Smoke barrier

A smoke barrier is a continuous surface, such as a wall, floor, ceiling assembly or damper in a duct, that is designed and built to restrict the movement of smoke. It may or may not also have a fire resistance rating, and it may contain protected openings such as doors. Smoke barriers divide a building into compartments, often called smoke zones, so that smoke from a fire stays in or near the zone where it starts. This approach is known as passive smoke management or compartmentation.

No smoke barrier is completely tight. Smoke passes through cracks, gaps around doors and holes around pipes, and the amount depends on the size of these leakage paths and on the pressure difference across the barrier. That pressure difference comes from the fire itself and from stack effect, wind and the ventilation system. Barriers are therefore often combined with fans that keep the protected side at a higher pressure, which is the basis of mechanical smoke control. In performance-based design, a barrier is described by its leakage area or leakage rate at a given temperature. Engineers also have to allow for defects such as unsealed service penetrations and doors left open.

Purpose

Smoke is recognised as the major killer in fires. Klote notes that smoke often flows to places far from the fire, so that stairs and lift shafts become filled with smoke and evacuation is blocked. Smoke barriers limit that spread. In the Australian verification method, they form part of a smoke containment strategy that keeps smoke out of adjacent enclosures and escape routes long enough for evacuation and fire brigade intervention.

Barriers are the oldest means of smoke management, together with smoke vents and smoke shafts. In the World Trade Center buildings, smoke barriers built into the architecture and the ductwork were one of two primary means of controlling smoke. The other was air movement equipment. The International Fire Engineering Guidelines list smoke barriers, together with natural smoke venting and mechanical smoke management, as protective measures in the sub-system for smoke development, spread and control.

The choice of barriers affects the whole fire analysis. In a zone model, a fire in an adjacent room that is not separated from the main space by a fire- and smoke-resisting construction is often the governing scenario. Smoke from such a room mixes with more air on its way out and produces a much larger smoke volume. In large open spaces such as atria and covered malls, however, barriers and pressure differences are not practical, and other strategies are needed.

Types of smoke barrier

Walls and floors are the basic elements. In Australia, the deemed-to-satisfy provisions of the National Construction Code allow smoke-proof walls of non-combustible construction, which may include safety glass. The Code does not set an acceptable leakage limit. In New Zealand, C/VM2 (second edition, 2025) speaks of smoke separations and requires the trial design to identify them together with fire separations and their closures.

Openings in a barrier need their own protection. Smoke doors must be built so that smoke does not pass from one side of the doorway to the other. Where ducts pass through a barrier, smoke dampers are used. A smoke damper is designed to stop smoke when closed and is closed by a signal from a smoke detector or the building control system. A fire damper is designed to stop fire and is closed by heat that melts a fusible link. Smoke dampers are typically designed to close if power to the actuator is lost. In the United States, smoke dampers are classified by leakage, from Class 1 (lowest) to Class 4, and by the elevated temperature at which they are tested.

Barriers do not always reach the floor. Draft curtains are barriers hung from the roof to limit the spread of smoke beneath it. They divide a large space into curtained sections, each forming a smoke reservoir in which a hot gas layer collects and from which it can be vented. If the layer drops below the bottom of the curtain, smoke flows out underneath it. NFPA 204, as cited by Cooper, recommends that such curtains reach down at least one fifth of the distance from the ceiling to the fire. Movable elements such as retractable screens, like doors and dampers, change position in a fire, and the guidelines ask for the delay of that change to be specified (see also fire curtain).

How smoke passes a barrier

Several forces drive smoke movement in a building: stack effect, buoyancy of the hot smoke, expansion of the heated gases, wind and the ventilation system. Each of these creates pressure differences across walls and floors. The hot gases in a fire room are lighter than the surrounding air. Above the neutral plane, where the pressures inside and outside are equal, this produces an overpressure that pushes smoke through any leakage path.

The pressures involved are small. For a fire room at 800 °C, the buoyancy pressure 1.52 m above the neutral plane is about 13 Pa. In full-scale fire tests, the highest pressure difference measured across the wall of the burn room at the ceiling was 16 Pa. Buoyancy weakens as smoke moves away from the fire, because the smoke cools and is diluted. Even so, small pressure differences are enough to push smoke through the many small gaps in ordinary construction.

Leakage depends mainly on workmanship rather than on the materials used. Klote gives typical leakage areas as a fraction of the wall or floor area, based on measurements by the National Research Council of Canada:

Typical leakage area ratios of walls and floors in commercial buildings

Construction element	Tight	Average	Loose
Stairwell walls (without door gaps)	0.14 × 10−4	1.1 × 10−4	3.5 × 10−4
Floors (including gaps around penetrations)	0.066 × 10−4	0.52 × 10−4	1.7 × 10−4

Doors add much more leakage. A door of 0.9 m by 2.1 m with an average gap of 3.2 mm already has a leakage area of 0.020 m². The Australian Building Codes Board notes that where smoke doors or dampers are present, leakage can be substantially greater than that of the barrier alone, even when they close properly.

Leakage through a barrier

The flow of air or smoke through a crack or gap follows the orifice equation. The flow rises with the leakage area and with the square root of the pressure difference:

$$\dot{V}=CA\sqrt{\frac{2\mathrm{\Delta }p}{\rho }}$$

Symbols

Symbol	Description	Unit
$\dot{V}$	volume flow through the leakage path	m³/s
$C$	flow coefficient, typically 0.6 to 0.7; 0.65 is commonly used	–
$A$	leakage area (flow area)	m²
$\mathrm{\Delta }p$	pressure difference across the barrier	Pa
$\rho$	density of the gas passing through	kg/m³

The equation applies to cracks and gaps of fixed geometry, for all but extremely narrow cracks. As an example, Klote gives a closed door with a crack area of 0.01 m². At 2.5 Pa it leaks about 0.013 m³/s of air, and at 75 Pa about 0.073 m³/s. For a new smoke-proof wall with service penetrations, the Board takes a leakage area ratio of 1.5 × 10⁻⁴. At 200 °C and 25 Pa this gives roughly 3 m³/h per square metre of wall.

Consider a wall 10 m long and 3 m high, so 30 m². Its leakage area is 30 × 1.5 × 10⁻⁴ = 0.0045 m². Smoke at 200 °C, or 473 K, has a density of about 353/473 ≈ 0.75 kg/m³. With C = 0.65 and a pressure difference of 25 Pa, the flow is about 0.024 m³/s, or 86 m³/h. A single-leaf smoke door meeting AS 6905 may leak up to 40 m³/h under similar conditions. The door alone therefore adds about half as much again as the whole wall.

Performance requirements and testing

Smoke barriers are specified for three temperature conditions: ambient (20 ± 10 °C), medium (200 ± 20 °C) and high, representing a fully developed fire. A wall that only meets the smoke-proof provisions is unlikely to remain an effective barrier at high temperatures. The Australian provisions are therefore read as aiming at ambient and medium conditions, unless a fire resistance level is also required. At high temperatures, leakage can be much higher, even when the fire resistance criteria are met, and the barrier itself may give off smoke.

The Board expects a smoke-proof wall to keep a leakage area ratio of no more than 1.5 × 10⁻⁴. The exposure is a temperature rising linearly to 200 °C over 30 minutes and then held for another 30 minutes. Under the Australian standard AS 6905, a single-leaf smoke door may leak 40 m³/h and a two-leaf door 65 m³/h at medium temperature and 25 Pa. The test follows at least 30 minutes of exposure at 200 °C. Smoke dampers under AS 1682.1 may leak no more than 360 m³/h per square metre at 300 Pa and must close within 30 seconds.

In New Zealand, C/VM2 (second edition, 2025) sets fixed modelling assumptions. Fire and smoke separations other than doors are taken to have zero leakage. A smoke control door is modelled with a 10 mm gap under the door, and a fire door without smoke seals with a 10 mm gap over its full height. Unrated lined walls get a leakage area of 0.1 % of their surface. Smoke separations that comply with the relevant product specification remain in place until their rated temperature or until flashover, whichever comes first. A separation not designed to a recognised standard, such as plain toughened glass, is assumed to disappear once the average upper layer temperature reaches 200 °C.

Barriers and pressurisation

Mechanical smoke control uses the barriers of traditional smoke management in combination with airflows and pressure differences created by fans. Air flowing outward through the small gaps around a closed door then prevents smoke from entering the protected side. Klote lists three advantages over barriers alone. Smoke control depends less on tight barriers. Stack effect, buoyancy and wind are less likely to overcome it. It can also prevent smoke flowing through a door that is open.

The pressure difference must be large enough to overcome the forces of the fire, but not so large that doors become hard to open. In the United States, NFPA 101 limits the force needed to open a door on an escape route to 133 N. For a fire close to a door in the barrier, Klote suggests a minimum pressure difference of 20 to 25 Pa. For smoke at 400 °C from a remote fire, 15 to 20 Pa is suggested, and 5 to 10 Pa where sprinklers cool the smoke. Field tests have shown pressure fluctuations of up to 5 Pa across partitions and closed doors, mostly attributed to wind.

Dampers in ducts link barriers and ventilation systems. In the World Trade Center towers, combination fire and smoke dampers were not required. NIST found that installing them might have slowed the development of hazardous conditions on the upper floors. It would probably not have improved escape, because the stairs were impassable.

Reliability and defects

Openings and defects largely determine how well a barrier performs. Among the factors that reduce reliability, the Board lists unprotected openings in fire and smoke barriers and fire and smoke doors that are wedged open. For doors and dampers, reliability is the probability that they close when needed. For walls with service penetrations it is more complex, because an unsealed gap may or may not matter for the outcome.

The Board therefore uses a distribution of leakage rather than a single reliability value. For a fully visible barrier without penetrations, it suggests a tight, average or loose wall with leakage area ratios of 0.15, 1.0 and 3.5 × 10⁻⁴ and probabilities of 25, 50 and 25 %. Where the barrier is partly hidden above a false ceiling, defects become more likely. Based on inspections of more than 5,000 passive fire protection systems in New Zealand, 17 % of small penetrations such as single cables and 37 % of large penetrations such as cable trays were taken as unprotected. Each unprotected large penetration adds an open area of 100 × 10⁻⁴ m², more than twice the leakage area of the whole 30 m² wall in the example above.

Limitations and misconceptions

A common misconception is that a smoke barrier stops smoke completely. All barriers leak, and the leakage grows with the pressure difference and with temperature. Smoke separations often have no fire rating. C/VM2 therefore treats a separation that does not meet a recognised standard as no barrier at all beyond flashover or 200 °C in the upper layer.

Doors are a particular weak point. They are opened during evacuation and are sometimes left or propped open throughout a fire, and without mechanical smoke control, smoke flow through these doors is common. Purging, the supply and extraction of large amounts of air, cannot keep the fire space breathable or stop smoke movement on its own. It can, however, significantly limit the smoke level in spaces separated from the fire by smoke barriers.

Finally, a barrier designed for one scenario may be bypassed in another. When fire scenarios are defined, whether doors and dampers are open or closed, whether smoke seals are fitted and whether leakage through barriers is controlled are treated as separate outcomes. The performance of a barrier must also be confirmed by smoke modelling for the specific building, because its efficacy depends on the other fire protection systems and the building layout.

See also

• Smoke and heat exhaust ventilation
• Stack effect
• Stair pressurisation
• Atrium smoke control

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Cooper, L. Y. (2002b). Smoke and heat venting. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-219–3-242). National Fire Protection Association; Society of Fire Protection Engineers.
• Evans, D. D., Peacock, R. D., Kuligowski, E. D., Dols, W. S., & Grosshandler, W. L. (2005). Active fire protection systems (NIST NCSTAR 1-4). National Institute of Standards and Technology.
• Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
• Klote, J. H. (2002). Smoke control. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-274–4-291). National Fire Protection Association; Society of Fire Protection Engineers.
• Milke, J. A. (2002b). Smoke management in covered malls and atria. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-292–4-310). National Fire Protection Association; Society of Fire Protection Engineers.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.