Images courtesy SAFTI FIRST
How radiant heat impacts rescue
Radiant heat is extremely dangerous to building occupants, since it can quickly reach a level that causes unbearable pain, followed rapidly by second-degree burns. The pain and burns sustained by building occupants from the uncontrolled passage of heat can be so intense the occupants are unable to exit the building safely. This is why NBC includes provisions that require use of building materials meeting CAN/ULC-S101 in critical areas such as exit enclosures, exit passageways, and fire-rated barrier walls and floors.
Depending on the building type or occupancy, these areas must be protected from smoke, flames, and radiant heat from one to four hours. This is intended to give building occupants a path of safe egress. They can also provide an area of refuge where occupants can await rescue, which can be necessary in hospitals, urgent care, long-term care, and other healthcare facilities where immobile patients make evacuation difficult or impossible.
How radiant heat can impact fire confinement
As mentioned, the firefighters’ secondary objective after rescue is fire confinement. Firefighters cannot effectively extinguish the fire if they cannot control how far it spreads. The heat generated from a building fire can be so intense it can ignite adjacent areas and surrounding buildings, making fire confinement much more difficult. As one firefighting training guide describes it:
Radiant heat moves away from the fire building in all directions; it is not affected by winds. Thus, fire may spread by radiation to any building near enough to the fire building to absorb sufficient heat… Radiant heat will also pass through transparent glass and ignite materials within a building. If the outside surface of a building is in danger of ignition from radiant heat, the areas within its windows constitute an equal hazard.
Two of the most effective ways to prevent radiant heat transmission from one building to the next are providing significant distance between the two structures and having unpierced fire walls. However, this is not always possible. In densely populated areas such as Toronto and Vancouver, buildings are constructed close together—either a few metres apart or right at the property line. While building codes allow some openings in exterior fire walls, there are restrictions to such openings because, as stated above, radiant heat will pass through the glass and ignite the combustible materials behind it.
NBC has provisions on the amount and size of protected and unprotected openings allowed in exterior walls. As fire-protective glass, such as ceramics and wired glass, does not protect against radiant heat, it is either limited in size or prohibited altogether, depending on the fire separation distance. However, these limitations do not apply to fire-resistive glazing that meets CAN/ULC-S101, because these materials are considered in the code as a wall capable of limiting radiant heat, even if it is transparent.
In other words, if an exterior fire wall incorporates fire-resistive glazing for vision or transparency, it is still considered a fire wall under NBC. As mentioned earlier, having unpierced fire walls is one of the best ways to prevent radiant heat from spreading from one building to the next—especially when there is very little distance between them. This is very important because, again, only when a fire is confined can firefighters effectively extinguish it.
‘Unpierced’ is not a defined term under NBC, but is used in this context as a fire wall that does not have openings. Fire walls with openings are essentially weaker compared to those without, because they contain areas where radiant heat can pass through. NBC limits the amount of openings in exterior fire walls depending on fire separation distance, occupancy type, and other factors, but using fire-resistive glazing in exterior fire walls can still allow designers to have vision and transparency without compromising the fire wall. In other words, fire-resistive glazed openings function in CAN/ULC-S101 like a fire wall that prevents the passage of smoke, flames, and radiant heat. (It is important for design and/or construction professionals to consult local codes and regulations for project-specific guidance.)
