Wood Wall Framing

Occupant experience is improved in wood buildings when party walls or exterior walls do not transmit sound between units. For walls in wood structures, sound isolation can be accomplished in two ways: Use partitions with a high mass (75 psf or greater) or use low-mass systems (2 to 5 psf) separated by air spaces of 3 to 6 inches. A rating of approximately STC 63 can be achieved when a double-stud wall is insulated with batt insulation and covered with two layers of gypsum wallboard on the outside faces of the studs. After double-stud construction, the next best framing solutions are staggered-stud and then single-stud construction. Acoustically, a single-stud wall creates a bridging point for sound to be conducted along every stud.

When the gypsum wallboard on one side of the wood stud is disconnected from the other side in a double- or staggered-stud wall system, the transmitted sound is reduced and the assembly has a higher STC rating. The advantage of the double-stud wall over the staggered-stud wall is twofold: First, the greater the separation between the gypsum wallboard on each face of the wall, the more the noise is reduced and the greater the STC rating becomes. The second advantage is that building utilities can be isolated from the stud system for the unit they serve. If the staggered-stud wall has plumbing or electrical run through the cavity, there is a greater chance that the studs will be connected together by the utilities, acoustically bridging the two lines of studs.

Please reference WoodWorks’ Acoustical Considerations for Mixed-Use Wood-Frame Buildings paper at www.woodworks.org/wp-content/uploads/Acoustics_Solutions_Paper.pdf for additional framing details.

Sheathing

In a light-frame wood building, the mass of the sheathing is just as important as the air space provided by the stud or joist cavity. In acoustical detailing, ⁵⁄₈-inch-thick Type X gypsum board is typically required, at a minimum. Alternatively, acoustically enhanced ⁵⁄₈-inch XP panels may be used where Type X gypsum panels are specified in some fire-rated wall and floor-ceiling assemblies. Used in the construction of high-rated STC wall assemblies, this ⁵⁄₈-inch-thick gypsum board consists of a layer of viscoelastic damping polymer sandwiched between two pieces of high-density mold-resistant gypsum board, providing constrained layer damping. Acoustically enhanced gypsum board can be used as a single-layer application or a component of multi-layered wall assemblies where sound transmission between rooms or dwelling units is a concern.

Three factors reduce the potential acoustical isolation:

• First is a smaller air space between the sheathing systems.
• Second is the common resonance of the two thinner wall systems. In this case, the walls radiate at the same frequency, coupling with each other, which reduces the wall system’s acoustical effectiveness.
• Third is the air space between the walls. In the case of lot-line walls for townhouses or row housing, the air space is typically sealed airtight. In the case of trapped air, 1–2 inches of air becomes very stiff, adding to the walls’ ability to couple together.

Insulation

The most cost-effective acoustical improvement to a sound-isolation system is the addition of batt insulation or any open-cell foam system to the stud or joist cavity. Batt or open-cell insulation reduces the sound that makes it into the stud cavity in the same manner as sound absorption works in a room. While closed-cell spray foams have higher R-values and offer improved building envelope energy performance by sealing the partition and improving airtightness, the closed cells do not allow the vibrating air molecules to interact with the insulation product so the sound attenuation is less. The cavity should be filled at least halfway to achieve a measurable improvement. In single-stud walls, a single-stud bay is filled; for staggered- or double-stud wall systems, insulation is only acoustically needed in one of the stud bays.

Resilient Connections

Finally, when double or staggered wood stud construction is not possible, decoupling the sheathing from the framing provides a similar form of isolation. Decoupling happens when the framing system functions like a spring. The spring system deforms as sound strikes the gypsum wallboard. As the spring or resilient system expands back to its original shape, it converts some of the acoustical energy from the sound into mechanical energy so there is not as much acoustical energy to transfer through the framing system.

Floor/Ceiling Systems

Noise reduction techniques for walls also apply to floor systems. While the main sources of noise complaints relative to walls are televisions or loudspeakers attached to party walls, footfall or impact noise from above can be an issue in multifamily projects if proper design steps are not taken. The second major noise complaint is floor squeaks.

Attenuation of impact sound, as opposed to airborne sound, is typically what governs for a floor assembly. As with most structural floor systems, floor finish material can have a significant effect on a floor assembly’s IIC rating. For horizontal applications (floors and roofs), additional materials will typically be applied on top of the wood structure, below it or both. As with all construction types, careful control of flanking paths is required.

Numerous studies of noise transmission through wood-frame floors have concluded that lack of sufficient mass is the major cause of poor sound insulation in these floors. Logically, mass has to be added to the wood-joisted floors to achieve satisfactory sound insulation, so floating a cementitious topping over an insulation underlayment on top of wood floors became popular practice. However, while this successfully controls the noise transmission through wood floor/ceiling assemblies, the solution can negatively impact the floor vibration performance, leading to occupant complaints about excessive vibrations in the wood-joist floors with concrete topping.

It appears that using mass alone to control noise transmission is not adequate. FPInnovations’ research on the effect of concrete topping on wood-joisted floors found that the floor spans should be reduced to improve stiffness. Therefore, the strategy for controlling the transient vibration and noise transmission of wood-joisted floors is to control the proper combination of floor stiffness (through span) and mass.

In wood-frame buildings, one effective floor/ceiling option features a base system construction consisting of the following:

• Gypcrete or lightweight concrete
• Impact-isolation matt
• Tongue-and-groove subfloor (glued and screwed to the joist)
• Joist system (with 6 inches of batt insulation)
• Resilient channel or puck system (resilient system)
• Two layers of ⁵⁄₈-inch Type X gypsum board

This system has a rating of STC 62, which is the highest rating possible without moving to construction methods found in recording studios and well above the required STC 50.

Impact Isolation

Impact noise can be reduced considerably with the use of soft floor finishes such as carpet. When carpeting is not practical or desired, the entire finish system must be considered. Floating engineered hardwood or tile floor systems offer the next best solution. A floating engineered hardwood floor offers the advantage of an isolation membrane that can be installed beneath the finished wood system. However, placing a resilient isolation mat or buffer under hard finish flooring systems requires coordination. Buffer systems include mats under the topping mass, such as foam, cork, or rubber mats made from recycled tires. Each of these buffer materials creates its own set of compromises on the installation process, with the total thickness of the finished floor system being one of the biggest coordination points.

Conclusion

Noise has a great impact on the built environment and the occupants that live and work in it, with numerous exterior and interior sources ranging from emergency vehicles and airplanes to HVAC systems, music, and talking. Acoustic design considers a number of factors, including building location and orientation, as well as the insulation or separation of noise-producing functions and building elements. You should now have a better understanding of how design teams can integrate acoustic design to create high-performance wood buildings that also enhance the health and well-being of occupants. Acoustical codes, green rating systems, and reference sources, such as the CLT Handbook and NLT Design and Construction Guide for mass timber buildings, can help you to decipher the myriad of considerations that go into optimizing acoustic design.

End Notes

Please reference WoodWorks’ Acoustical Considerations for Mixed-Use Wood-Frame Buildings for architectural details and additional information regarding this topic.

WoodWorks provides free one-on-one project assistance related to the code-compliant design, engineering, and construction of nonresidential and multifamily wood buildings. Contact help@woodworks.org.

American Wood Council provides code support to assure safe and efficient wood building design. Contact info@awc.org.