Combustibility & the Building Code

Combustibility is an extremely important topic when it comes to building construction, as it relates directly to the fire safety of the structures we live and work in. It is essential to restrict a fire’s ignition, spread, and severity, maximizing the time of occupants to safely evacuate a space, and minimizing the chance of flames spreading to neighbouring buildings. This is a critical part of maintaining safer cities.

How is combustibility enforced?

Each and every material that goes into a structure must be considered when addressing combustibility in construction. Part 3 of the BC Building Code applies to larger buildings that carry higher occupancy, with a part 3 building being defined as:

“buildings classified as Group A(Theatres, Libraries, Arenas etc.), B(Jails, Hospitals, etc.) or F-1(Light Industrial); or exceeding 600 m2 in building area or exceeding three storeys in building height and have major occupancies: Group C (residential), Group D (office/service), Group E (retail), or Group F-2, F-3 (medium- and high-hazard industrial)”

The use of combustible building materials is restricted for part 3 buildings in both the provincial and national codes. The intent of this restriction is obvious. Any material used in a part 3 non-combustible building must be classified as “non-combustible” through a furnace test (CAN/ULC-S114), be deemed “limited combustible” through cone calorimeter test (ULC-S315) or be specifically exempt. Although UPVC (Vinyl) has been found to be surprisingly fire resistant, it does not meet the standard to be considered non-combustible and is therefore deemed combustible. This label has restricted the use of vinyl windows into the exemption category. [3.1.5.4(5)] of the code lays out 3 sets of criteria that must be met for exemption:

• Each window in an exterior wall face is an individual unit separated by noncombustible wall construction from every other opening in the wall
• Windows in exterior wall in contiguous stories are separated by not less than 1M of noncombustible construction
• The aggregate area of openings in an exterior wall face of a fire compartment is not more than 40% of the area of the wall face

Meeting this criteria is not always practical and in turn removes the option for vinyl windows in many scenarios.

What are the different interpretations of combustibility in the Building Code?

These restrictions were brought into the 1965 edition of the building code. It has been admitted these restrictions are somewhat arbitrary. For instance, the 1M vertical separation between windows was not written to consider an overhang such as a balcony between patio doors, and the 40% wall area does not consider sprinkler use, resistance rating of surrounding wall, or proximity to an adjacent building. Over the years exceptions to these requirements have been made on specific projects after careful review.

Until recently, aluminum window frames have been the default material used in noncombustible buildings. Although it is true that aluminum does not burn, it is a particularly terrible insulator. As society has continued to focus more on the importance of energy efficiency, so too have our building codes. The insulating performance of windows substantially affects the carbon footprint of the entire building. As local and national codes continue to tighten their energy requirements, the need for alternatives to aluminum increases. Some of the most energy-efficient window frames in Canada today are PVC. Not only are PVC windows a far better insulator – they also come at a much lower price point compared with aluminum.

How is this requirement actually enforced?

In 2014 it was acknowledged that building code needed to be modernized, and deep study into the true fire resistance of composite windows was launched by the industry. Through multiple combustion tests and the full-scale vertical burn test, it was proven that PVC window frames perform just as well as aluminum in a fire. Interested readers can learn more here. A code change request was filed on August 30, 2016, backed by the observed test results. Following the public review, the proposed change was approved at the April 2019 Standing Committee for Fire Protection meeting with a single amendment:

               3.1.5.4.(5) as it is present in the 2020 NBC:

Combustible window sashes and frames are permitted in a building required to be of noncombustible construction, provided they are vertically non-contiguous between storeys.

Note that this means that there are very little restrictions. The only restriction is that the combustible window sashes and frames cannot be contiguous between storeys. “Contiguous” simply means touching. Technically, as long as there is any separation between storeys, even 1mm, that would meet the requirement to be non-contiguous. These changes have been adopted in BCBC 2024 (BC) and NBC AE 2023 (AB). These come in force on March 8 for BC and May 1 for AB.

This welcome change enhances the ability for both codes and industry to continually evolve and create more energy-efficient buildings without sacrificing safety. Centra has utilized exemptions and recent national code changes to use vinyl windows on larger buildings and save clients a lot of money in the process. Stay tuned to learn more.

Do note that a code section for encapsulated mass timber has been added in the NBC 2020 as well as the various provincial codes (BCBC 2024, NBC AE 2023). The section on combustible window sashes and frames within (3.1.6.8.) matches the old code requirements. This is unchanged, likely due to the code change introducing encapsulated mass timber construction being processed before the code change for combustible window sashes and frames. This is likely to be unified in the next code cycle.

Limitations

Although it may seem that, in typical construction, combustible window sashes and frames can be used with very little restrictions, it is important to note that all the requirements for fire-rated windows are still in place for the locations they were required previously. Fire-rated windows are still required in the locations they were previously required. Please speak to your code consultant for details.

Implications/Summary

Purely looking at combustibility of window sashes and frames, there is no reason to rush to get your building permit application in before your local code’s change in force (March 8 2024, for BC, May 1 2024 for AB). There may be other parts of the new code that you would like to avoid, but those will not be discussed here.

If your project is permitted based on the old/outgoing code, all the old restrictions for combustible window sashes and frames apply. It may be possible to comply to the new, less restrictive requirements via an alternative solution which simply points to the new code. Please consult your code consultant and/or Authority Having Jurisdiction for their opinion.

If your project is permitted based on the new/incoming code, for a typical building, the only requirement for combustible windows and sashes is that there is some separation between storeys. All the old requirements for combustible window sashes and frames apply to encapsulated mass timber construction in the new/incoming code. Knowing the history and the reasoning behind why it is unchanged in the new code, you may also be able to comply to the new, less restrictive requirements for other buildings via an alternative solution pointing to the section of the code with those requirements and the reason that the old requirements are still there in the encapsulated mass timber section. However, it would likely be significantly more difficult to convince an Authority Having Jurisdiction, as the restrictions are present in the new code. If you would like to take this approach, please reach out to your code consultant as early as possible for their opinion. If you have an upcoming project you need help with, be sure to speak to the Window Geeks.