May 20, 2015
By Tim Preager
Wood has long been regarded as a viable material for construction thanks to numerous factors, namely design flexibility, efficiency, and affordability—reasons making it especially popular for smaller housing projects. Add in a smaller carbon footprint than concrete or steel, and wood can also be seen as a much more sustainable choice.
On January 1, 2015, wood-frame construction became even more attractive when changes to the Ontario Building Code (OBC) raised the limit from four to six storeys for wood-frame buildings. This is a catch-up since most European Union and several North American jurisdictions—including British Columbia—already allow six-storey wood-frame buildings. While the changes still do not permit use of mass timber construction such as cross-laminated (CLT) and glued-laminated (glulam), the implications are still evident.
Though wood construction provides an opportunity to builders, it is expected the changes will also benefit the Ontario economy. Cheaper construction costs should result in an increase in affordable housing. A report commissioned by the Building Industry and Land Development Association (BILD) estimated the costs savings of using wood frame for six-storey buildings could amount to about $30 to $40 per square foot—15 to 20 per cent less than a concrete structure.
Since wood-framed buildings are faster to construct, it would result in quicker occupancy for residents. With the construction industry using more wood, the increased demand for forestry products could offer a tremendous employment opportunity across Ontario.
Like many other changes to codes, there are questions and concerns around the safety and quality of wood-framed buildings. Critics claim expanded wood-frame construction still pose fire risks, but new safety rules requiring stairwells to be built with non-combustible materials and combustion resistant roofs make it much safer.
From an acoustical standpoint, there are also concerns that cannot be ignored. How soundproof are wood-framed buildings? Can residents come to enjoy the same privacy in wood structures as they have come to expect from steel and concrete buildings? The short answer to both of these questions is ‘yes,’ but only when the building is properly designed.
Challenges with wood
Wood construction buildings offer significant acoustical challenges, in particular related to sound isolation between adjoining spaces and throughout the building. The material tends to be lighter than concrete, which makes it harder to stop the transmission of sound.
Just as water can easily travel through tiny cracks, sounds does the same thing. Noise can travel through not only the air, but also the structure itself and through flanking paths. Flanking is sound transmitted through an indirect path such as through ducts, duct walls, the floor, or even gaps and cracks. Any small hole or path that may not consist of enough elements to stop a sound can help carry that sound. 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, and ceiling cavities. This means occupants are more susceptible to noise issues when units are not correctly designed.
Further, wood-framed buildings tend to be rigidly connected to other support structure. Once sound hits one of these structures, it can travel quite easily and efficiently throughout the building.
According to the new building code, units must be designed to have a minimum sound transmission class (STC) of 50. In older buildings that have been converted to lofts, the STC could be anywhere from 35 to 45 if the acoustical design is not considered in the retrofitting. As a result, neighbouring conversations and activities are easily carried from one unit to the other.
Compounding the issue, high STC ratings alone do not always guarantee a problem-free situation. Sound travels not only through the air from one room to another, but also through the walls, floor, and ceiling. The STC tests done in a laboratory do not take into account this flanking noise, so even when high STC partitions are constructed, it is still possible to have problems in the final product.
In wood-framed buildings, lower frequency sounds transfer more easily through walls than in a concrete building. This can be prevented by following some of the strategies outlined in this article. With proper acoustical design, residents can have the same privacy in wood structures as they have come to expect from steel and concrete buildings.
Strategies to minimize sound transmission
The following are some considerations to help minimize sound transmission in wood structures.
Proper stud spacing
Generally speaking, the farther apart studs are, the better the sound reduction—up to 5 dB more in performance. The typical standard for stud spacing in single stud walls is 400 mm (16 in.) on centre (oc); this is measured from the centre of one stud or joist to the centre of the next. However, 610-mm (24-in.) oc spacing is becoming more common and recommended when trying to isolate sound; it can often be a simple modification to the design.
Floor-ceiling assembly design
The same concerns raised about noise transferring through walls also apply to floors. While most noise complaints come from televisions, speakers, or people talking, there are also issues with impact sounds—overhearing people walking on hard surfaces. Soft surfaces (e.g. carpeting) can reduce impact noise by as much as 20 dB, but this is not an option in every unit. Other solutions include a noise barrier ceiling, rubber floor underlay, and a drywall ceiling supported on resilient channel.
Amount and type of insulation in the walls
One of the most cost-effective ways to minimize sound transfer is to include insulation in the walls, namely glass fibre batts. Generally, a minimum of 50 mm (2 in.) of glass fibre batts is sufficient; however, when sound performance is critical, it is best to fill the entire air cavity.
Amount of air space
When dealing with lightweight materials, one must consider the amount of air space between finished wall panels. Increased air space in a floor-ceiling or wall construction can result in noise reduction.
Single or isolated stud wall
In wood-frame construction, one of the most effective ways to prevent noise transfer is using an isolated stud wall. This is two sets of studs in a wall cavity that are not touching each other. As a result, the air space is also increased, which further improves sound isolation.
All of these factors were considered within the design and construction of the Wood Innovation Design Centre (WIDC), a 4645-m2 (50,000-sf) building of cross-laminated timber and glulam. The Prince George, B.C., facility is the tallest all-wood building in North America, and posed some unique acoustical challenges.
Since many of the rooms would be used for distance learning by the University of Northern British Columbia (UNBC), there were specific requirements around sound isolation to prevent sound bleeding between rooms, causing a distraction for students and teachers. The acoustics of the rooms had to be carefully considered to ensure speech intelligibility.
To add to the challenge, there was no room for error on this project. As a design-build public-private partnership (P3) model, there was a set of strict requirements that had to be adhered to in construction. Failure to meet these requirements at the completion of the project would result in significant monetary penalties.
Given there have not been many wood structures of this size constructed to date, there were limited practical examples to follow; there were no field test measurements to reference or extensive design guides to follow when it comes to the acoustics and sound isolation.
With little existing acoustic testing available on the WIDC’s unique structure, a theoretical analysis had to be conducted to accurately determine what the design of the walls, floors, and ceilings would be to achieve the required sound isolation and room acoustic performance. The Forestry Products Innovation (FPI) then directed field mockup tests based on the theoretical design to confirm performance.
The National Research Council Canada (NRC) had recently performed preliminary laboratory STC tests of a sample of wall and floor assemblies made with CLT; it shared this unpublished data with the designer. The theoretical, laboratory, and field test results were then compared, with the designs adjusted as needed before proceeding with construction.
To ensure sound isolation in the WIDC building, several elements were incorporated into the acoustical design.
Isolated double-stud wall
Wood studs tend to transfer sound more easily than steel so there was a need to break the transmission path that exists with a single stud connection where the two sides of the drywall are connected through the screws and stud. Using an isolated double stud wall resolved this issue and significantly increased the sound isolation between spaces.
Wall assembly design
The amount of air space, stud spacing, and number of layers on the wall on each side were considered. For walls between video conferencing rooms and lecture theatres, an isolated double stud construction designed to meet STC 55 was used. A 25-mm (1-in.) air gap separated the wood studs spaced at 610 mm (24 in.) oc and filled with 90-mm (3.5-in.) glass fibre insulation.
Numerous designs were created to find the optimal floor-ceiling construction with a combination of wood materials with resilient channel and drywall: Floor-ceiling assemblies had to be designed to STC 50 throughout the building in noise-sensitive spaces, and an impact insulation class (IIC) rating of 70 or higher. Sometimes referred to as ‘impact isolation class,’ the IICs rating measures the assembly’s resistance to transmit structure-borne or impact noise—for example, how well the impact noise of a heel dropping on the floor above is reduced to the space below. These ratings were critical for the video conferencing rooms.
Any openings where sound could transfer were sealed. The stud wall separating the sensitive spaces did not reach the bottom of the structural floor, leaving open holes between rooms allowing noise transfer. The same applied with ceiling chases. In both cases, the space had to be properly covered and filled with acoustic insulation, then sealed and caulked to reduce any sound transmission.
To confirm the acoustic design met all the requirements, testing was performed at the WIDC with support from FPI. The tests confirmed the theory was applied successfully and met all of the requirements for compliance. As a result, the WIDC became the first wood building built to strict acoustical requirements.
As acoustics can have an impact on design, constructability, cost, and performance over multiple disciplines (i.e. structural, architectural, and mechanical) the importance of designing it correctly at the outset is paramount. Noise-control elements are often hidden so correcting any issues would require breaking down walls to locate the source. It is critical that the acoustical design is considered in the preliminary stages of a project because fixing elements behind the wall such as stud placements and spacing can be challenging, messy, and costly to fix after the fact.
With expected changes to the National Building Code of Canada (NBC) in the coming months, there has been an increase in the discussions and interest around reducing sound transmission in wood-framed buildings. It is anticipated the NBC changes will incorporate a new rating to address flanking noise or airborne sound transmitted through the wall or floor between dwelling units—and it will only be a matter of time when the OBC adopts this as well. (Other provinces may follow.)
This rating would include apparent sound transmission class (ASTC), which is a more realistic measure of the actual sound level perceived by occupants, as it includes noise transmitted through wall, ceiling, and floor junctions. With a move to ASTC on the horizon, there will be increased pressure for construction professionals to reduce noise through flanking paths which can be a challenge for wood-framed buildings.
The reality is noise is more than bothersome for many people. It can rob residents of their right to peace of mind in their own homes. Wood-framed buildings have great advantages for construction professionals and occupants. With proper acoustical design, noise-sensitive spaces can exist beside each other in a retail or commercial application. High levels of acoustic comfort and isolation between sensitive spaces may be achieved, allowing for occupant privacy and contentment, and fewer post-occupancy issues such as noise complaints.
Tim Preager is a principal with Aercoustics Engineering Ltd., which has specialized capabilities in all aspects of acoustics, noise, and vibration control. He has been involved in numerous innovative projects, including the Wood Innovation Design Centre and Maple Leaf Gardens. Preager is also the project engineer for the Toronto Transit Commission, providing services in acoustics, vibration, and noise control for existing and proposed projects. He can be reached at firstname.lastname@example.org.
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