Within the U.S., mass timber is rapidly gaining popularity as a building material, and for the right reasons. Mass timber has numerous benefits, but the most notable is its environmental impact. An alternative to steel and concrete mass timber has the capacity to store huge quantities of carbon.
“Mass timber is actually grown on trees. When it’s paired with sustainable forestry techniques, the mass timber can have the potential to be a sustainable resource,” says Denis Blount, who is Associate Principal at Arup.
The benefits aren’t over there.
With an excellent strength-to-weight ratio, mass timber’s lightweight weight translates into lower costs for foundations. Prefab mass timber components need lesser workers in the field. The product is able to give an advantage in the market.
Then there’s the indisputable attractiveness of wood exposed, and research has shown its positive effects on health. “People like the feel of wood, and its visual, tactile nature can provide benefits to occupants,” Blount adds.
Although the mass-timber industry has been in use in Europe for many years and more recently, U.S. project teams have been adopting the extremely durable material, usually as cross-laminated lumber (CLT). This has been fueled by the inclusion of higher-quality mass timber structures within the 2021 International Building Code. In December 2022, more than 1,700 mass timber buildings for commercial, multifamily, and institutional projects were constructed or in the process of being designed across the U.S.
Mass timber’s Achilles’ heel is acoustics
For all its advantages, mass timber is not widely praised for its acoustic issues. Ceilings and floors that are exposed to wood have resulted in issues with high levels of noise. “Acoustics are somewhat of an Achilles heel for mass timber systems,” says Rose Mary Su, Principal Acoustician at Acentech Acentech, an acoustics, technology, and vibration expert based in Cambridge, Mass. Arup’s Blount adds, “I don’t think mass timber necessarily leads to poor acoustics, but it is an inherent challenge.”
To overcome this hurdle, Here are five related issues with mass timber and the solutions to them:
The first issue is that wood is lower mass than concrete or steel, and this means that the sound can pass through wood faster.
Although wood’s weight is light and can reduce the cost of construction, it means that it does not provide the same sound or vibration isolation as steel or concrete. The greater the density and mass the better the quality of the acoustics. A material that is more dense, like concrete, performs better at blocking the transmission of sound. “Whenever you don’t have enough mass, sound goes through the structure,” Su claims.
Solution: Add volume to mass timber. In reality, due to the poor acoustic quality, the timber that is completely untreated for walls, floors, and ceilings is extremely rare. “You must do something on the timber structure to increase the sound-isolation capabilities. Layers, elements or design elements over the timber structure base,” Blount says.
These components include a laid-down concrete layer or gypsum-based layer over the flooring and flooring underlayments that are resilient, usually between the top layer as well as the floor. In addition, when you have an insulated floor that is soundproof and beautiful ceiling-side wood is able to be left exposed.
“Almost all mass timber buildings have additional material like concrete or gypcrete,” says Chris Pollock, Associate Principal, Americas Technical Services Leader, Arup. “Resilient underlayments have a significant benefit for mitigating footfall noise and a marginal improvement to airborne sound separation.”
Arup utilized this strategy for its 80 Street South (80 M) project, which is a vertical extension that was built on the top of an existing offices building located within Washington, D.C. Arup employed the concept of what Blount describes as “a mass timber sandwich” that is a floor made of timber with a concrete topping, and an underlayment. “We were able to design a structural system that performed well acoustically using a concrete topping,” Blount states.
The 80 M Street SE (80 M) project in Washington, D.C., involved building a mass timber addition on top of an existing office building. To improve acoustic performance, Arup designed a “a mass timber sandwich”: a mass timber floor with a concrete topping and an underlayment. Photo: Roy Blunt
Problem 2 Problem #2: A gypcrete or concrete layer reduces noise but can also cover up the wood.
“It’s pretty easy to achieve sound isolation by putting materials on top of mass timber,” Blount states. However, the technique of putting concrete on top of timber can also block out the wood, thereby reducing one of the main advantages.
Solution: The wood is covered with wood. Even if they employ an underlying layer of concrete or gypcrete on the floor, some team members restore the look of timber by incorporating wood pieces and decorative elements into the design. “Add wood details that have sound-absorption finishes to hide the fact it’s not mass timber,” Su adds.
As an acoustic consultant, Acentech employed wood-based sound-absorbing surfaces on the John W. Olver Design Building located at the University of Massachusetts in Amherst. The initial CLT educational building to be built in the U.S.
Problem #3: Vibrations and impact descend from the unit that is above.
It’s not just noise that moves through wood. “A lot of times when people tell us they can hear noise, it’s not just voices traveling through the material, it’s also impact such as footfall,” Su adds. This is particularly relevant to commercial structures, which don’t have the same requirements for sound quality for residential construction, Su adds.
Solution: Use air space to act as a buffer. Air space can be used as an audio buffer, which can reduce the sound of floor and airborne sound. Air space is created, for example, by using drop ceilings or an engineered wood floor that is floating placed on top of a flooring pad.
Modular units benefit from the advantages of air spaces built into the units. Arup was the provider of acoustic as well as fire-design services to a mixed, affordable housing project that was designed by Forterra, the land conservation organization based in Seattle. In the apartment module of the project design, Arup built an acoustic layer over the floor with a top and underlayment.
“What’s really nice about modular construction is the stacked modules have air space with insulation material between them, which is good both acoustically and from a thermal protection standpoint,” Blount adds.