Acoustics

As with any issue of building performance, the acoustics of a mixed-use wood-frame structure can be designed to meet or exceed minimal requirements, depending on the expectations of the developer, buyers and tenants.

For wall systems, sound isolation can be accomplished in two ways. One is to use partitions with a high mass (75 pounds per square foot, psf, or greater) or to use low mass systems (2 to 5 psf) separated by air spaces of 3 to 6 inches.

The goal in party walls or exterior walls is to keep other people's noise out of, and tenant noise in, the unit. In lightweight wood structures, this is achieved by separating the materials with an air space (e.g., stud or joist construction). In terms of acoustical performance, the most effective wood-frame wall is a double stud wall, followed by staggered stud and then single stud.

In mid-rise wood-frame buildings, options for improving acoustic performance include:

Sheathing. 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.

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. While closed cell spray foams have higher R-values and offer improved building envelope energy performance by sealing the partition and improving air tightness, the closed cells do not allow the vibrating air molecules to interact with the insulation product so the sound attenuation is less. It is this interaction that helps reduce the sound.

Resilient channels. When double or staggered stud construction is not possible, decoupling the sheathing from the framing provides a similar form of isolation.

Floor finishes. Impact noise can be reduced considerably with the use of soft finishes such as carpet. When carpeting is not practical or desired, floating wood or tile floor systems offer the next best solution.

For a more detailed introduction to these concepts, a technical paper, Acoustical Considerations for Mixed-Use Wood-Frame Buildings, is available at woodworks.org/wp-content/uploads/Acoustics_Solutions_Paper.pdf.

Innovative Wood Products and Building Systems

A number of innovative technologies and building systems are increasing the opportunities for mid-rise wood buildings. For example:

Prefabricated Systems

Specifying prefabricated or factory manufactured wall and roof panels can provide better quality wall construction since the panels are constructed off-site in moisture controlled environments. Wall panelizing is particularly useful for jobsites that don't have adequate space to construct the walls and can speed the erection time considerably. Wall panels can also optimize stud design and increase sound proofing and energy efficiency. Panels may range from 4 to 30 feet long, and are lifted into place by crane.

Mass Timber

While traditional wood-frame construction is a proven solution for mid-rise structures up to six stories, mass timber products such as cross laminated timber (CLT) are creating new possibilities. CLT is a multi-layer wood panel in which each layer is oriented crosswise to its adjacent layer for increased dimensional stability and structural performance. Widely used in Europe and now available in North America, it is considered viable for buildings up to 12 stories and even higher.

In Australia, CLT has been used to create a 10-story all-wood building, while eight-story examples exist in the U.K. and Austria. North American applications include (among others) a two-story elementary school in West Virginia, a six-story CLT structure in Quebec, a five-story heavy timber/CLT hybrid building at the University of British Columbia , and an eight-level heavy timber/CLT hybrid (six stories plus a penthouse and mezzanine), also in British Columbia.

Among CLT's attractive structural characteristics are high dimensional stability, high axial load capacity, high shear strength, rigidity around openings and negligible settlement effects. CLT assemblies also offer inherent fire resistance due to thick cross-sections that, when exposed to fire, char at a slow and predictable rate. The industry is also conducting research on the ability of CLT structures to resist lateral loads caused by earthquakes or high winds. Other benefits include speed of construction, thermal performance, and the environmental advantages offered by all wood products—including a low carbon footprint.⁹

The 2015 IBC identifies CLT as a structural product and recognizes it for use in Type IV exterior walls, floors and roofs. The 2015 National Design Specification® (NDS®) for Wood Construction, referenced in the 2015 IBC, also includes new structural and fire design provisions for CLT. However, while the 2015 IBC won't go into effect in most jurisdictions until 2016, designers can pursue the use of CLT under the alternate means and methods approach in the current code. More information is available from the American Wood Council (awc.org) or APA (apawood.org), which developed the American National Standard, ANSI/APA PRG 320 2011: Standard for Performance Rated Cross-Laminated Timber.

Conclusion

The last few years have seen a trend toward taller wood buildings, driven by their acceptance in building codes and the value they provide. Design professionals are capitalizing on wood's ability to offer high density at a cost that is typically less than other materials. They also appreciate wood's versatility, adaptability and light carbon footprint. However, while today's building codes recognize wood's safety and performance capabilities in buildings that are five and six stories—and these are becoming increasingly common—innovative technologies and products can be expected to propel designers of mid-rise wood buildings to even greater heights.

Endnotes

1. Life Cycle Environmental Performance of Renewable Building Materials in the Context of Building Construction, Consortium for Research on Renewable Industrial Materials, Phase I 2005, Phase II 2010; A Synthesis of Research on Wood Products and Greenhouse Gas Impacts, Sarthre, R. and J. O'Connor, 2010, FPInnovations; Werner, F. and Richter, K. 2007, Wooden building products in comparative LCA: A literature review, International Journal of Life Cycle Assessment
2. See endnote 1.
3. The Wood Carbon Calculator for Buildings was developed by FPInnovations, WoodWorks and the Canadian Wood Council; available at woodworks.org
4. Case study, http://woodworks.org/wp-content/uploads/Stella-CaseStudy-WEB.pdf
5. O'Connor, J., 2004, Survey on Actual Service Lives for North American Buildings, FPInnovations
6. Case study: All-wood Podiums in Midrise Construction, APA, http://www.apawood.org/SearchResults.aspx?tid=1&q=n110
7. The ShakeOut Scenario – Woodframe Buildings, URS Corporation, prepared for the U.S. Geological Survey and California Geological Survey, 2008
8. youtube.com/watch?v=c25HuZeQsyo&context=C3f612acADOEgsToPDskLH3EQs-aodM9-NsZgF2lGi
9. U.S. CLT Handbook, http://www.rethinkwood.com/masstimber

The reThink Wood initiative is a coalition of interests representing North America’s wood products industry and related stakeholders. The coalition shares a passion for wood products and the forests they come from. Innovative new technologies and building systems have enabled longer wood spans, taller walls and higher buildings, and continue to expand the possibilities for wood use in construction. www.rethinkwood.com