Also, simply caulking and weather stripping your old windows can have a dramatic effect on reducing noise, and it improves the energy efficiency of your house. In turn, your heating and cooling system won’t need to use as much energy.
When planning a new bedroom, do some research regarding the STC (sound transmission class) rating for various types of wall construction. A typical uninsulated interior wall with drywall on each side of 2-by-4 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. These openings are also culprits for drafty rooms, so using an inexpensive outlet insulation kit is one more way to improve energy efficiency. Adding insulation inside the wall increases the STC by very little—normal conversation would still be easily heard. At the other extreme, with an STC of 66, yelling is barely audible in adjacent rooms.
Your first step in soundproofing interior walls is to get out the caulk gun and seal any gaps in the walls and at joints. It won’t help energy efficiency much, but it does block the vibrations that create sound.
If it’s normal household sounds and voices, many standard soundproofing methods are effective. If you want to block deep bass vibrations from music or a home theater, a thicker wall is best. You can create that by installing two layers of drywall or using a high-density wallboard. If using drywall, you can nail them tightly together or leave them slightly separated for the benefits of decoupling.
Decoupling the two surfaces of a wall is critical to block sound transmission. That means drywall on one side is not attached to the same wall studs as the drywall on the adjacent wall. One simple method is to install a second layer of drywall over the existing one. Make sure not to screw it into the wall studs or very tightly to the existing drywall so it stays decoupled. This method also increases the thickness of the wall for blocking bass vibrations.
Another method is to stagger the studs on wider headers and footers in the wall cavity. The drywall on each wall is attached to every other stud, so there is no direct path for the sound to travel.
James Dulley is a nationally recognized mechanical engineer writing about home energy issues for the National Rural Electric Cooperative Association. If you have a question for Jim, please email firstname.lastname@example.org, or mail to James Dulley, Michigan Country Lines, 2859 W. Jolly Rd., Okemos, MI 48864. Be sure to let us know which electric co-op you receive service from. Visit dulley.com for more home improvement and do-it-yourself tips.