Soundproofing Interior Walls2

During building or remodeling, an effective and affordable way to improve the soundproofing performance of walls and ceilings is to put batt or blanket insulation between studs or joists.

Major insulation manufacturers, including CertainTeed, Johns Manville, Knauf Fiber Glass, and Owens-Corning, market 3 1/2-inch-thick fiberglass or rock wool batts specifically for this purpose. They are both excellent at absorbing the sound that would otherwise travel through the air.

Designed to fit between studs, acoustic batts are 14 1/2 inches or 22 1/2 inches wide and 3 1/2 inches thick. Most are the same as R-11 or R-13 insulation batts.

Kraft-faced batts are friendliest to handle and easiest to fasten in place (a vapor barrier is not needed for interior walls). They should be installed tightly between framing members, and snugly around pipes, electrical boxes, wires, and heating ducts with as few hollows or gaps as possible.

Leaving only a small portion of a wall or ceiling uninsulated can dramatically reduce its sound-reducing performance. Batts can be friction-fit in wall cavities; if temporary support is needed, two or three bands of drywall tape may be stapled horizontally across studs. In ceilings, batts should be installed just above the backside of the ceiling material.

 A conventional wood-stud wall packed with insulation yields an STC (Sound Transmission Class) of about 38, better than the 15 to 35 STC of an uninsulated wall but still considered low. Boosting performance to recommended levels calls for additional measures. Using metal studs helps; the same wall, built with 2 1/2-inch metal studs, yields an STC rating of 45.

Sound Wall Construction

Another way to achieve better performance is to apply a second layer of 1/2-inch gypsum wallboard to one side of the wall. This gives the surface more mass, making it less prone to vibrate and transfer sound waves. Adding this layer to one side of an insulated wall increases the STC rating to 40; adding it to both sides pushes the STC to 45.

An even more effective way to build an interior wall is to mount 1/2-inch gypsum wallboard on special resilient channels that run horizontally across the wall. These channels absorb sound so it isn’t conducted through the wall studs, resulting in an STC rating of about 46. Typically, the drywall is screwed to a flange on the channels, not to the studs. Combining insulation, channel-mounted wallboard, and a dual layer of 1/2-inch gypsum on one side achieves an excellent STC rating of 52.

In roughly the same category is a wall with staggered wall studs. Though this requires more labor and framing material, a wall of 2-by-4 studs, staggered along 2-by-6 bottom and top plates with two thicknesses of fiberglass insulation, produces an STC of about 50. Because the wall surfaces are each fastened to an independent set of studs, noise can’t travel through the studs from one surface to the other.

Where codes and safety allow, consider eliminating fireblocking in interior walls; these short blocks, mounted horizontally between wall studs, transmit noise readily from one wall surface to the other. If you’re thinking about doing this, be sure to check with your local building department.

Ceiling and Floor Construction.

A floor-ceiling construction that produces an STC of 53 is achieved by mounting 1/2-inch gypsum wallboard to resilient channels fastened to 2-by-10 ceiling joists with 3 1/2-inch-thick batts between the joists. In this scenario, the floor above has a plywood subfloor, particleboard underlayment, carpet pad, and carpet.