Why ductwork needs fire protection
Ventilation systems link rooms, storeys and fire compartments together. In a fire, an ordinary steel duct quickly heats up and loses its airtightness and shape, and fire and smoke travel along it to places they should never reach. That is why the codes require transit ducts passing through other fire compartments, as well as smoke control ventilation ducts, to have a specified fire resistance rating.
The fire resistance rating of a duct is expressed as an EI class with the time in minutes: E — integrity, I — insulation. The specific value (for example, EI 30, EI 60, EI 120 or higher) is set by the design on the basis of the codes in force for the type of system and building concerned.
Protection methods
Stone wool mats
Wired mats stitched to galvanised mesh, often foil-faced, are wrapped around the duct and secured with wire, pins or banding. They are well suited to circular ducts and also provide thermal and acoustic insulation.
Mineral wool boards
Rigid boards faced with reinforced foil, for rectangular ducts. They are bonded with a fire-rated adhesive and/or fixed with welded pins; the joints are glued and covered with foil tape.
Thin wrap materials
Inorganic fibre mats encapsulated in foil. Being thin, they are convenient in confined spaces and are widely used on commercial kitchen extract ducts, where grease deposits place special demands on fire protection.
Fire-resisting enclosures and ducts made from boards
Low-density calcium silicate boards are used to build self-supporting ducts without a steel casing, or to encase a steel duct. This solution is chosen for high fire resistance ratings, for smoke extraction systems, and in shafts and tunnels.
Fire protection coatings
Coatings for ductwork also exist, but what matters here is an approved solution specifically for ducts of the relevant type, cross-section and sheet metal thickness. A paint tested only on load-bearing structural steel cannot be used on ducts.
How to choose a solution
- System type. General ventilation, smoke control ventilation, kitchen extract — the requirements differ.
- Required EI class. This comes from the design; the number of layers and the material thickness come from the manufacturer’s approved solution.
- Cross-section shape and size. Circular and rectangular ducts are protected with different materials; for large cross-sections the range of solutions is limited.
- Duct wall thickness and construction. Many solutions require a certain minimum sheet thickness and type of joint.
- Hangers and supports. These heat up in a fire too. Approved solutions specify how to protect the hangers and at what spacing to install them.
- Penetrations through walls and floors. Where a duct passes through a fire-separating element, the penetration is sealed to a separate detail — see “Fire stopping of penetrations”.
- Space. In tight shafts and ceiling voids, thin wrap materials have the advantage.
Common mistakes
- Ordinary thermal insulation instead of a fire protection system — “the wool is there, the fire resistance isn’t”.
- Fire protection stopping short of the fire-separating element or not continuing into the penetration.
- Unprotected hangers and trapeze bars.
- Loose joints, missing overlaps and no foil tape over the seams.
- Boards or mats from different manufacturers in one system, adhesive “from the shop next door”.
- Painting a duct with a fire protection paint tested only on load-bearing structures.
Materials from leading manufacturers
- ROCKWOOL Conlit Fire Mat — ROCKWOOL — wired stone wool mat on mesh for circular ducts.
- ROCKWOOL Conlit Ductrock — ROCKWOOL — stone wool board with reinforced foil facing for rectangular ducts.
- 3M Fire Barrier Duct Wrap 615+ — 3M — thin inorganic fibre mat for ventilation and kitchen extract ducts.
- PROMATECT-L500 — Promat — calcium silicate boards for self-supporting fire-resisting ducts and for encasing steel ducts.
- PROMATECT-H — Promat — high-density boards for shafts and fire-resisting partitions through which ducts pass.
- Hilti CFS-S ACR — Hilti — acrylic fire protection sealant for sealing gaps in penetrations.
- Nullifire FS702 Intumastic — Nullifire (Tremco CPG) — acrylic sealant for joints and penetration seals.
- Promat DURASTEEL — Promat — steel-faced fibre cement board for fire and blast resistant ductwork.
- Protecta FR Damper — Protecta (Polyseam) — non-mechanical graphite fire damper for circular ducts Ø63–1250 mm.
Frequently asked questions
Can fire protection and thermal insulation of a duct be combined?
Yes — mineral wool fire protection mats and boards insulate the duct at the same time. But the reverse is not true: ordinary thermal insulation does not provide fire resistance.
What is different about fire protection of a kitchen extract duct?
Commercial kitchen extract ducts can accumulate grease deposits, which may themselves catch fire inside the duct. Such ducts call for special systems tested by dedicated methods.
Do duct hangers need protecting?
As a rule, yes — or the hangers are designed with a safety margin. Hanger requirements form part of the manufacturer’s approved solution.
Which is better for smoke extraction — a mat or a board?
It depends on the cross-section shape, the required class and the location. Boards are more often used for rectangular ducts, mats for circular ones, and calcium silicate enclosures for high ratings and shafts.