Insulation does more than create a thermal barrier between exterior and interior of a home. Insulation can also be used in interior walls to control sound vibrations originating from an adjacent room.
Play rooms, great rooms, and home theaters are known for echoing loud noises through the ceiling, floor and walls of nearby rooms. Referred to as nuisance noises, these sound waves can travel easily through a hollow space, like an uninsulated wall or ceiling.
Nuisance noises are generally referred to as either impact noise or airborne noise, referring to the method the sound uses to travel. Impact noises travel through materials in direct contact, while airborne noises travel across the air, like your voice.
Fiberglass batts, rigid foam boards, spray foam, mineral wool, and cellulose provide interior wall sound control to varying degrees, but some are more effective than others. Here we will discuss the usefulness of interior wall insulation for sound control and how each insulation type is used.
How Does Interior Wall Insulation Control Sound?
Heat, cold, and sound are transferred from one place to another in much the same way. Heat, cold, and sound can move from one material to another through direct contact, also known as conduction. Sound, heat, and cold can also travel from one place to another through the air, a process known as convection.
Insulation in all its forms resists this transfer of heat, cold, and sound by trapping air pockets inside the material. Air is a very effective thermal insulator, so all insulation traps air using fiberglass fibers, cellulose, or chemicals and uses them to prevent the thermal transfer.
Because sound travels in much the same way as heat and cold, insulation is effective at preventing the sound waves from traveling through an interior wall. The tiny pockets of air and solids within the insulation help break up the amplitude of the sound, effectively deadening it.
What Insulation Is Best For Interior Wall Sound Control?
Insulation achieves sound control by absorbing the sound waves as they travel through the insulation. Sound wave energy is dissipated through the insulation because the energy is used to vibrate the tiny cells or fibers within the insulation instead of passing through unobstructed.
Fiberglass batts, spray foam, rigid foam boards, and mineral wool are often used as interior wall insulation, but some work better than others. Here is a list of the most common insulation materials used in interior walls for sound control, and how they are used:
Fiberglass Batts
Fiberglass batts are likely the most common insulation used for interior wall sound control. Fiberglass batts are usually installed with staples and attached directly to the wall studs using the included vapor barrier. Batts without the vapor barrier, known as unfaced batts, can be used to seal small gaps and cracks within the framing.
Fiberglass batts are great for interior walls because batts are available in the same thicknesses as standard wall studs. Interior walls built with 2” x 4” wall studs will hold a 3 ½” thick batt (R-13), while 2” x 6” walls will contain a 5 ½” thick batt (R-19-21).
Cellulose Batts and Rigid Boards
Cellulose batts and rigid boards are used for interior wall sound control because they can be stacked together to achieve a very precise R value. Cellulose batts and boards offer between R-2 and R-6 per inch depending on the thickness.
Cellulose batts and rigid boards are chosen for their thickness and corresponding R value, just like fiberglass batts. Some cellulose batts are faced for easy interior wall installation, while unfaced cellulose batts are common for insulating irregular shaped gaps and openings.
Cellulose rigid board insulation can be installed with mechanical fasteners or adhesive, and are often spaced ¼” apart to increase the sound deadening effect. In these installations, small rubber grommets are attached to the fasteners, which helps absorb the sound waves between the boards.
Mineral Wool
Mineral wool, also known by the brand name Rockwool, is made from the natural by-product of stone smelting, known as slag. Mineral wool uses compression or mechanical fasteners to stay in place instead of stapling the mounting tabs directly to the interior studs.
Mineral wool insulation is often placed in two, 2” thick layers, ½” apart, which can reduce sound transmission by over 50 decibels. For reference, human speech is about 60 decibels, so someone speaking in the next room would barely be audible through an interior wall insulated with mineral wool.
Spray Foam
Spray foam reduces sound transmission through an interior wall unlike other traditional forms of insulation. Cellulose, mineral wool, and fiberglass trap air inside the insulation just like spray foam, but these materials will compress over time from the effects of gravity. Often, this settling leaves a gap at the top of the wall through which sound can travel.
Spray foam insulation is great for insulating interior walls because it will never lose its shape or effectiveness. Open cell spray foam expands significantly (3x-5x), while low rise spray foam, also known as closed cell foam, will only expand about double its original size. Be careful to purchase the appropriate version for your interior wall to avoid over-expansion of the foam.
Rigid Foam Boards
Rigid foam boards provide about R-3 per inch and can be stacked like cellulose boards and mineral wool. Rigid foam boards are great for interior wall insulation because they are relatively inexpensive and easy to install.
Rigid foam boards can be installed with mechanical fasteners or adhesive, often using sound deadening caulk like this one. As with rigid cellulose board insulation, the effectiveness of rigid foam boards can be increased by adding sound absorbing rubber grommets to the fasteners.
Alternate Methods Of Interior Wall Sound Control
If adding insulation to an interior wall is not feasible, you can still help the wall absorb nuisance noises. Sound professionals insulate interior walls to absorb sound waves, not reflect them. To reduce the travel of sound through an interior wall, your wall should absorb the sounds, not reflect them like an echo.
Sound Absorbing Wedges
Sound waves will bounce off of smooth surfaces and reflect in the form of an echo, making the sound seem louder than it is. Acoustic professionals have been known to install sound absorbing foam wedges like these to break up and absorb the sounds instead of reflecting them.
Extra Layer Of Drywall
For maximum interior wall sound control, audio professionals install whichever insulation or foam is appropriate for the thickness of the wall. Then the pros install not one, but two layers of drywall perpendicular to each other, adding ½” sound insulating washers to the fasteners.
It might seem contradictory to add more drywall since we have concluded that smooth surfaces reflect sound waves through interior walls. However, the effect is that the sounds will travel through the first layer and become trapped in the ½” space between the layers, deadening the sounds.
Wall Treatments
Recording studios commonly use wall treatments instead of or in addition to interior wall insulation to keep a space quiet. Thick tapestries, curtains, and other dense fabrics perform the same function as the insulation, just on the outside on the wall instead of the inside.
