Understanding the acoustical challenges of mass timber buildings

by sadia_badhon | December 20, 2019 11:20 am

By Tim Preager

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Second only to stone, wood use in construction dates back thousands of years. In fact, the first timber home[2] discovered in Britain was built more than 10,000 years ago. In Canada, there are many all-wood buildings that are over 100 years old. Nevertheless, changes to building codes, fear of fire risks, and the introduction of steel and concrete have caused a decline in wood building construction.

However, it appears old is new again. Wood has made a big comeback and new changes being introduced to the National Building Code (NBC) in 2020 might result in the rise of tall wood buildings.

Starting next year, NBC will allow the construction of tall wood buildings up to 12 storeys from the current limit of six. The impact is already seen in British Columbia, where the province obtained permission from the National Research Council (NRC) to adopt the rules right away.

With the country’s vast forest resources, wood is an obvious material for construction, and it has less impact on the environment compared to manufacturing steel or extracting and crushing rock from quarries to make concrete. Being a natural resource, it is readily available, renewable, environmentally sustainable, biodegradable, economically feasible, and carries the lowest carbon footprint[3] of any comparable building material.

Wood stores carbon dioxide (CO₂), and some reports predict replacing steel with mass timber could reduce CO₂ emissions[4] by 15 to 20 per cent. For example, the wood used in the 18-storey University of British Columbia (UBC) Brock Commons residence[5] tower will store over 1750 metric tonnes of CO₂-equivalent greenhouse gases (GHGs), which is similar to taking 511 cars off the road for a year.

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The biggest hindrance to wood construction in the past has been the fear tall wooden structures are a fire hazard. However, the wood industry has evolved, and mass timber is actually fire-resistant. Mass timber buildings can be defined as one in which the primary loadbearing structure is made of either solid or engineered wood. This can include glue-laminated (glulam) timber, cross-laminated timber (CLT), and nail-laminated timber (NLT). This new technology means wood buildings can move into a new era.

Notable examples

In recent years, several new and innovative wood buildings have been developed in the United Kingdom, Australia, and North America. These structures demonstrate the true potential of mass timber construction. Canadian construction is being led mainly by builders of educational facilities, but mass timber is also explored for a mix of projects.

The Wood Innovation and Design Centre (WIDC) in Prince George, B.C., initially held the title of the world’s tallest, modern, all-timber structure but was soon surpassed by the UBC Brock Commons Building, which was an exception to the building codes’ six-storey limit. Several others in design include the Arbour at George Brown College in Toronto—the 12-storey building will be Ontario’s first tall wood institutional structure.

Sidewalk Labs is looking at constructing a collection of mass timber buildings with the tallest one topping out at 30 storeys in Toronto. If successful, it will be the first neighbourhood constructed entirely from mass timber. On the West Coast, there are intentions to seek similar-sized tall mass timber buildings up to 40 storeys.

Even without the changes to NBC, there is a plethora of advantages to mass timber construction. However, the use of mass timber in multifamily and commercial buildings presents unique acoustic challenges.

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Acoustical challenges

The most attractive aspects of mass timber—lightness and structural characteristics—is actually its greatest downfall in terms of acoustics. The structural material is lighter than concrete, making it harder to stop the transmission of sound. Therefore, low-frequency sounds transfer easier through walls in a wood-framed building than a concrete one. With acoustics, more mass like concrete typically means better noise control. This includes impact noises from footfalls or running children, or from airborne sound from a stereo system or loud voices.

Sound transmission class (STC) rating is the ability of a material to reduce transmission of sound between rooms. Generally, the higher the STC number, the less sound is transmitted. Typically, an STC rating of 50 is desirable, and also required by code between residential spaces. However, a single number does not tell the whole story.

It is important to note the sound transmission requirements, acoustical challenges, and potential solutions will slightly differ between residential and commercial buildings. In commercial buildings, it is not uncommon for the landlord to request an exposed ceiling to showcase the mass timber construction. The irony is this is similar to the office spaces of the early 19^th century when buildings were built with layers of 2x4s. Today, the modern wood-look, combined with the ability to showcase a building’s low-environmental impact, may allow the landlord to charge a premium for the space. While it looks great esthetically, it poses major issues around sound transmission. There are no requirements or building code recommendations on sound transmission in commercial spaces, but some tenants might require a quieter space, so landlords need to consider this in the design process.

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Most building codes mandate a specific sound performance or STC rating for walls and floor systems in residential spaces. However, high STC ratings alone will not guarantee ideal soundproofing as sound not only travels through the air from one room to another, but also via indirect paths such as ducts, duct walls, floors, ceilings, or even gaps and cracks. Known as flanking noise, this type of sound transfer is unaccounted for in STC ratings determined in laboratories.

Flanking paths can be a critical issue in mass timber construction, and without proper acoustical planning, result in a lot of complaints from residents. Mass timber spectral characteristics are less desirable in comparison to traditional concrete or concrete/steel assemblies for certain types of sound. Due to the heavier assembly, concrete has a significant advantage when it comes to minimizing the transmission of low-frequency sounds, such as those from small home theatres, banging noises, mechanical equipment, and noisy neighbours.

In comparison to a concrete STC 50 partition, with a mass timber assembly—built-up with additional layers to achieve the same rating—it can easily be twice as loud (5 dB worse) in low-frequency sound areas. Relying on a single STC 50 for code reasons will not result in performance similar to concrete condominiums in 95 per cent of the buildings out there. With that said, for other types of sounds such as normal voices, general living room/kitchen activities, the addition of concrete on top of an air space filled with a soft material such as insulation or a rubber matt, can provide significant benefits.

It is very critical to consider apparent sound transmission class[9] (ASTC), a more realistic measure of the actual sound level transmitted between occupants since it includes noise transmitted through wall, ceiling, and floor junctions, as opposed to STC, which is a laboratory test made under controlled conditions and construction requirements. The updated building code is expected to require the calculation of ASTC in all permit submissions and this will have a significant impact on mass timber buildings. This building code change should produce better performing buildings and is a great thing for the industry, but like anything that increases performance, the cost associated with engineering and construction will also rise. In addition to an STC rating of 50 for walls, all residential assemblies and their short-circuit paths will have to show, on paper, they can achieve an STC rating of 47 or higher. This means the details of connection points, holes, how things get tied together, and construction sequencing becomes far more important than ever with mass timber construction. Previous to this, for typical construction, ASTC tests indicate most assemblies typically test within approximately 5 dB of the lab-rated STC value (Figure 1).

This does not mean wood buildings will be noisy. With proper acoustical design, residents can have similar privacy in wood structures as they have come to expect from steel and concrete buildings. However, like many engineering trade-offs, this does not come without additional cost and implications on services, floor-to-floor heights, and performance. There are several ways to minimize sound transfer in mass timber buildings.

Determine the right mass timber option

There are various options when it comes to mass timber, such as CLT, NLT, mass plywood panel (MPP), as well as dowel-laminated timber (DLT). Controlled laboratory STC testing, where the sound transmission of a material is accurately measured between rooms, has found CLT performs slightly better as the laminates are cross-oriented in a panel, have less susceptibility for small holes and cracks, and is generally more monolithic than DLT and NLT.

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Increase mass wherever possible

Due to its light weight, using mass timber exclusively will not be great at stopping noise from transferring through it. A single 152-mm (6-in.) panel of concrete provides enough mass to achieve typical residential requirements, but a 152-mm piece of CLT does not. A hybrid solution combines the mass timber with other materials that are better at stopping noise (e.g. concrete). Many designs incorporate the use of thin, 50 to 76-mm (2 to 3-in.) thick concrete layers or fibrous cement panels on top of the wood base assembly. Another option incorporates a dropped ceiling below the wood structure. For this, care would have to be taken to allow accessible areas for services and/or penetrations for HVAC, sprinklers, and electrical systems. Either option will impact floor-to-floor heights, so it is critical to look at these sound mitigation issues at the planning stages (Figure 2).

Minimize flanking

Just like water, sound can also travel through tiny cracks, holes in walls, pipe penetrations, or any other path other than the main demising partition. Any small hole or path without enough elements to stop a sound can help carry it. In wood buildings, a lot of flanking paths can exist where sound travels through assemblies other than the wall itself, such as the floor, joists, cavities, pipe penetrations, junctions between floors and walls, and ceiling cavities. This means occupants are more susceptible to noise issues in poorly designed units.

The main factor affecting flanking paths is the floor system itself and the materials used and sequenced onsite. For horizontal sound transmission, the main driving factor in achieving performance across the same floor plate will be the flanking paths. For example, if the base system is CLT, and the target is STC 50 or higher, then wall assemblies need to be placed down directly to the base of the CLT floor slab before concrete pour or require the installation of an acoustical under mat or additional layers. This is particularly important once the new ASTC requirements come into effect. The same applies for commercial spaces with performance mandates between rooms. Even if the wall has a rating of STC 60, performance will suffer if the flanking path only achieves STC 45. For raised floor systems, similar to standard office floor plates with under-floor air distribution systems, walls needing to achieve an STC rating of 50 should be installed first and placed down to the base mass timber structural slab.

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For vertical sound transfer between floors, a common design consideration is to raise the flooring by placing underlays and mats between the mass timber panel and concrete. Another option is to hang ceilings below the base wood assembly to achieve medium performance between floors. Combining these approaches can achieve even higher performance where it might be necessary as more mass timber spaces are used. Regardless of the option chosen, it is imperative these requirements are outlined at the outset of the project design. While floor raises and drop ceilings can be incorporated later, buildings teams risk losing height, adding weight, and going over budget if these decisions are made later in the project schedule.

Key considerations

When considering a mass timber construction, it is critical for a developer to have a realistic sense of goals. Do they want to use this building to showcase the possibilities of building strategies for the future? Can they attract high valued tenants and residents to spaces where the cost implications are higher in comparison to traditional buildings? Are they willing to pay more money to invest in a more sustainable project? The higher capital spent on tall mass timber structures may be offset by the reduced project timeline as they are quicker to construct than concrete buildings. Mass timber structures[12] are often built as components offsite and assembled at the project site, thereby contributing to faster construction timelines.

With a greater focus on environmental sustainability, the many changes to building codes, and technological advances in wood construction, it is not far-fetched to assume mass timber construction may become a standard in the building industry over the next five to seven years. British Columbia’s Wood First Act (WFA) has mandated any building in the province has to first consider mass timber construction and show why it cannot work prior to choosing concrete or steel. This may eventually become the expectation across the country.

It may take some time to fully establish deep roots within the construction industry because the supply chain and logistics for mass timber product delivery is still in its infancy. Currently, it is still more cost-effective to purchase products from outside of Canada but this is changing, thanks to recent government grants and plans by large construction companies to build and operate mass timber plants locally. As building codes evolve and developers understand the many benefits of mass timber construction, what is now considered a design trend could soon become the standard.

[13]Tim Preager is a principal with Aercoustics Engineering with a focus on acoustical design. Preager also leads wood building design projects for residential, commercial, and institutional sectors. He can be reached at timp@aercoustics.com[14].

Source URL: https://www.constructioncanada.net/understanding-the-acoustical-challenges-of-mass-timber-buildings/

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