Fire safety

Safety and sustainability take centre stage with growth in tall wood construction

15 November 2019

Encapsulation with noncombustible stone wool insulation an advantageous solution.

Mid rise wood frame building in Canada

As sustainable construction gains traction across North America and around the world, tall wood mass timber construction is increasing as a result.  This construction type has garnered a lot of interest—and some concern.  However, it does have a strong and growing contingent of supporters within Canada’s architectural and design community.  This article will define mass timber construction, examine its advantages, explore code developments in Canada, discuss the vital role of encapsulation, outline potential solutions as well as highlight notable projects.

"Mass timber " or "tall wood” construction can be defined as construction using timber products that have been engineered for strength to support loads similar to traditional structural materials such as concrete or steel.  As such, it differs from light wood or stick-frame or even heavy timber post-and-beam structures.  Several common types of mass timber products include cross-laminated timber (CLT), nail-laminated timber (NLT), and glued-laminated timber (GLT) also known as glulam. 

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A new code for a new era in tall wood construction

Code sections specific to tall wood construction are set for inclusion in the 2020 National Building Code of Canada (NBCC) and will outline requirements for the use of mass timber construction for structures up to 12-storeys (42 metres in height).  In addition to combustible, heavy timber and noncombustible construction, it will introduce a new construction type known as “Encapsulated Mass Timber Construction” (EMTC)—a type of construction where the mass timber is comprised of engineered wood product that has a fire and strength rating. EMTC is neither ‘combustible construction’ nor ‘heavy timber construction’ nor ‘noncombustible construction’, as defined within the NBC. 

Code word: Encapsulation

The definition will be set forth as follows: Encapsulated Mass Timber Construction is a “type of construction in which a degree of fire safety is attained by the use of encapsulated mass timber elements with an encapsulation rating and minimum dimensions for the structural timber members and other building assemblies.”  

Encapsulated means the mass timber is fully covered and protected from fire exposure by gypsum or other materials to resist fire and prevent its spread.  EMTC is required to have an encapsulation rating, which is the time, in minutes, that a material or assembly will delay the ignition and combustion of encapsulated mass timber elements when it is exposed to fire under specified conditions of test and performance criteria, or as otherwise prescribed by the NBCC. EMTC buildings would use mass timber with a minimum thickness of 96 mm and would require a minimum 50-minute encapsulation rating.

Advantages of tall wood construction driving growth

Canada’s federal government entities have demonstrated enthusiasm for mass timber construction, given the size and scope of Canada’s forest products industry, the potential economic benefits, and its seamless fit with the government’s prioritization of climate action and carbon reduction strategies.   Green builders and owners looking for more sustainable solutions are also embracing tall wood construction. 

The diverse range of benefits and advantages of wood construction compared to traditional construction methods cannot be overlooked, making it a competitive option.  Those in favour have noted the following as just a few reasons tall wood construction should be a growing part of the mix in North America’s built environment.

  • Environment: As noted above, mass timber represents a shift toward the use of rapidly renewable resources. This is seen by many as an important adaptation to climate change.  Life cycle assessments demonstrate that wood outperforms steel and concrete in terms of embodied energy, air pollution and water pollution. (Source: https://www.woodworks.org/why-wood/life-cycle-assessment-lca/life-cycle-assessment-lca-resources/).  Less energy is required for its manufacturing process which results is less greenhouse gas emissions.  It also has a lighter carbon footprint since wood continues to store carbon absorbed during its natural life. 
  • Efficiency: Mass timber construction can be part of a very tight and efficient building envelope. Panels can be manufactured to precise specifications, resulting in tighter joints. Thicker panels also offer inherently higher thermal resistance, reducing heat transfer for a more comfortable indoor environment and potentially reducing energy consumption.
  • Cost-effective: Mass timber buildings can often be completed in less time than a traditional build due to off-site fabrication. Shortened construction schedules can reduce financing time and speed up occupancy, while smaller footings (since wood is lighter than concrete) and smaller crews can also represent potential savings. 
  • Fire protection: Because it chars slowly, mass timber provides valuable fire resistance. Once formed, char protects the wood’s inner structure from further degradation. When used in Part 3 buildings, mass timber CLT assemblies also have fewer concealed spaces, which reduces a fire’s ability to spread undetected. In addition, CLT offers increased compartmentalization if used for interior walls.
  • Seismic performance: CLT panels perform exceptionally well in seismic testing in multi-storey applications due to their dimensional stability and rigidity. Demonstrated to be ductile, with good energy dissipation, seismic testing has shown no residual deformations. In fact, a seven-story CLT building was tested on a shake table in Japan, subjected to 14 seismic events with almost no damage.  While offering strength that compares similarly to steel, it lighter weight reduces load on the foundation, improving safety and resiliency.
  • Acoustic performance: The mass of the wall in mass timber buildings helps to mitigate sound transmission, as does sealants and membranes used to improve air tightness and insulation installed between the floor and wall. When stone wool insulation is used for encapsulation in tall wood structures for encapsulation to improve fire resilience, it also offers the secondary benefit of excellent sound absorption.

With so many obvious benefits, one might wonder why mass timber construction hadn’t caught on sooner.  The primary concern surrounding the use of mass timber construction in tall buildings has been fire safety.  Another considerable obstacle has been that codes and standards simply did not exist to provide proper guidance or an easy path to compliance.  The tall wood buildings in Canada that have been constructed to date—including those over 12 stories in height—have been approved through the alternative solutions path, as provided by the National Building Code of Canada. To date, taking his path has been necessary as there have been no specific code provisions that allow tall wood buildings. The 2020 national building code, which is being finalized, has codified the regulatory language to allow up to 12 stories without having to go through the alternative solutions method. The alternative solutions path is included in the code because there are times were designers want to build structures that are not explicitly allowed by the code. This is done by engineering analysis while demonstrating alignment with the objectives and functional statements of the code. 

Over the last 20 years, tall wood mass timber construction has gained greater traction in Canada and Europe than in some other parts of the world.  Building officials, fire services, architects, engineers and industry experts have been extensively studying fire behaviour in mass timber buildings and weighing in on fire and life safety considerations. In Canada, a significant amount of testing has been conducted to demonstrate the fire safety of wood structures. As a result, codes and standards have now been developed—and, as noted, are most likely to be implemented in the 2020 National Building Code of Canada.

Early adoption

Wood construction hasn’t always been controversial. It’s interesting to note that many heavy timber buildings were built in Toronto, Montreal, and Vancouver from around 1859 through to approximately 1941.  Similar buildings were also constructed throughout the U.S. during this period.  Known as brick and beam buildings, many of these historical structures have stood the test of time.  Most were built for industrial use as factories or warehouses.  Today, many have been converted to office or residential space and are highly coveted for their detailed exteriors, exposed wood beams and columns, high ceilings, sandblasted brick and often forged metal connectors.  Typically, there are between four and nine stories tall, with a height of up to 100 ft. (30 m), and total floor space up to 312,000 sq. ft. (29,000 sq. m.). The timber used in these historical buildings was usually Douglas Fir or northern species (Grade #1 or better).  There have been some known cases of fire events in historical brick and beam buildings. Heavy timber over the years has demonstrated adequate structural fire resistance thanks to the slow rate of charring during fires. In one event, a consultant noted beams were refinished after a fire when they were judged to be structurally sound.  (*Source: https://fpinnovations.ca/Documents/a-study-on-historical-tall-wood-buildings-in-toronto-and-vancouver.pdf)

The resurgence of tall wood

For centuries, people have looked to the past for inspiration, and in architecture and design, this is no exception.  The appetite for the aesthetic beauty of wood remains strong.  The desire for natural materials in construction is making significant inroads amid the sustainability movement now influencing many—if not most—industries worldwide.  This can largely be attributed to a greater awareness of climate and environmental issues and an increased sense of urgency to stimulate change.  Even among those for whom this is not a considerable driver, the appeal of wood, and more importantly, its ability to attract a premium among buyers and tenants is also a factor.

What separates today’s tall buildings from those built of heavy timber, is more sophisticated construction and fabrication techniques, a greater understanding of building science, better testing and building modelling, and stronger, much more developed codes to protect building occupants.

Fire Safety in tall wood construction: A new standard

A new standard test method has been developed to evaluate fire performance and determine if the assembly complies with the required fire rating per the NBCC.  The test method has been designated as ULC S146, the “Standard Method of Test for the Evaluation of Encapsulation Materials and Assemblies of Materials for the Protection of Mass Timber Structural Members and Assemblies”.

The following fire rating requirements will be set forth for EMTC buildings, in addition to the following restrictions:

  • Encapsulated mass timber must have a two-hour fire resistance rating
  • Floors must have a two-hour fire resistance rating.
  • Mezzanines must have a one-hour fire rating.
  • Load-bearing walls, columns and arches should have a fire rating not less than the fire-resistance required for the supported assembly.
  • Sprinklers will be a requirement throughout in accordance with NFPA 13.
  • A maximum of 12 storeys and 6,000 square metres is proposed for buildings with a residential occupancy (residential occupancy is classified as Group C occupancy per the National Building Code of Canada)
  • A maximum building area of 7,200 square metres would be applicable to a building with a business and personal services occupancy (business and personal services occupancy is classified as Group D occupancy per the National Building Code)
  • Exterior balconies are exempt but mass timber used must be a minimum of 96 mm
  • The total exposed combined area of mass timber beams, columns, arches, and walls should not exceed 35 per cent of the total wall area of the suite’s perimeter. (Mass timber beams, columns and arches unencapsulated within a suite can’t exceed 10 per cent of the wall area and the fire spread rating (FSR) of 150). 
  • Mass timber ceilings can be exposed if walls within the structure face the same way (not opposing) and the exposed wood has a FSR of 150.
  • Encapsulated mass timber construction is permitted on all storeys of Group C and D buildings, while it may only be used on limited storeys in Group A, E, and buildings with storage garages.
  • An increased fire resistance rating may be required between some major occupancies.
  • To remain in compliance with the NRCC, any damaged or removed encapsulation materials must be repaired or replaced so that the encapsulation rating of the materials is maintained.

There are restrictions on the use of exterior cladding elements in EMTC.  Exterior cladding elements are defined as elements or components that are exterior to the structure of a building.  There are also other restrictions on the use of combustible materials and components in a variety of other building elements including: roofing materials, window sashes and frames, components in exterior walls, nailing elements, flooring elements, stairs, interior finishes, elements in partitions, and concealed spaces.

While it is expected that the 2020 NBCC will allow for prescriptive solutions, it is anticipated that it will also leave room for performance-based designs using EMTC as long as the EMTC structures meet the same ratings as noncombustible buildings.     

Safe and resilient: Research and fire testing of mass timber construction

Within Canada, two major mass timber fire research studies are referenced for EMTC:

  • Fire Safety Challenges of Tall Wood Buildings – Phase 2: Task 2 &3 - Cross-Laminated Timber Compartment Fire Tests
  • Fire Testing of Rooms with Exposed Wood Surfaces in Encapsulated Mass Timber Construction

Other prominent and referenced testing/research includes: Fire performance of mass-timber encapsulation methods and the effect of encapsulation on char rate of cross-laminated timber (Hasburgh et al., 2016).

Comprehensive testing has allowed researchers to draw a number of important conclusions:

Mass timber structures have been shown to have acceptable performance when exposed to fire.  This is because there is sufficient mass to allow a char layer to form, insulating the remaining wood from heat penetration and protecting its structural integrity.

There are adequate solutions to achieve code-compliant fire performance.  Requirements for one-hour fire resistance ratings are readily achieved with mass timber construction, including CLTs. However, high-rise buildings typically taller than six stories, are required to achieve a fire resistance rating of at least two hours for floors and other structural elements. To obtain higher fire resistance ratings, guidance exists based on conducted research.  The Technical Guide for the Design and Construction of Tall Wood Buildings in Canada by FPInnovations presents various encapsulation methods for full or partial protection of timber elements. 

 

Encapsulation, as a form of passive fire protection, has been shown to protect structural elements against the impact of fire exposure and mitigate the effects of fire on the underlying mass timber.  Encapsulation aims to ensure that the protected mass timber does not contribute to the fuel load since the encapsulation material delays its involvement for a period of time. 

Materials tested for effectiveness in encapsulation of mass timber include Type X gypsum (1/2 inch and 5/8 in single and/or multiple layers, stone wool insulation (also known as mineral wool or rock fibre insulation), intumescent, and spray-applied fire-resistive materials (such as Portland cement) or a combination of encapsulation materials, with or without cavities, and with varied attachment methods.

The research and testing has demonstrated that materials differ in their ability to prevent heat transfer, and as a result, a range of encapsulation times and char rates were achieved.  Each of the materials tested demonstrated the ability to provide some protection, but the best performing materials proved to be stone wool (mineral wool/rock fibre) due to the fact that it remains in place for long periods of time, while continuously protecting the wood. Building industry professionals, code officials and other stakeholders continue to amass data through ongoing research which may contribute to improved encapsulation solutions and/or the potential for code updates.

Encapsulation Methods: The distinct advantages of stone wool

To achieve an encapsulation rating of not less than 50 minutes, there are several encapsulation options that can be employed including:

  • Two layers of not less than a 12.7-mm-thick Type X gypsum
  • Not less than 38-mm-thick gypsum-concrete or concrete topping
  • Other noncombustible material or assembly of materials that provide an encapsulation rating of 50 minutes

The use of stone wool insulation as an encapsulation material falls into the latter option.  The material is well suited to the protection of structural elements.  In fact, stone wool insulation is commonly used to protect elements in curtain wall assemblies and as fire-safing insulation in perimeter fire containment systems, as well as other applications where exceptional fire protection is required. 

Third-party testing confirms that stone wool insulation products do not ignite, burn, support combustion or release flammable vapours when subjected to fire or heat as per CAN/ULC S114.  Additionally, certain stone wool insulation products have been shown to achieve a 0/0 flame spread and smoke development rating, further demonstrating desirable characteristics when exposed to fire.

Stone wool insulation supports the need for fire protection redundancies to be built into wood buildings.  The passive fire protection offered by stone wool—especially if it incorporated throughout the building—can help protect escape routes and support effective compartmentalization by potentially slowing the progression of fire.  It facilitates safe egress and contributes to improved occupant safety.

Testing has shown that stone wool stays in place for a long time (well beyond 2 hours). This is significant, as the stone wool material remains in place to protect the wood from radiation long after the encapsulation time has been exceeded.  Some other encapsulation materials have been shown to fall off after approximately 50 minutes, leaving the wood bare and unprotected. In fact, in the research report, Fire performance of mass-timber encapsulation methods and the effect of encapsulation on char rate of cross-laminated timber (Hasburgh et al., 2016), it was noted that “rock fibre insulation was able to improve encapsulation time by nearly 20 min, over a standard gypsum board application, and stay in place over 2 h.”  This research also look at acoustic performance of the EMTC assemblies tested.  It noted that rock fibre “can also provide the opportunity to improve acoustic performance and when used with a dropped ceiling will conceal building services without the need for sprinklering the concealed spaces.”

Of note to building industry professionals, as a general rule, it cannot be assumed that all mineral wool products offer the same fire performance.  Architects and specifiers are encouraged to refer to manufacturers regarding product composition and fire testing data.  For example, it is noteworthy that although stone wool (specifically ROCKWOOL stone wool insulation products) falls in to the category of rock fibre or mineral wool, its higher quality composition (optimized fibre chemistry, low shot content) offers a low linear shrinkage rate of no more than five per cent in the length dimension when tested in accordance with ASTM C356 at 1,000˚C.  This (dimensional stability) is important when considering an encapsulation material, as it reduces potential exposure to the wood structure beneath due to shrinkage or gaps forming when exposed to fire.  Conversely, slag wool, while also classified as a mineral wool, may not achieve the same performance, exhibiting greater linear shrinkage when exposed to fire.  This is a result of uncontrolled fiber chemistry as a consequence of greater quantities of slag.

Stone wool’s excellent drying potential provides an important benefit during the construction phase, as well, protecting wood from the elements.  Should stone wool get wet, either during construction or in service, the moisture-resistant product will dry out, retaining its full R-value and fire-resilient properties.  Because it’s comprised of inorganic materials, stone wool is resistant to mold and mildew.  This is a distinct advantage over hydrophilic products, which once exposed to moisture need to be replaced to prevent mold from forming.  Using stone wool as an encapsulation material or as insulation for various applications in wood buildings allows construction to move forward with less concern around rain and weather. 

Time on the job site is an important consideration, as well.  Stone wool is light weight and easy to handle.  Stone wool products, like ROCKWOOL COMFORTBOARD™, for example, come in a variety of thicknesses and can be applied as an encapsulation material in a single layer to achieve a high level of fire protection.  Two layers of gypsum, meanwhile, are required to achieve a 50-minute encapsulation rating.  There’s no drying time for stone wool or requirements for specific temperatures or conditions to exist prior, during or after install such as with other encapsulation products like spray-applied fire-resistive materials (SFRM), which is also not suitable for surfaces exposed to moisture or high humidity levels.  In addition, the moisture may cause mold growth due to the porous nature of SFRM.

Project profiles: Stone wool already quietly protecting tall wood buildings across Canada

Origine Condominium (Pointe-aux-Lièvres, Québec) - Origine is a 13-storey, 92-unit residential tower located on the banks of the Saint-Charles River in Quebec City. With fire safety a top concern, particularly since Origine was laying the groundwork for future sustainable wood buildings, the project was extensively fire-tested with mockups at the National Research Council in Ottawa. ROCKWOOL COMFORTBOARD™ 110 was installed to provide a layer of continuous exterior insulation, offering important fire resistance while improving energy efficiency.  To that end, the COMFORTBOARD™ 110, combined with Origine’s massive wood panels, reduce thermal bridging to a minimum.  It is estimated that a wood building’s energy costs can be about 60 per cent of a standard building. Choosing lumber for the structure and other sustainable materials, like the ROCKWOOL stone wool insulation, will substantially reduce the building’s environmental impact in terms of embodied energy, as well as its carbon footprint.  Stone wool was also chosen due to its dimensional stability, vapour permeance, moisture resistance and excellent drying potential (given the humid riverfront location), and its sound absorption properties.

Brock Commons at UBC (Vancouver, BC) - Completed in 2017, Brock Commons Tallwood House is an 18 storey, LEED Gold target, 404-bed student residence building located at the University of British Columbia in Vancouver, BC. The world’s tallest mass wood tower at the time of its completion, Brock Commons Tallwood House’s timber structure and prefabricated facade went up in only 66 days. The structure’s wall panels, fabricated by Centura, integrate ROCKWOOL COMFORTBATT™ R24 insulation and 2” CAVITYROCK® with Cascadia Clips. Stone wool insulation was specified to its noncombustibility and will add important fire protection for the wood panels. The stone wool will also serve to protect the wood elements from moisture during construction, reducing the risk of mold that might exist with other types of encapsulation materials when exposed to moisture. In addition, the insulation will assist with moisture management of the wall assembly throughout the life of the building due to the product’s moisture resistance and high drying potential.  The wall system satisfies the criteria when subjected to testing in conformance with CAN/ULC-S134, Standard method of Fire Test of

Exterior Wall Assemblies. At 2,233 cubic meters, the building utilizes an extraordinary amount of timber that stores an impressive 1,753 metric tons of carbon dioxide and avoids production of 679 metric tons of greenhouse gas emissions.  

Academic Wood Building (U of T) – The Academic Wood Tower at U of T is planned as a 14-storey mass timber and concrete hybrid tower to be built above the Goldring Centre for High Performance Sport at the University of Toronto.  Stone wool insulation is anticipated to be used for the walls.

Conclusion

Strong evidence exists demonstrating the strength, durability and safety of tall wood construction including EMTC.  It is supported by comprehensive testing that demonstrates that encapsulated mass timber offers adequate fire resilience to meet the provisions as they will be set out in the 2020 NBCC.   Encapsulation materials, such as stone wool, play a vital role in achieving acceptable fire resistance ratings.  Gypsum or concrete coatings can be used, although alternate encapsulation material such as stone wool insulation may provide a number of additional advantages including water repellency—providing protection both during and post construction—increased acoustic benefits, and better thermal properties. 

Similar fire testing in other countries is also bearing out the safety and performance of EMTC.  In fact, the support for taller timber structures is evidenced throughout the globe, with trailblazing and iconic projects that literally take the construction of EMTC to new levels.  In the United States, the International Code Council approved 14 changes to the International Building Code (IBC) in early 2019, permitting mass timber construction up to 270 feet (~80 metres or 18 storeys). 

Revolutionary projects and others in the planning stages underscore the possibilities that exist for tall wood construction and suggest that Canada’s conservative approach to its acceptance and permissibility, as it is expected to be included, under the new code may leave room for further development and potentially greater heights.  An 80-story timber tower has been proposed by PLP Architecture at the Barbican complex in London.  It would create more than 1,000 residential units in a one million sq. ft. mixed-use tower and mid-rise terraces.  As part of its ongoing Riverline community project, Perkins+Will has proposed an 80-storey, 300 unit residential skyscraper made from timber called the River Beech Tower. Anders Berensson Architects has unveiled conceptual plans for Stockholm's tallest building—a 133-metre wooden skyscraper covered in numbers, which would be erected on top of a 1960s car park in the city centre. Completed and in-process projects are also reaching new heights: the 24-storey HoHo project in Vienna by RLP Architects, 14-storey TREET residential tower in Norway, Australia’s 10-story Forte tower (as the tallest wood building in the world at the time), and the 24-story HoHo project in Vienna designed by RLP Architects.

Moving forward, sustainability, economic and efficiency benefits are expected to continue to fuel the proliferation of encapsulated mass timber construction.  These new buildings will be celebrated for their innovation and splendor, much like their historical brick and beam counterparts are coveted today, proving that what is old is new again—but much better, as Canada takes tall wood construction to a new level.