Proposed Change 1870

Over the past decade, fire research on mass timber structures, specifically those constructed using cross-laminated timber (CLT), has shown that in certain fire scenarios (i.e., no automatic sprinkler activation and no fire department intervention), a secondary fire growth and even a secondary flashover can occur, due to the delamination of the CLT elements. As a result of the concerns arising from these fire phenomena, the encapsulation requirements and special exceptions for encapsulated mass timber construction (EMTC) were introduced in the National Building Code of Canada (NBC) 2020, which include restrictions on exposed mass timber surfaces as well as exposed surface flammability (via flame-spread ratings) for the different elements in Article 3.1.6.4.

The exceptions were based on the research conducted up to that point using CLT product that was manufactured in conformance with ANSI/APA PRG 320-2014, "Standard for Performance-Rated Cross-Laminated Timber." Since that time, additional fire research was performed that resulted in the 2018 edition of the standard, which was referenced in the NBC 2020, and includes additional fire-performance requirements for the adhesives used in CLT product to eliminate delamination in a fire.

Also, research testing was recently performed by the National Fire Laboratory of the National Research Council of Canada (NRC) using CLT product conforming to ANSI/APA PRG 320-2018, as well as nail-laminated timber (NLT) elements, and additional mass timber elements, such as glued-laminated timber (glulam) beams and columns. Furthermore, research on real-scale fire tests was conducted by the Research Institutes of Sweden (RISE) on compartments constructed of CLT product that is also compliant with ANSI/APA PRG 320-2018. These tests have demonstrated that the existing provisions in the NBC 2020 are now conservative in nature and can be safely expanded to provide additional options to designers and builders.

The current provisions for small areas of exposed ceilings are restrictive in that they are only allowed within suites, not in fire compartments. This can be confusing and restrictive, especially in cases where the entire storey is a single occupancy/tenant or in the case of classrooms where the concept of suites is not always clear. Research has shown that allowing for increased use of exposed ceilings would not result in an undue fire risk. As such, the requirements in Article 3.1.6.4. on the encapsulation of mass timber elements do not reflect the latest research and need to be revised to reflect the performance of the materials that are now available in the market.

This proposed change revises the exceptions to encapsulation of structural mass timber elements for EMTC in Article 3.1.6.4. of Division B of the NBC, based on new research performed by the Fire Laboratory at the NRC and RISE, as well as the recent changes made to ANSI/APA PRG 320 in 2018 that require the use of adhesives with additional fire performance characteristics in the manufacture of CLT.

In most cases, an automatic sprinkler system will ensure that, if a fire starts in a building, the fire never grows large enough to challenge the structure. This is equally true in a high-rise mass timber building with exposed mass timber. In the case that the sprinklers are unable to control the fire, the fire department is expected to arrive and extinguish the fire long before the structure is compromised, given that a two-hour fire-resistance-rated structure is required for buildings over six storeys in accordance with Articles 3.2.2.48. and 3.2.2.57. The primary concern regarding exposed mass timber in a high-rise building is what happens in the event the fire is not controlled by either the sprinkler system or the fire department, even though the probability of such an event occurring is acceptably low. Therefore, the question becomes: what happens when the fire continues to burn and the room’s combustible contents are consumed? More specifically, the question is whether the mass timber structure provides enough fuel for the fire to continue to burn at a level that will eventually lead to structural failure or the fire will decay as would be expected in a noncombustible building. The concern for this low-probability scenario is the primary reason the EMTC provisions in the NBC 2020 have taken steps to limit the amount of exposed mass timber in Article 3.1.6.4.  

As a result of the perceived fire hazard concerns arising from the potential for delamination in CLT (assuming no sprinkler activation or fire department intervention), recent changes were developed and adopted in ANSI/APA PRG 320-2018. The new requirements mandate the use of adhesives that must be pre-qualified through large-scale fire testing that is intended to evaluate the propensity for delamination of the CLT wood laminations.

Between 2018 and 2021, additional fire research was conducted by the NRC and RISE using CLT products manufactured using the “new generation” adhesives (qualified under the ANSI/APA PRG 320-2018 protocol) and other mass timber products such as NLT and glulam. Primarily, this research was initiated to specifically evaluate the Code provisions for EMTC that were introduced in the NBC 2020 as well as the limits prescribed in American codes, which relate to the percentages of permitted exposed mass timber surfaces using the CLT product with new generation adhesives. This research also explored other mass timber element scenarios with different amounts of mass timber elements exposed.

The NRC and RISE reports are listed below:

• "Fire Testing of Rooms with Exposed Wood Surfaces in Encapsulated Mass Timber Construction" (NRC CLT Report)[1]
• "Nail Laminated Timber Compartment Fire Tests" (NRC NLT Report)[2]
• "Fire Safe implementation of visible mass timber in tall buildings – compartment fire testing" (RISE Report)[3]

The NRC CLT Report describes a total of five CLT compartment fire tests (2018 NRC CLT compartment tests) that included varying amounts of exposed CLT wall and ceiling surfaces; two of the tests also incorporated glulam columns and beams.

The test rooms were relatively small (4.5 m × 2.4 m × 2.7 m tall) due to the test laboratory constraints. This test room size represents a severe scenario from the point of view of re-radiation of heat between surfaces, which is critical for the mass timber surfaces to continue to burn once the movable fuel load is consumed. The test room also had a door opening that provided a ventilation condition similar to the more severe one used in earlier tests performed by the NRC for the Fire Protection Research Foundation at the National Institute for Science and Technology (NIST) laboratory.[4] The smaller ventilation condition resulted in a longer duration and, therefore, more severe fire exposure to the compartment boundaries (walls and ceiling), as well as any beams and columns included. Table 1 provides a summary of the tests.

Table 1. Summary of the 2018 NRC CLT compartment test results by configuration[1]

With respect to the overall results of the various degrees and configurations of exposed mass timber surfaces, the NRC CLT Report states that “[i]n all tests with exposed mass timber, the peak room temperatures were similar to the baseline (fully encapsulated).”

With respect to the performance of the CLT, which used an adhesive compliant with ANSI/APA PRG 320-2018, the NRC CLT Report concludes that “[a]ll tests in this series showed that the second generation CLT with the thermal resistant adhesive improved significantly in fire performance to resist the char layer fall-off.” This means that the changes made to ANSI/APA PRG 320 in early 2018 do ensure that adhesives that are approved for use in CLT (because they are compliant with that edition of the standard) will not delaminate, thereby significantly increasing the fire performance of the CLT panels. This increased performance was demonstrated in particular in Test 4 of the recent NRC tests, where a fire in a compartment that had an exposed glulam beam and column and an exposed CLT ceiling still experienced the decay phase once the moveable fire load was consumed. This result is significant since the test set-up was rather challenging (i.e., severe). The room was small, thereby increasing the radiation feedback between burning surfaces, and the ventilation opening was a single doorway, resulting in a longer and more severe fire exposure than typically occurs in actual construction.

These results are for CLT products that conform to ANSI/APA PRG 320-2018, which is the latest version of the standard and is referenced in NBC Sentence 3.1.6.3.(3). The results are equally applicable to other mass timber products that are known not to delaminate, such as glulam, when exposed to fire temperatures.

The NRC NLT Report describes a total of four NLT compartment fire tests (2019 NRC NLT compartment tests) that included varying amounts of exposed NLT wall and ceiling surfaces; two of the tests also incorporated glulam columns and beams.

The same set-up was used for the test rooms as in the NRC CLT Report in terms of the size of the compartments and door ventilation openings. The compartment configurations were also similar to Tests 4 and 5 of the CLT compartment tests. Table 2 provides a summary of the NLT compartment tests.

Table 2. Summary of the 2019 NRC NLT compartment test results by configuration[2]

Again, with respect to the overall results of the various degrees and configurations of exposed mass timber surfaces, the results of the NRC NLT Report show that the peak room temperatures were similar to the CLT baseline (fully encapsulated).

With respect to the performance of the NLT, the NRC NLT Report concludes that “NLT panels typically have some small gaps between laminations … These small gaps provided passages for the flame and hot pyrolysis gas to travel in the NLT panels. Test NLT-3 and Test NLT-4 demonstrated that, in the absence of operationally effective sprinklers, to reach full decay of the fire three layers of 12.7 mm thick Type X gypsum board were necessary for NLT rooms with partially encapsulated walls and fully exposed ceilings to limit undue contributions of the protected NLT elements to the compartment fires, while still keeping the same total area of exposed surfaces as in the two test configurations." It was also stated that, based on the test results from both NRC reports, “second generation CLT generally performed better than NLT in these compartment fire tests to limit contributions of timber to the fire.”

The NRC NLT Report also notes, "it is reasonable to expect that, had the three-layer gypsum board protection been used on Wall A and Wall C in Test CLT-5, the recurrent fire at 2020 minutes would not have occurred in that CLT test." This would mean that if a third layer of 12.7 mm thick Type X gypsum board were added to CLT Test 5 the additional regrowth in that test would not occur and the fire would reach full decay.

The RISE Report describes another five CLT compartment fire tests (2021 RISE CLT compartment tests) that included varying amounts of exposed CLT wall and ceiling surfaces and glulam columns and beams.

The test compartments were larger outdoor structures (7.0 m × 6.85 m × 2.73 m tall) with two ventilation opening factors (0.062 m^1/2 or 0.25 m^1/2) representative of typical openings in tall residential and office buildings respectively. The CLT used in these tests were also products manufactured using the new generation adhesives that was qualified under ANSI/APA PRG 320-2018 protocol. Table 3 provides a summary of the tests.

Table 3. Summary of the 2021 RISE CLT compartment test results by configuration[3]

Again, with respect to the overall results of the various degrees and configurations of exposed mass timber surfaces, the results of the RISE Report show that the peak room temperatures were similar to the baseline (fully encapsulated).

With respect to the performance of the CLT that used an adhesive compliant with ANSI/APA PRG 320-2018, the RISE Report showed similar performance to the NRC CLT Report where thermal resistant adhesive improved significantly in fire performance to resist the char layer fall-off. This result means that the changes made to ANSI/APA PRG 320 in early 2018 do ensure that the adhesives approved for use in CLT (because they are compliant with that edition of the standard) will not delaminate, thereby significantly increasing the fire performance of the panels.

Therefore, the findings of the NRC and RISE research would suggest that the current requirements are too restrictive and can be relaxed, while continuing to maintain a sufficient level of fire safety.

Table 4 provides a comparison of the NBC 2020 requirements and proposed changes for the NBC 2025 based on the most recent fire research.

Table 4. Comparison of NBC 2020 EMTC provisions and the proposed changes for the NBC 2025

Notes to Table 4: CL = ceiling; W = walls; B/C/A = beams/columns/arches; S = suite; FC = fire compartment; FSR = flame-spread rating; NP = not permitted

Sentence 3.1.6.4.(3) on Beams, Columns and Arches

The proposed change to Clause 3.1.6.4.(3)(a) is based on Test 3 of the NRC CLT Report, which included three beams and two columns that were fully exposed, representing 36% of the perimeter wall area. Therefore, this proposed change increases the maximum permitted exposed surface area for beams, columns and arches from 10% to 35% of the perimeter wall area.

Sentence 3.1.6.4.(4) on Walls

This proposed change revises Clause 3.1.6.4.(4)(a). Instead of requiring all exposed surface areas for mass timber walls to face one direction, the proposed change aligns with the research that shows it also suffices to ensure that facing or adjacent surfaces are a minimum distance apart, so re-radiation between exposing surfaces is sufficiently minimized. 

Cost

This proposed change is not likely to entail significant costs because it would not increase the stringency of the requirements for encapsulating mass timber elements. The proposed change would permit more timber elements to be left exposed, without requiring more of them to be left exposed. Not being required to install encapsulation materials reduces costs; however, the thickness and size of the exposed mass timber elements might need to increase, which could add cost.

A design complying with the NBC 2020 would remain in compliance with this proposed change.

Benefit

This proposed change would allow designers to leave exposed more of the mass timber construction elements of a building, an interest in which has been expressed by several regions of Ontario and British Columbia that have requested more biophilic construction (i.e., the use of natural finishes). Not being required to install encapsulation materials also provides benefits during the construction process, which can be challenging as relates to managing moisture effects.

Many Code users, including developers, architects, engineers and builders, across Canada are seeking additional options for the use of products such as wood that have demonstrable environmental benefits (e.g., low embodied carbon) in buildings of a greater variety of heights and sizes.

This proposed change can be enforced by the current Code enforcement infrastructure. This proposed change would not introduce conflict or create enforcement issues since it is generally a relaxation of the existing NBC 2020 provisions.

Architects, designers, developers, owners and engineers would benefit from the increased flexibility provided by this proposed change.

Authorities having jurisdiction, including fire departments, would need to continue to evaluate their operating procedures in accordance with the Code requirements for EMTC.