November 28, 2016
By Robert (Bob) Marshall, P.Eng., BDS, LEED AP, and Robert Hartogsveld, CTR, CCCA
The 2015 edition of the National Building Code of Canada (NBC) introduced 360 technical revisions that address contemporary concerns and raise standards even as Canada is in the midst of a construction boom.
The two most timely and impactful code revisions for building and design professionals are Part 3, “Fire Protection, Occupant Safety and Accessibility,” and Part 9, “Housing and Small Buildings.”
Part 3 provides an affordable housing option for Canada’s rapidly growing population. It permits the construction of residential, business, and personal services buildings up to six storeys in height using gypsum and traditional combustible construction materials—particularly wood framing assemblies. Advances in wood research and building technology over the years have led to stronger, safer, more sophisticated fire-protected products that make mid-rise wood construction a more viable option.
Part 9 of NBC 2015 raises the bar for project sound control standards with the introduction of the apparent sound transmission class (ASTC) rating. This rating gives a more comprehensive description of noise between dwellings—including sound transmitted through flanking paths—than the longstanding sound transmission class (STC) rating.
This article outlines the importance of each of these code revisions, and demonstrates how professionals can create smart sound-control strategies that exceed code requirements and reduce project costs.
Wood-framed mid-rise construction: An affordable housing solution
Canada is one of the fastest-growing developed nations in the world, largely due to the continuous wave of immigrants seeking new homes in the country in recent years. According to Statistics Canada, the nation welcomed 320,932 immigrants as permanent residents between July 2015 and July 2016—its highest annual total in more than a century. These numbers are expected to increase.
Metropolitan areas across the nation, from Vancouver to Halifax, are receiving the lion’s share of this population influx—as well as the accompanying rising real estate prices. The record-breaking detached home prices in many of these areas are prompting more people to look into townhouses and condos, also skyrocketing in price. The Toronto Real Estate Board recently reported the benchmark price for a detached home in the Greater Toronto Area (GTA) was $952,983 in July—a 21.1 per cent increase from last year. The same report showed the average condo price for the metropolitan area was $402,865, increasing 9.2 per cent from the same month in 2015.
This high equity is beneficial to some—current homeowners can complete residential upgrades to create an income-producing secondary suite within their homes. However, for those looking for a new home, the nationwide need for more affordable housing options is clear. One very workable solution is mid-rise multi-family residential construction—specifically, construction of fire-protected wood-framed buildings of up to six storeys nationwide, and cross-laminated timber (CLT)-framed buildings of 12 or more storeys in Québec and British Columbia. Since its announcement, the new code revision has been adopted by the building codes of those two provinces, along with Ontario and Alberta.
The use of wood framing in mid-rise construction has a number of advantages. Most importantly, it costs less than poured concrete for mid-rise buildings. A recent report from the Building Industry and Land Development Association (BILD) estimates wood framing can save project teams $300 to $400/m² ($30 to $40/sf), or up to 20 per cent more than steel and concrete. Those numbers translate to $20,000 to $25,000 savings for a 100-m2 (1000-sf) home. (Paul J. Bedford’s article, “Unlocking the Potential for Mid-Rise Buildings: Six-Storey Wood Structures” was published in 2013 by the Building Industry and Land Development Association (BILD).) This means mid-rise construction can create more affordable prospects for property owners, developers, and home buyers in desirable neighbourhoods that normally have high-priced housing.
As the only renewable construction product, wood has many advantages over other materials. Its processing requires less energy than concrete or steel in terms of resource extraction, manufacturing, and transportation, and makes less of a carbon footprint. According to a 2005 study, that was led by the Consortium for Research on Renewable Industrial Materials (CORRIM), the life cycle of steel and concrete results in 26 per cent to 31 per cent more greenhouse gas (GHG) emissions than that of wood.
However, in a wall assembly, wood framing is only as good as the surrounding materials. These include insulation, gypsum drywall, sealants, and putties. Such components are especially important given the requirements of NBC’s new ASTC rating to address flanking paths. Due to the many flanking paths typical in high-density projects—such as vertical and horizontal wall intersections—acoustic control can be a challenge to address. When discussing any multi-family dwelling, sound control is a must for ensuring an acceptable level of peace and privacy for residents.
ASTC and sound flanking paths
For many years, NBC set minimum levels for the control of noise transmitted between dwellings by defining sound insulation in terms of the STC rating. This rating measures the ability of a separating wall or floor assembly to hinder noise passing between two rooms or residential units. The higher the STC rating, the more the airborne sound transmission is restricted. Likewise, lower STC ratings mean more sound is transmitted through the wall assembly, increasing the ambient noise level of the space and resulting in a noisier interior environment with less privacy.
However, over time, code authorities discovered the STC rating was not as accurate a description of sound insulation as it needed to be—the metric overlooked noise that passes through sound flanking paths such as those from the common wall to the other walls, the ceilings, and the floor within dwellings. The more comprehensive ASTC rating was developed to address this issue. It describes the sound insulation of the complete building system and includes not only direct noise transmission through the common wall or floor, but also through the flanking paths. It is anticipated the ASTC rating will eventually overtake STC as the code’s acoustical standard.
Currently, STC and ASTC ratings are derived from sound attenuation values evaluated in the 1/3 octave bands between 125 and 4000 hertz (Hz). Acoustical engineers fit these values to the appropriate transmission loss (TL) curve to determine a final STC rating. The 2015 NBC still requires low-rise and high-rise residential buildings to achieve a minimum STC rating of 50 in wall assemblies separating two residential units, and a rating of 55 for elevator shaftwalls that border a living space. It also now demands a minimum ASTC rating of ≥47 for wall assemblies.
Fortunately, there are simplified strategies for meeting and even exceeding these numbers.
Smarter sound control strategies
Many sound control methods used in wall assemblies over the years have become known for being either too costly or difficult to install. Even minor installation errors can compromise the acoustic abilities of the entire wall assembly.
Resilient channels provide a good example. These sound-dampening metal strips are screwed in perpendicular to studs to acoustically isolate the wallboard, after which the wallboard must be very carefully screwed into the resilient channels. Any simple mistake, even something as small as positioning the resilient channels too close together or upside down, or using screws that are too long, can result in the creation of a new sound transmission ‘short circuit’ path, and thus degrade the assembly’s acoustical performance.
Another common sound control technique involves building double-stud walls or installing multiple layers of gypsum board. However, this technique is often expensive. Additionally, multiple layers of wall assembly configurations often increase the footprint of the wall, which results in a less efficient use of limited, high-priced floor space.
In response to these issues, building product manufacturers have been able to introduce smarter, more practical options for sound control in wall assemblies. One example is laminated-core, noise-reducing gypsum board.
Laminated-core, noise-reducing gypsum board
Developed to be a smarter alternative to more common sound control methods on interior walls and ceilings, this specialized gypsum board is a single-panel product that contains a viscoelastic polymer core applied between two specifically formulated thin layers of gypsum board. The final product ends up being 12.7 to 15.9 mm (1/2 to 5/8 in.) thick—the same as traditional gypsum board. This material is ideal for new construction or renovations over both wood and steel framing.
One key advantage of laminated noise-reducing gypsum boards is their ability to dampen sound transmission. The polymer core acts as a shock-absorber that dampens board vibrations and dissipates the sound energy into thermal energy. The boards perform well acoustically over an extended range of frequencies, resulting in higher STC and ASTC ratings for wall assemblies.
This type of gypsum board is an excellent acoustic solution to specify to meet NBC’s ASTC requirements, without the need for materials such as resilient channels and isolation clips. It can, however, be used to achieve extra sound transmission control in wall assemblies where resilient channels or isolation clips are desired. In such cases, the material helps reduce the negative impact of any short circuits.
Replacing traditional multi-layered gypsum systems with a single laminated noise-reducing gypsum board on one side allows contractors to build effective noise-reducing wall assemblies with a National Research Council (NRC) rating of ASTC 48. Use of this material cuts construction time and material and labour costs, and helps the project gain valuable floor space. The minimal amount of material required also contributes to the structure’s sustainability and compliance with green building practices.
In tests, assemblies utilizing a single layer of laminated-core, noise-reducing gypsum board on both sides have achieved a preliminary NRC rating of ASTC 51. This means they exceed the acoustic performance of double layers of traditional gypsum board. This gypsum board application is therefore quite effective where acoustic performance is essential, such as in condominiums, hospitals, retirement homes, and hotels.
Sealing sound flanking paths
Sound flanking paths are usually cutouts or gaps in assemblies allowing the transmission of sound between two spaces. In an acoustical structure, any gap that leaks air also leaks sound. This can render useless an otherwise effective sound barrier. Typical flanking paths include joints between walls and ceilings, floors, and other walls. Certain best practices help project teams reduce sound flanking paths in their wall assemblies.
When working with laminated noise-reducing gypsum board, installers may follow traditional interior gypsum board application and finishing methods, but should apply a little more attention to detail to ensure acoustic performance. For instance, the board layout should stagger joints from one side of the wall to the other. Next, sound-absorbent fibreglass batt insulation can be installed in wall cavities for higher STC ratings where needed. Again, resilient channels are not necessary to achieve performance in this application, except in very acoustically sensitive situations. However, it is appropriate to specify putty pads—tested per ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements—or acoustical sealant at electrical outlets.
To achieve the sound ratings provided in code tables and NRC tests, acoustical sealant is required at electrical boxes and at junctions of intersecting walls and floors. The checklist at the top of this page can be used to apply sealants before fire and sound separations inspection, which is required by most municipalities. One should specify an acoustical sealant to seal the perimeter gaps and wall penetrations per ASTM C919, Standard Practice for Use of Sealants in Acoustical Application.
Eliminating other sound flanking paths
It is important to eliminate other sound flanking paths.
Hollow-core doors are poor sound-blockers. When privacy and noise control are key considerations, doors must be solid wood or have insulated cores. Further, they should be gasketed so as to help prevent sound from passing between the door and its perimeters.
Double-pane and/or storm windows reduce sound transmission, and the addition of weatherstripping can also help. Windows facing exterior noise sources should be small in both size and number. With double-hung windows, only those that can be tightly closed should be specified.
Wiring and piping
Holes for wiring or conduits should be sealed or caulked, with holes cut out for piping should be sealed with caulking. Merely stuffing these holes with insulation is insufficient, as sound can easily pass through porous insulation. However, insulating the holes and then caulking over the insulation can be effective.
Wood-framed mid-rise residential construction is a great solution to this country’s affordability challenges. It will help accommodate and sustain population growth by offering cost-efficient, high-performance structures meeting the requirements of the National Building Code of Canada (NBC). Applying innovative sound control ASTC strategies, as required in the 2015 code, will help design and construction professionals create a more affordable living space and enhance the wellbeing of occupants.
Robert (Bob) Marshall, P.Eng., BDS, LEED AP, is the building science manager for CertainTeed Saint-Gobain Canada. He has been appointed by the National Research Council (NRC) to the Standing Committee on Energy Efficiency in Buildings. He can be reached at firstname.lastname@example.org.
Robert Hartogsveld, CTR, CCCA, is the architectural solutions manager for CertainTeed Gypsum & Insulation Ontario. He holds designations through CSC and serves on the board of directors of the organization’s Toronto chapter. He may be reached at email@example.com.
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