Most bedroom remodels treat soundproofing and access to services as unrelated topics addressing separate concerns that can be taken on independently. In truth, both areas must be addressed holistically for either to succeed. The wall system includes the access to services within it. An STC 55 wall can turn out to be only STC 30 when someone decides to use a typical plastic access cover over a plumbing shutoff.
Pinpoint Sources of Unwanted Bedroom Noise
Before even considering who to hire or what materials to buy, spend ten minutes compiling a list of all possible noise sources, classifying each as airborne or structure-borne. This seemingly simple task is neglected all too often, wasting both time and money.
Sound falls under two general categories: airborne and structure-borne.
Methods of controlling each type are significantly different, and using one for the other type of sound is often a wasted expense. Pay close attention to the times of the day and night when noises are especially bothersome. Are they coming through the shared wall or from above or below? Are they coming through ducts or windows? An en suite bathroom is a likely source of airborne flanking noise due to conversations and structure-borne noise from the plumbing. A shared wall with the living room is likely to be dominated by airborne sound, while a bedroom below that shares a floor assembly is likely to be dominated by structure-borne sound (and will require an IIC rating for the floor assembly, rather than an STC for the room).
The Weakest-Link Rule and Minimizing Penetrations
Like water, sound will find any opening through which to escape. Any penetration in the system that allows sound to pass through diminishes the STC of the system. This is why it is said that an assembly is only as good as its weakest component. In the context of noise control, that means a single small hole in an otherwise great wall assembly is often enough to greatly diminish its STC rating.
Every service penetration in a given structural assembly presents an avenue for sound to pass through, and every such penetration has an impact on the STC of the assembly. The STC of an entire assembly is dictated by its penetrations in the same way that the STC of a wall is dictated by the STC of its drywall layers. The STC of a wall assembly that has been finished with double drywall plus resilient channels is only as good as the STC of whatever penetrations that wall has.
Damping compounds such as Green Glue increase the STC rating of a given assembly or structure by introducing a mechanism by which sound is converted to heat as it travels through the assembly.
However, adding Green Glue or similar compounds to a wall penetration that has not been finished with acoustic sealant will do little to improve the STC rating of the overall assembly. Every penetration must be properly sealed, and all service access points must have their mass and airtightness maintained in order to preserve the integrity of the overall assembly. Acoustic sealant must be used wherever there is an opening, and wherever there are service access points, the wall’s ability to resist sound must be maintained using special access panels.
Decoupling versus Access: The Basic Design Conflict
Decoupling the drywall from the framing dramatically increases the STC rating of a given assembly. Using RC-1 resilient channels or similar sound-isolating clips provides excellent noise reduction without adding significant weight to the assembly. However, such systems do not allow for the addition of “service” access panels, as mounting such a panel directly on the wall frame defeats the purpose of the decoupling.
The only way to mount such access panels while preserving the overall integrity of the assembly is to mount them on blocking that is itself decoupled from the overall wall framework. The blocking and the wall framing that isolates the access panel must be isolated from one another with isolation pads, with neoprene, or some other material that allows decoupling while maintaining the ability to secure the panel in place. The STC of such a panel can rival that of the assembly as a whole, but the panel is more difficult to finish and install.
Acoustic Integrity of Bedroom Access Panels
Basic access panels make for poor acoustic hatches. The access panels and covers sold in hardware stores have low mass and offer almost no acoustic integrity. They are rarely even tight enough to serve their basic function as service ports, allowing both air and noise through the opening. Putting such an access panel in an otherwise good acoustic wall is like leaving a small window open.
However, maintaining the integrity of the wall STC or IIC ratings requires that the panel not compromise either the mass or the airtightness of the assembly. It is not enough for an access panel to be thick or to have its access cover made of metal. The panel must be of sufficient mass to contribute to the integrity of the assembly, and it must make an airtight seal when its cover is mounted.
Some specialized access panels available from Access Panels Direct provide sufficient mass, airtightness and acoustic integrity, including continuous perimeter seals that prevent acoustic leakage by maintaining the seal when the panel cover is mounted. Panels should also be heavy enough that they contribute meaningfully to the mass of the overall assembly. The STC rating of such an access panel should also be specified for comparison to the overall STC of the acoustic wall. This rating should, ideally, be found on the product documentation for the panel. This should be chosen before deciding on other materials used in the remodel, not as an afterthought when access panels are being installed.
HVAC Considerations and Plenum Noise
Bedrooms next to or below plenums present unique challenges, as both airborne noise from the HVAC system and structure-borne noise from its fans and compressors find ways to penetrate the bedroom. Any ducts that pass through the bedroom wall become direct conduits for noise, no matter how good the overall acoustic integrity of the wall may be.
Flexible connectors should be used wherever possible to allow vibration from the HVAC to be isolated from the bedroom wall. Where the ductwork passes through, wrapping the duct in mass loaded vinyl can help isolate it from the surrounding space, reducing noise without adding excessive mass to the overall assembly.
The location of access panels relative to the ductwork should also be carefully considered: putting a panel on the wall behind the headboard of a bed puts the most acoustically sensitive part of the overall assembly right next to the noisiest part of the room. Wherever possible, access panels should be placed close to the HVAC equipment itself and away from sleeping areas. In the rare case that this is impossible, the considerations for the panel itself take on increased importance, including choosing the appropriate acoustic hardware.
Flanking Paths Around Structural Penetrations
No matter how good the overall acoustic integrity of a bedroom or living area wall is, it will always be undermined by flanking paths. There are several types of flanking paths, each of which can undermine a wall’s ability to resist sound. Penetrations offer flanking paths around the assembly through hollow cavities in the surrounding structure.
Where pipes or other services penetrate a wall, the empty space around the pipe acts as a direct path for airborne sound. Packing mineral wool tightly around the penetration point can dramatically reduce airborne flanking while also reducing structure-borne sounds transmitted through the pipe. Since the mineral wool must be fitted around the service penetration, and the holes around it must be sealed with acoustic sealant, this can be a time-consuming process. It is important not to use expanding foam to seal such penetrations, as it can act as an acoustic bridge and transmit structure-borne noise directly.
The top and bottom plates of a stud wall also present flanking paths, especially around the edges of the wall. Applying a bead of acoustic sealant along these edges can help seal the cavity, reducing airborne flanking without introducing a rigid element that will conduct structure-borne sounds into the wall. Spending an extra hour or two on this step can dramatically improve the integrity of the final assembly.
Fire Ratings and Acoustic Requirements
In multi-family buildings or in buildings with otherwise attached units, many common walls have fire ratings. An hour rating is typical, meaning that a given wall must withstand one hour of fire before failing. Any access panel mounted on such a wall must maintain the integrity of that rating, and attempting to use a standard access panel defeats the purpose of the one-hour fire rating.
Dual-rated access panels exist specifically for this purpose, having been tested for both fire resistance and acoustic integrity. Using an acoustic access panel on a one-hour fire-rated assembly negates the one-hour fire rating. Using a fire-rated access panel on a wall with acoustic requirements negates the acoustic integrity of the assembly. Both are serious code violations, and both occur with surprising frequency due to poor communication on site.
Whenever working on a bedroom in an apartment or a multi-family building, consult local codes to determine the fire rating, if any, for each shared wall. The technical data provided by the access panel manufacturer will indicate both the STC and fire ratings for a given access panel. This, in turn, must be compared to the acoustic requirements of the assembly.
Getting the Sequence Right
Making an STC 60 wall requires more than just choosing the right materials and access hardware. It requires holistic thinking about the assembly as a whole. Every element, from the type of wood framing to the number of layers of drywall to the specific type of access panel must be chosen with care. It is no less important to choose the right sequence when building the assembly. Just as standard access covers compromise an otherwise good acoustic wall, so too can improper sequencing of elements when building the wall assembly.
Acoustic sealant, perimeter gaskets, and specific access hardware must be chosen before drywall is installed.
Adding the right access covers at the wrong time can be both time- and budget-intensive.
