Definition of an “A” Class Division

 “A” class divisions are those divisions formed by bulkheads and decks which comply with the following:

.1  they shall be constructed of steel or other equivalent material;

.2  they shall be suitably stiffened;

.3  they shall be constructed as to be capable of preventing the passage of smoke and flame to the end of the one-hour standard fire test;

.4  they shall be insulated with the approved non-combustible materials such that the average temperature of the unexposed side will not rise more than 140oC above the original temperature, nor will the temperature, at any one point, including any joint, rise more than 180oC above the original temperature, within the time listed below:

                          class “A-60”                  60 min

                          class “A-30”                  30 min

                          class “A-15”                  15 min

                          class “A-0”                     0 min


.5  the Administration shall require a test of a prototype bulkhead or deck “in accordance with the Fire test Procedures Code” to ensure that it meets the above requirements for integrity and temperature rise.

Source:  SOLAS – Part A:  General, Regulation 3, Definitions

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Restricted vs. Unrestricted Bulkheads

The question often arises, what is the difference between an unrestricted and restricted bulkhead?

It all depends which direction a bulkhead is most likely to face a fire.  An Unrestricted bulkhead is insulated with materials that allow fire to approach either side of the bulkhead – either the insulated side or the un-insulated side.  Either way, the bulkhead provides the rating of the insulation (i.e. A-60).

In a restricted bulkhead configuration, the insulation must be placed on the side of the bulkhead that is most likely to face a fire source.



A division where an engine room adjoins a sleeping quarters is an excellent example.  Obviously, the potential for a fire comes from the engine room.  If the insulation used on the dividing bulkhead is rated as “unrestricted” application, either side of the bulkhead can be insulated – in the engine room or in the sleeping quarters.  However, if an insulating material is approved only in a “restricted” application, the insulation must be applied to the bulkhead where the likely source of fire might be – in this case, the engine room.


For years, we used 2” of 8 pcf mineral wool to achieve an A-60 rating.  The original fire test consisted of a steel bulkhead, insulated with the mineral wool.  The bulkhead was placed in the furnace, with the insulation facing the furnace.  Thermocouples were attached to the non-exposed side (in this case, bare steel) and the test was run for 60 minutes.  The “backside” temperature was only allowed to rise 140oC.

In the 1990’s, testing showed that if the test was reversed – that is the bare, steel side was placed to the furnace and the insulated side was left un-exposed, it generally took 50% more insulation (3”) to meet the same temperature rise.  Additionally, it required stiffeners to be included in the test.  It was determined that this configuration with the insulation away from the furnace was the most onerous test to pass and it was adopted as the new standard for A-60 insulation when the USCG adopted the IMO SOLAS code.

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Frequently Asked Questions

What about decks?

                In original USCG testing, once an insulation passed the bulkhead test, the material in the same density and thickness was allowed to be used on the underside of decks.  The IMO SOLAS code now requires that a separate test be conducted for insulating decks.

             Testing of decks seems to be a simpler task as the material is usually applied to the underside of a deck (which is towards the furnace), therefore mimicking a “restricted” application.  The main change was the inclusion of insulated stiffeners on the exposed side.  In practice, the original 2” of insulation suffices as we would expect.

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Insulating stiffeners

                Stiffeners must be insulated in the same fashion as were tested; in the case of Rockwool’s mineral wool, vertical boundaries (bulkheads) had 3” of insulation on the flat bulkhead, and 1-1/2” of insulation surrounding the stiffeners.  For decks, Rockwool achieved their A-60 rating with 2” of insulation on the flat deck, with 1” surrounding the stiffeners.

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What about A-30 and A-15 Insulation?

             This is an ever popular question.

             Going back to original USCG guidelines, once a material was approved for an A-60 boundary, NVIC 6-80 allowed a “formula” to determine A-30 and A-15 insulation without additional testing.  A-30 was allowed at ¾ of the original A-60 thickness, and A-15 was allowed at ½ of the original A-60 thickness.  For example, if 2” 8pcf mineral wool passed an A-60, then the USCG would allow 1-1/2” for an A-30 boundary (2” x .75 = 1.5”) and 1” for an A-15 boundary (2” x .50 = 1”).  However, the adoption of the IMO SOLAS code did not recognize this formula and now requires individual tests for an A-30 rating and an A-15 rating.  Since the testing is extremely expensive and must be conducted in bulkhead and deck configurations, most manufacturers have decided not to offer A-30 and A-15 rated products at this time.

             As an interesting side note, in 2003 we worked with the USCG in testing the “rule of thumb” formula of ¾ of an A-60 thickness would yield an A-30 and ½ of an A-60 thickness would yield an A-15.  We tested a multitude of different insulations.  Indeed, the testing proved that the rule of thumb held true.  The USCG presented the information to the IMO, but it was rejected.


Let’s make it even more confusing….

             In some circles, it was believed under the old USCG requirements that one used A-15 insulation for beam wrap for an A-60 bulkhead.  That was an understandable, but incorrect assumption.

             NVIC 6-80 required beams to be wrapped with ½ the thickness of the base A-60 insulation.  In the case of mineral wool, 2” met A-60, so 1” was used for beam wrap.

             Coincidentally, A-15” insulation was defined as ½ the thickness of the base A-60 insulation.

             Therefore, many beamwrap insulation drawings called for beams to be wrapped with A-15, when they actually were being wrapped for an A-60.  At the time, it was a moot point since the insulation was the same type and thickness.  However, in insulating today, some people still specify their beams with A-15 insulation on an A-60 boundary.  A-15 is not used; the correct wrap for beams is what was used and approved during the test.

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Installation of Fire Insulation

             Fire insulation must be installed in the same fashion as it was tested.  Pin and washer type and pin spacing must follow similar patterns (spacing from edges of insulation and spacing between pins) as was tested.  Additionally, some flexible insulation is “overlapped” at the joints.  Always request and follow the installation recommendations from the manufacturer to ensure a solid fire boundary.

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What the “h” is an H-60?

                “H” stands for hydrocarbon.  An H-60 bulkhead or deck is rated for a larger intensity fire, frequently found in the offshore industry.  The fire test is more severe - for an A-60 rating, the furnace temperature gradually ramps up to about 1,750oF over 60 minutes.  For an H-60 rating, the furnace is ramped to 2000oF in 5 minutes and is held there throughout the remainder of the test.


Is there such a thing as an Unrestricted H-60 bulkhead?

             To achieve an A-60 rating, the bulkhead must first be rated to A-0 which by definition is a suitably stiffened, steel (or equivalent) material, capable of preventing the passage of smoke and flame to the end of the one-hour standard fire test.  However, standard steel cannot withstand the 2000oF fire.  Therefore insulation must be applied to protect the steel (insulation towards the furnace).  As such, successful completion of the fire test with the insulation towards the fire would yield a “restricted” bulkhead rating.

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Non-Combustible Materials
Some materials can be installed that don’t have USCG certification.  Fiberglass lagging tape, some untreated fiberglass lagging cloth, and metal jacketing just to name a few.  These materials (and others) are exempt automatically.  See below:      

 Annex 2 - Products Which May Be Installed Without Testing And/Or Approval

 Paragraph 1    Non-Combustible Materials

 "In general, products made only of glass, concrete, ceramic products, natural stone, masonry units, common metals and metal alloys are considered being non-combustible and they may be installed without testing and approval."


 Paragraph 5    Low Flame-Spread Surfaces

    5.1    "Non-combustible materials are considered to comply with the requirements of Part 5 of annex 1.  However, due consideration shall be given to the method of application and fixing (e.g. glue)."

Source: IMO FTP Code, Annex 2

Therefore, fiberglass lagging tape is automatically approved under SOLAS' FTP Code.  However, the lagging adhesive must have appropriate approvals.

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Marine Insulation Specialists