A-60 Marine Insulation: How It Works + Thickness Guide

A-60 is the fire rating that ship designers, builders, and classification societies talk about most. It defines a bulkhead, deck, or enclosure that must stop flames and smoke for 60 minutes while keeping the unexposed side within safe temperature limits. The insulation behind that rating is what makes it work.

This guide explains what A-60 marine insulation is, how it works, and how thickness is selected for different shipboard locations. It is written for naval architects, marine engineers, shipyards, and procurement teams who need to specify fire-rated insulation that passes class approval.

What A-60 Means Under SOLAS

A-60 is a fire-resistance class defined by the International Maritime Organization (IMO) Safety of Life at Sea (SOLAS) Convention and tested under the IMO Fire Test Procedures (FTP) Code.

The letter "A" means the division is made of steel or equivalent material and is capable of preventing the passage of smoke and flame for the rated time. The number is the minutes of fire resistance:

For A-60, A-30, and A-15 divisions, the FTP Code requires that the average temperature rise on the unexposed side does not exceed 140 °C above the initial temperature, and any single point does not exceed 180 °C. That temperature control is the insulation's job.

How A-60 Insulation Works

An A-60 rated assembly works through three mechanisms that work together during a fire.

Integrity — stopping flame and smoke

The division must prevent flames and hot gases from passing through for the full 60 minutes. That depends on:

• Steel plating — usually 4.5 mm or thicker on steel ships, providing the primary barrier.
• Mineral wool or ceramic fibre insulation — fills the cavity and blocks heat transfer.
• Fire-resistant cladding — aluminium or galvanized steel sheets that protect the insulation and hold it in place.
• Seals and penetration systems — certified fire stops at joints, cable transits, pipe penetrations, and damper openings.

Without proper sealing at penetrations, even the best insulation layer will fail the integrity criterion.

Insulation — limiting temperature rise

The insulation layer slows heat transfer from the fire-exposed steel to the unexposed side. Mineral wool is the most common core material because it combines low thermal conductivity with a high melting point. Typical marine-grade rock wool has:

• Thermal conductivity around 0.035–0.040 W/m·K at room temperature
• Service temperature up to approximately 650–750 °C
• A1 non-combustible classification under EN 13501-1
• Melting point above 1000 °C

In a hydrocarbon fire or very high-temperature zone, ceramic fibre or a composite system may be used instead of, or in addition to, rock wool.

Structural stability — staying in place

The assembly must not collapse or displace during the fire. Stability comes from:

• Mechanical fixing — stainless-steel pins, washers, and support rings spaced according to the approved drawing.
• Steel framing — stiffeners and frames that resist deformation under thermal load.
• Density and compression — higher-density boards or blankets are used where vibration, impact, or airflow could displace the insulation.

Typical A-60 Construction

A standard A-60 bulkhead or deck on a merchant vessel usually consists of:

1. Fire-exposed steel plate — 4.5 mm minimum on steel ships.
2. Insulation layer — rock wool board or blanket, density 80–150 kg/m³, held by pins and clips.
3. Vapour-barrier or facing — aluminium foil or glass tissue where moisture control is needed.
4. Outer cladding — 0.5–1.0 mm aluminium or galvanized steel sheet, mechanically fixed.

The total thickness of the insulation depends on the fire rating, the location, the density of the wool, and the tested system. Shipyards do not design A-60 thickness from first principles for each project — they use systems that have already been type-approved by a classification society.

A-60 Thickness Guide by Ship Location

The table below shows typical thickness ranges used in approved marine insulation systems. These are indicative; the actual thickness must come from an approved system drawing for the specific vessel and class society.

Decks

A-60 decks must carry personnel and sometimes equipment, so the insulation is usually a rigid rock wool board rather than a flexible blanket. Density is higher to resist compression, and the top surface is protected by steel or aluminium cladding.

Bulkheads

A-60 bulkheads separating accommodation from machinery spaces are often built with a single layer of rock wool board fixed to steel stiffeners. At joints between boards, a 50 mm minimum overlap is standard practice. Staggered double-layer systems improve joint performance and are common in high-risk boundaries.

Engine rooms

Engine rooms have the highest heat load and vibration. The boundary insulation is often 60–120 mm thick, sometimes in two layers, and may combine rock wool with ceramic fibre at exhaust casings or turbocharger areas. Steel support framing is heavier, and pins are spaced closer together than in accommodation areas.

Fuel tanks and hazardous boundaries

Where an A-60 division forms the boundary of a fuel tank or other high-risk space, the insulation thickness is usually at the upper end of the range. The density is also higher to improve structural stability under hydrocarbon fire conditions.

Ventilation ducts

When a ventilation duct passes through an A-60 division, the duct itself must be insulated for a length of at least 450 mm on each side of the division. A certified fire damper must also be fitted. This prevents the duct from becoming a chimney that carries fire and smoke between spaces.

Pipe and cable penetrations

Cable and pipe penetrations are among the most common failure points in fire tests. Each penetration must use a certified transit or sealing system:

• Cable transits — intumescent or ablative seals that expand under heat.
• Pipe penetrations — steel sleeves, mineral wool packing, and approved sealant.
• Large ducts — fire-rated collars, dampers, or insulated bulkhead penetrations.

Certification: IMO FTP Code and Class Societies

A-60 insulation cannot simply be bought "off the shelf" and installed. It must be part of a system that has been tested and approved.

IMO FTP Code

The IMO FTP Code, adopted by resolution MSC.307(88), defines how fire-resistance tests are carried out. The main parts relevant to A-60 insulation are:

• Part 1 — Non-combustibility test for insulation materials.
• Part 3 — Fire-resistance tests for A, B, and F class divisions, including the 60-minute standard fire test.
• Part 5 — Surface flammability and smoke-toxicity tests for interior finishes.

A standard A-60 test exposes the sample to the ISO 834 cellulosic time-temperature curve, which reaches approximately 945 °C at 60 minutes. The unexposed side is monitored for temperature rise, and the sample is checked for integrity.

Classification society type approval

After IMO FTP Code testing, marine insulation systems are submitted to class societies for type approval. Common approvals include:

• DNV — DNV-ST-A-053 and related marine rules
• Lloyd's Register (LR) — Marine & Offshore type approval
• Bureau Veritas (BV) — NR216 and related rules
• ABS — Rules for Materials and Welding
• CCS — China Classification Society guidelines

The type-approval certificate specifies the exact product, thickness, density, fixing method, and cladding. Any deviation from the approved drawing invalidates the rating.

A-60 vs H-120: Different Fire Curves

A-60 and H-120 are both fire-resistance ratings, but they are not interchangeable because they use different fire curves.

The hydrocarbon curve heats up much faster than the cellulosic curve. Materials that pass A-60 may not pass H-120 without additional thickness, ceramic-fibre layers, or intumescent coatings. Offshore projects usually specify H-120 or H-60 for hydrocarbon-exposed areas, while merchant ships use A-60.

For more detail on marine fire standards, see our guide to marine fire insulation materials and standards.

Material Selection: Rock Wool, Ceramic Fibre, or Composite?

For most A-60 applications, marine-grade rock wool is the default. It is cost-effective, widely approved, and easy to install.

Ceramic fibre is used where:

• Continuous surface temperatures exceed 650–750 °C
• Hydrocarbon fire ratings such as H-120 are required
• The insulation must be thin but high-performing

Composite systems use layers of rock wool and ceramic fibre to balance cost and performance.

Rosewool supplies marine-grade rock wool blanket and rock wool board for A-60 divisions, as well as ceramic-fibre products for high-temperature and offshore applications. See our marine & offshore application page for an overview of fire-rated insulation solutions for ships and platforms. All materials are supported by ISO 9001 quality management and tested to marine-market standards.

Common Specification Mistakes

• ** Designing from first principles instead of using an approved system.** A-60 thickness must match a tested and class-approved system, not a general heat-loss calculation.
• Ignoring penetrations. Even perfect bulkhead insulation fails if cable or pipe transits are not certified.
• Mixing A-60 and H-120 requirements. Offshore hydrocarbon zones need the hydrocarbon curve, not the cellulosic A-60 curve.
• Using non-marine-grade products. Products sold for building HVAC often lack the density, facing, and certification needed for shipboard fire divisions.
• Omitting the vapour barrier or cladding. The insulation must be protected from mechanical damage, oil, and moisture for the life of the vessel.

Bottom Line

A-60 marine insulation is a system, not a single product. The rating depends on the steel structure, the mineral-wool or ceramic-fibre layer, the cladding, and the sealing of every joint and penetration. Thickness varies by location: around 40–60 mm for accommodation bulkheads and decks, 60–120 mm for engine-room boundaries, and more for high-risk fuel-tank divisions.

Always specify from approved system drawings and class-society type-approval certificates. At Rosewool we supply A-60-grade rock wool board and blanket for marine and offshore projects, with technical support for density, thickness, and certification matching. Contact our team for project-specific recommendations.