A1 non-combustible ductwork and EN 1366 fire-resisting ductwork solve different fire-safety problems. One classification describes how a material reacts to fire; the other demonstrates how a complete ductwork system performs during a fire test. They should not be treated as interchangeable.
The essential distinction
| Requirement | What it describes | What it does not prove |
|---|---|---|
| A1 or A2-s1,d0 | Reaction to fire: the extent to which a material contributes to fire, smoke and flaming droplets. | It does not establish a period of duct integrity, insulation or smoke leakage performance. |
| E, EI or EIS with a time period | Fire resistance: performance of a complete tested or assessed construction for a stated duration. | It should not be inferred merely because the duct is made from steel or another non-combustible material. |
Sheet steel is non-combustible, but an ordinary steel ventilation duct can distort, separate at joints, lose its supports or allow hot gases and smoke to pass during a fire. Fire-resisting performance depends on the complete arrangement, including the duct construction, flanges, stiffeners, sealants, fixings, supports, penetrations, fire-stopping and any insulation or protection system.
Where does the 11 metre rule come from?
For England, Approved Document B gives height-related guidance for the reaction-to-fire performance of external walls. For residential purpose groups with a storey more than 11 metres above ground, the external surface guidance is class A2-s1,d0 or better. It also gives A2-s1,d0 or better guidance for certain insulation and filler materials used in the external wall of residential buildings with a storey 11 metres or more above ground.
This is probably the origin of references to “12 metre” or “A1 fire-rated” apartment ductwork. The relevant figure in the current England guidance is generally 11 metres, measured to the floor level of the top storey in accordance with Approved Document B—not simply to the roof or parapet.
For a relevant building with a storey at least 18 metres above ground, Regulation 7(2) applies more widely to materials becoming part of the external wall or a specified attachment. Subject to the stated exemptions, those materials must achieve A1 or A2-s1,d0.
Important: this does not create a blanket rule that every ventilation duct throughout an apartment building over 11 metres must be A1, nor that every such duct must be fire resisting. The exact requirement depends on whether the product forms part of the external-wall construction, the building use and height, the proposed wall build-up, the project fire strategy and the Building Regulations route being followed.
Façade ventilation ductwork
Apartment ventilation and MVHR systems often use plastic ductwork internally. Where a duct, adaptor, insulation product or terminal enters the external-wall build-up, the design team should establish the reaction-to-fire performance required for every relevant component in that zone.
- Confirm whether the duct, transition, insulation and terminal become part of the external wall.
- Check the building use, relevant storey height and applicable national guidance.
- Review the complete wall system, including cavity barriers and the position of compartment floors.
- Obtain product classification evidence rather than relying on the description “fire rated”.
- Coordinate condensation control, thermal bridging, weathering and airflow as well as fire performance.
An A1 or A2-s1,d0 external grille or airbrick is not automatically a fire damper. Unless supported by separate fire-resistance evidence for the intended installation, it should not be assumed to maintain the fire resistance of a compartment wall or floor.
When is EN 1366 fire-resisting ductwork needed?
Fire-resisting ductwork is normally considered because of the duct function and route, not simply because of building height. It may be required where the duct must continue operating during a fire, or where its route would otherwise compromise fire separation.
- A ventilation duct crosses a compartment wall, compartment floor, protected shaft or protected escape route and the selected method of maintaining compartmentation requires fire-resisting ductwork.
- A smoke extract duct serves more than one compartment or passes through another fire compartment, requiring the BS EN 1366-8 route where applicable.
- A single-compartment smoke extract system requires evidence to BS EN 1366-9 and the associated smoke-control classification route.
- A stair, lobby or firefighting-shaft pressurisation system must continue delivering air for the required fire period.
- Kitchen extract ductwork crosses fire compartments and ordinary damper arrangements are inappropriate because grease accumulation can affect their operation.
- The fire strategy or project specification requires a stated E, EI or EIS classification and duration.
Approved Document B identifies several possible methods for maintaining the performance of a fire-separating element where air-handling ducts pass through it: fire dampers, a fire-resisting enclosure, fire-resisting ductwork, or suitably activated fire-and-smoke dampers. A project may use a combination. Selection must reflect the duct function, protected route, maintenance requirements and fire strategy.
Which evidence should be requested?
The phrase “A1 fire-rated duct” is ambiguous. Before pricing or manufacture, the specification should identify whether the project needs non-combustible materials, a fire-resisting system, or both.
- For reaction to fire: classification evidence to BS EN 13501-1 for the relevant product or material.
- For fire-resisting ventilation ductwork: applicable BS EN 1366-1 test and classification evidence, normally expressed using E, I and S criteria with a duration and installation orientation.
- For smoke extract ductwork: the appropriate BS EN 1366-8 or BS EN 1366-9 evidence and classification route, together with the product-standard requirements where applicable.
- For penetrations: tested or assessed fire-stopping and installation details compatible with the supporting construction and duct system.
- For the installed arrangement: confirmation that sizes, orientation, pressure, supports, joints, access doors and protection remain within the applicable field of application.
Questions to resolve before specifying
- Is the requirement concerned with the external wall, compartmentation, continued system operation during fire, or more than one of these?
- Has the consultant requested A1/A2 reaction-to-fire performance, EN 1366 fire resistance, or both?
- What classification and duration are required: E, EI, ES or EIS?
- Does the duct serve one compartment, multiple compartments or a protected escape/firefighting system?
- Do the duct size, orientation, pressure and support arrangement fall within the chosen system evidence?
- Have transitions, insulation, terminals, flexible connections, access doors and fire-stopping been included in the review?
Common specification mistakes
- Using “A1 fire rated” without saying whether this means reaction to fire or fire resistance.
- Assuming that steel ductwork automatically provides 30, 60 or 120 minutes of fire resistance.
- Specifying an A1 grille while overlooking combustible duct, insulation or adaptors within the external-wall zone.
- Assuming a non-combustible airbrick performs as a fire damper.
- Applying the 11 metre external-wall guidance as a universal internal ductwork rule.
- Failing to state the required fire exposure direction, orientation, leakage criterion or duration.
Practical rule
A1 describes how a material reacts to fire. EN 1366 evidence demonstrates how a complete ductwork system resists fire. A project may require either one—or both—but one should never be assumed to prove the other.
Source basis: Approved Document B Volume 1 for England, including Sections 9 and 10; BS EN 13501-1; BS EN 13501-3; BS EN 13501-4; BS EN 1366-1; BS EN 1366-8; and BS EN 1366-9. Requirements differ across the UK and project-specific fire strategies may set additional provisions. This is an original practical summary and does not reproduce or replace the standards, statutory guidance, classification reports or professional fire-engineering advice.
Guidance review: September 2026. This is an original practical summary and does not reproduce or replace the relevant standards or project evidence. How this guidance is reviewed.