Fire-Rated Calcium Silicate Ductwork for EI 120 Applications: A Technical Guide for European Projects

When a ventilation duct passes through a fire compartment boundary in a German hospital, a French data center, or a Dutch shopping mall, it stops being a simple piece of sheet metal. It becomes part of the building's passive fire protection strategy — and under European regulation, it must be proven to behave like one for 120 minutes of fully developed fire exposure.

That single requirement, abbreviated as EI 120, drives the entire specification conversation for fire-rated ductwork in European construction. This guide is written for mechanical consultants, fire engineers, and procurement specialists who need to understand exactly what calcium silicate ductwork can deliver, how it is tested, which system configuration fits a given project, and what to demand from a manufacturer before a single board is shipped.

What EI 120 Actually Means

The classification "EI 120" is not a marketing number. It comes from EN 13501-3, the European standard that converts fire test data into a product rating.

E (Étanchéité / Integrity) — the duct must not develop cracks, openings, or sustained flaming on the unexposed face.

I (Isolation / Insulation) — the average temperature on the outer surface of the duct must not rise more than 140 °C above ambient, and no single point above 180 °C.

120 — both criteria must hold for 120 minutes under the ISO 834 standard fire curve defined in EN 1363-1.

A duct that only meets integrity (no insulation requirement) is classified as E 120 and is dramatically cheaper — but it will transfer enough heat through its walls to ignite combustibles in adjacent compartments. For buildings where life safety, evacuation, and fire service operations depend on compartmentation, EI — not E alone — is the correct specification.

The Classification Suffixes German Engineers Actually Check

Most B2B catalogues stop at "EI 120" and leave the rest ambiguous. German inspectors and TÜV-certified consultants do not. A compliant classification string reads something like:

EI 120 (ho i↔o) S 500 multi

Each element matters:

CodeMeaningWhy it matters

ho / ve

horizontal / vertical orientation tested

Vertical shafts and horizontal runs behave very differently in fire

i → o

fire from inside only

Kitchen extract, where fire starts inside the duct

o → i

fire from outside only

Typical ventilation supply running through another compartment

i ↔ o

fire from both sides

The demanding case — required for most multi-compartment applications

S

smoke leakage ≤ 5 m³/(h·m²) at 300 Pa

Mandatory for smoke control ducts

500 / 1000 / 1500

pressure differential in Pa

Must exceed the fan's design negative pressure

single / multi

fire compartments the duct may pass through

Drives whether EN 1366-8 or EN 1366-9 applies

For most German projects we quote — particularly mixed-use buildings, hotels, and industrial facilities — the target classification is EI 120 (ho i↔o) S 500 multi, tested to EN 1366-1 (ventilation) or EN 1366-8 (multi-compartment smoke extraction). Anything less should be questioned during technical review.

Why Calcium Silicate Is the Dominant Material for EI 120 Ducts

Three material properties drive the choice:

1. Non-combustibility (Euroclass A1). Calcium silicate board, when formulated from hard calcium silicate (xonotlite/tobermorite phase) rather than fiber-cement fillers, tests as A1 non-combustible under EN 13501-1. This is a prerequisite for use in EN 1366-8 smoke extraction systems, which explicitly restrict materials to A1 and A2-s1, d0 classes.

2. Dimensional stability at elevated temperature. Unlike gypsum-based boards, calcium silicate does not release crystalline water in a way that causes structural collapse. The board retains its geometry through the full 120-minute ISO 834 curve, where furnace temperatures exceed 1,050 °C. This is what allows the duct to maintain its cross-section and prevent the integrity failure triggered by a 10 % dimensional reduction under test.

3. Low thermal conductivity. With λ-values typically between 0.17 W/(m·K) and 0.25 W/(m·K) at room temperature, calcium silicate provides the thermal barrier that keeps the unexposed face below the 140 K insulation limit without requiring a separate mineral wool layer.

Sheet steel, by contrast, reaches structural failure within 10–15 minutes of EI 120 exposure without cladding. This is why every proven European EI 120 system — Promat PROMADUCT, etrex, Thor Duct, and others — is ultimately built around a calcium silicate component.

Two System Configurations: Which One Fits Your Project?

When we quote German clients, we present two fundamentally different system architectures. The choice depends on installation conditions, budget, and structural load limits at the supporting slab.

Option A — Self-Supporting Structural Calcium Silicate Duct

A duct fabricated entirely from high-density calcium silicate boards (900–1,000 kg/m³), typically 30–50 mm thick, mechanically connected with stainless steel screws and fire-rated sealant at all joints. No internal steel liner.

Technical profile:

Density: 900–1,000 kg/m³

Compressive strength: ≥ 12 MPa

Board thickness for EI 120: typically 40–50 mm (system-dependent)

Weight: approximately 55–70 kg/m² of duct surface

Maximum span between supports: typically 1.2–1.5 m

Best suited for:

Kitchen extract applications where grease fire (i → o) is the dominant threat

Projects where internal cleanability and a smooth calcium silicate inner face are preferred

Retrofits where access is restricted and prefabricated steel ducts cannot be brought in

Trade-offs: Higher dead load on structural supports. Skilled fabrication labour required on site or at a prefab shop.

Option B — Galvanised Steel Duct with Calcium Silicate Cladding

A conventional galvanised steel duct (built to DIN EN 1507 for rectangular ducts or DIN EN 12237 for circular) clad externally with lower-density calcium silicate boards (600–800 kg/m³), typically 25–40 mm thick, mechanically fixed to the steel substrate.

Technical profile:

Cladding density: 600–800 kg/m³

Cladding thickness for EI 120: typically 30–40 mm

Total weight: approximately 30–45 kg/m² of duct surface

Steel duct fabricated to standard HVAC specifications

Cladding installed after duct installation and pressure testing

Best suited for:

Standard office, hotel, hospital, and commercial ventilation

Projects where HVAC contractors already fabricate sheet metal ductwork and want a separate fire-protection trade

Applications where lower dead load is critical

Trade-offs: Two trades on site (ductwork + fire protection). Interface quality between steel duct and cladding must be controlled, especially at supports and penetrations.

For the German market, we recommend Option B for the majority of projects. It matches the established German workflow where HVAC fabricators (SHK / TGA contractors) produce the steel ducts per DIN EN 1507, and a passive fire protection contractor installs the calcium silicate cladding in a clearly defined scope. It also keeps weight manageable for long horizontal runs and simplifies interaction with fire dampers certified to EN 15650.

The Procurement Checklist: What to Demand Before Ordering

A fire-rated duct is only as compliant as its documentation. When we ship to European projects, the following technical file travels with every order. If a supplier cannot produce these, treat that as a red flag.

Classification report to EN 13501-3 (for ventilation ducts) or EN 13501-4 (for smoke extraction ducts), issued by an EU-notified laboratory. Reports from non-EU laboratories do not satisfy CE/UKCA marking requirements.

Test reports to EN 1366-1 / EN 1366-8 / EN 1366-9 as applicable, showing the exact tested construction — board density, thickness, fixing spacing, joint treatment, support details.

Declaration of Performance (DoP) under Regulation (EU) No 305/2011 (CPR), identifying the notified body.

CE marking on the boards themselves, or UKCA for Great Britain projects.

For German projects specifically: confirmation of DIBt (Deutsches Institut für Bautechnik) acceptance, either through a national technical approval (abZ/aBG) or through direct reference in the Bauregelliste.

Material safety data sheet confirming zero asbestos content and, for European hospital/school projects, formaldehyde-free formulation.

EXAP (extended application) report if the project's duct geometry falls outside the directly tested range. EN 15882-1 defines the rules for extending ventilation duct test results; note that no EXAP standard exists for smoke extraction ducts — if the duct is outside the tested scope, it must be retested or separately assessed.

Common Specification Mistakes We See in European Tenders

After quoting dozens of fire-rated duct projects across Germany, the Netherlands, and Austria, three recurring errors stand out:

Specifying "fire-rated" without the suffix. A specification that reads "fire-rated ductwork, EI 120" with no mention of orientation, fire side, or pressure leaves the manufacturer free to supply the cheapest classification. Always specify the full string (e.g. EI 120 (ho i↔o) S 500 multi).

Mixing test standards. A duct tested to British BS 476 Part 24 is not directly equivalent to EN 1366-1. German inspectors will reject BS 476 reports unless a separate EN 13501-3 classification has been issued. For European projects, specify EN test standards only.

Ignoring support and penetration details. The fire rating applies to the tested assembly, including its supports and its penetrations through walls and slabs. Specifying a certified duct and then fixing it with non-certified threaded rod to a non-certified slab assembly invalidates the classification. Penetration sealing must be tested to EN 1366-3.

How HR Rhino Delivers EI 120 Ductwork to European Projects

Qingdao Henry Calcium Silicate Board manufactures both the high-density structural boards used in Option A and the medium-density cladding boards used in Option B, produced under ISO 9001 quality control with asbestos-free, formaldehyde-free formulations. For German-market projects, we supply:

Boards with Euroclass A1 classification to EN 13501-1

Density grades of 450, 600, 800, 900, and 1,000 kg/m³

Thicknesses from 6 mm to 50 mm

Project-specific cutting and edge treatment for prefabrication

Technical documentation package including test reports, DoP, and installation details

Our engineering team works directly with consulting engineers to select the correct density, thickness, and system configuration for the project's classification target, produce shop drawings, and quote FOB Qingdao or DAP to any EU port.

If you are specifying or procuring fire-rated ductwork for a European project and want a technical proposal tailored to your EI rating, duct geometry, and local certification requirements, contact our technical sales team at sales@hrcalciumsilicateboard.com or WhatsApp +86 156 8913 8837. We typically return a full proposal with system drawings, weight calculations, board schedule, and certification summary within three working days.

About the author: This article was prepared by the technical sales team at Qingdao Henry Calcium Silicate Board New Materials Co., Ltd. (HR Rhino), a manufacturer of calcium silicate boards for passive fire protection applications exporting to European and international markets since 2012.

Further Reading

Understanding EN 1366-1 and EN 13501-3 classifications

Calcium silicate board density guide: 450 vs 600 vs 900 vs 1000 kg/m³

Fire-rated ductwork vs fire damper strategy: which approach fits your project?

Kitchen extract fire protection: calcium silicate duct requirements under EN 1366-1