Fire Ratings for Construction Materials

Introduction

In the Middle East and South Asia there is a lot of confusion over Fire Safety of Construction Materials to an extent that could endanger lives. This article has been written to explain the differences in classification and examines the new legislation released by Civil Defence in Dubai. Descriptions commonly used by building material manufacturers to describe their products include terms like noncombustible, non-flammable, ignition resistant, Class A rated, fire-resistant and fire safe. Whilst Fire Safe is not a defined standard and is purely marketing nomenclature,, all the other terms relate to the description of the relative combustibility of materials. Firstly, do not lump noncombustible, non-flammable, ignition resistant, and fire-resistant into the same “good” category; there is a big difference? The only materials that will not burn in a fire or contribute to a fire are non-combustible materials. In insulation, this means using glass or rock mineral wool. All other materials will burn and, in high rise buildings, where the control of fire spread is critical, best practice is to use nothing but non-combustible materials to prevent the loss of lives in the event of a fire. There is a particular risk if the fire can spread over the façade of the building or along the building services and pipes that connect various fire compartments in a building. If combustible material is used in these applications they represent a considerable source of danger in a fire due to flame and/ or smoke spread.

Having said this, not all combustible materials should be lumped into a “bad” category, and there a way to evaluate differences in anticipated performance between two combustible materials? The objective of this article is to describe how building codes and associated standards have defined and used these terms, and to provide ways to evaluate differences between combustible materials.

Definitions

Building codes and test standards have provided definitions for some terms commonly used to describe how a given material or assembly will perform in a fire. Terms that have been defined include:

  • Combustible
  • Noncombustible
  • Fire-resistant or Fire-resistance
  • Ignition-resistant

Combustible and noncombustible refer to the performance of a material (e.g., wood, stucco, steel). Fire-resistant can refer to a material or an assembly (e.g., all the component s in a wall – siding, insulation, and sheathing products). Ignition resistant can refer to a material or an assembly (e.g., when discussing ignition resistant construction). Definitions of these terms have been developed by a number of groups.

How the Terms are Used?

Combustible

Combustible materials are those that readily ignite and burn. Many common construction materials are combustible, including wood and wood-plastic composite and petrochemical based plastic and rubber products. In insulation, all EPS, XPS, PIR, PUR XLPE (Cross-linked polyethylene), NBR (Nitrile rubber insulation) are classified as combustible.

A number of tests have been developed that evaluate the fire performance of combustible materials. Two key properties are helpful in characterizing the relative combustibility of different materials – flame spread index and heat release rate.

The flame spread rating of a material is determined by subjecting material placed in a horizontal tunnel to a gas flame. A combustible material will be rated as Class A, Class B, or Class C based on its performance in this test. A material rated as Class A would have a lower flame spread, and therefore a better performance rating, than a Class C material. The results of the flame spread test results in a numeric rating. If the numeric value is less than 25, then a Class A flame spread index is assigned. Numeric values for Class B are between 25 and 75. Values above 75 fall into the Class C category. Another method used for comparing the combustibility of materials is to evaluate the heat release rate. This can be done by measuring the mass (weight) loss of a burning material or by measuring the total and/or rate of energy released while a material is burning. The heat release rates have been published for common construction materials. The heat release rate of a material is determined by collecting the gases of combustion (oxygen, carbon dioxide and carbon monoxide) in an oxygen depletion calorimeter. The heat of combustion per unit mass of oxygen consumed is nearly constant for a wide range of materials (Quintiere 1998) and therefore the heat release rate of a material (HHR) is directly proportional to the rate at which oxygen is consumed during combustion. To measure the HRR of assemblies and larger-component sections, they are burned under a large hood that is tied into an air collection system. The heat release rate of small samples can be measured in a smaller calorimeter called a Cone Calorimeter. Smaller values of heat release rate reflect lower combustibility than larger values. The flame spread rating and heat release rate of materials have been used to characterize combustible materials. This information is becoming available for materials commonly used on the outside of buildings, and is being used to compare performance of combustible construction materials.

Noncombustible

In Europe and the UK, when it comes to providing a definition for non-combustibility we need look no further than the Building Regulations which defines it as: "Any product classified as non-combustible when tested in accordance with the requirements of BS476: Part 4: 1970 - Fire tests on building materials and structures - Non-combustibility test for materials, or;

Any product classified as Class A11 in accordance with BS EN 13501-1:2002 - Fire classification of construction products and building elements. Classification using data from reaction to fire tests when tested to BS EN ISO 1182:2002 Reaction to fire tests for building products - Non-combustibility test and BS EN ISO 1716:2002 Reaction to fire tests for building products - Determination of the gross calorific value."

A product qualifies for a classification of non-combustible as defined by the building regulations if it meets the requirements of either of the above clauses.

At Knauf, you can be assured that when `Euroclass A1` or `non-combustible` is assigned to one of our glass or rock mineral wool products, the products have been tested and classified by the relevant method, from the relevant standard(s) referred to above.

In the USA, a noncombustible material is one that is not capable of undergoing combustion under specified conditions (ASTM E 176). Non-combustibility can be assessed by a standard test method, ASTM E-136, Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 degree C. The test described in ASTM E-136 uses a furnace. The test begins with four samples of a given material. To be considered noncombustible, three of the four replicate test specimens must meet one of the following two sets of criteria:

When the weight loss of the specimen during the test is 50% or less, then

  1. The recorded temperature of the material is not greater than 30°C (54°F) above a temperature measured in the test apparatus.
  2. There is no flaming from the specimen after the first 30 seconds of the test.

Fire ratings of construction materials

Figure 4. A diagram of a furnace used to evaluate whether or not a material can be considered “noncombustible”. Figure based on Figure 1, ASTM Standard E 136.

When the weight loss of the specimen during the test exceeds 50%, then

  1. The recorded temperature of the material is not greater than the temperature measured at a specific location in the test apparatus.
  2. There is no flaming from the specimen at any time during the test. Criteria No. 2 is provided for materials that contain large quantities of combined water or other gaseous components, a condition that would not apply to current exterior-use construction materials. Criteria No. 1 is the most useful for characterizing construction materials. Note that a material complying by this criteria can be deemed noncombustible even though some limited flaming may occur. The conditions given in criteria No. 1 were based on research conducted by Setchkin (1952). If coatings or facings are applied on the surfaces, for the product to be classified non-combustible, the entire product must conform with the strict requirements of the A2-s1,d0 reaction to fire class.The A2 rating confirms that the tested product does not show any sustained flaming for more than 20 seconds in the non-combustibility test. The s1,d0 subclasses mean that such products do not produce burning droplets and significant smoke emissions. The A2-s1,d0 is principally different from B-s1,d0 class, because A2-s1,d0 product cannot produce significant amounts of heat or smoke when subjected to a full-size fire exposure; combustible products rated “B” to EN 13501-1 can be capable of producing significant amounts of heat and smoke when subjected to a higher level of flame or heat than the 31 kW small fire used in the SBI test (Single Burning Item).

For this reason, and unique to Knauf, the Façade product range uses non-combustible facings which do not contain Kraft paper such as the standard FSK facing used by our competitors.

Ignition-resistant

In most parts of North America, “ignition-resistant” is not defined so it can mean different things to different people. The International Code Council’s International Wildland-Urban Interface Code and the California Building Code have defined ignition-resistant materials as those meeting a minimum flame spread rating after being subjected to a specified wetting-drying weathering cycle. The duration of “ignition resistant” test is 30 minutes compared to the 10 minute duration used to evaluate flame spread. In California, a material labeled “ignition resistant” has passed the 30 minute test. An example of an ignition-resistant material is lumber that has been pressure impregnated with a fire retardant that has been rated for use on the exterior of a building.

Combustible insulation products that qualify as an ignition resistant material have been treated with a fire retardant.

Fire Resistant

Fire resistance ratings and tests provide guidance on fire safety issues. They are designed to evaluate the capability of a material or assembly to contain a fire within a compartment or building, or continue to provide a structural function in the event of an (internal) fire (Beitel 1995). For example, fire resistance ratings will help determine if a given building construction will allow enough time for people to exit a burning building before it collapses (Kruppa 1997). A common fire test to evaluate fire resistance of walls uses a large vertical furnace to expose a wall to radiant heat from gas burners. NFPA285 is the test commonly used in Dubai to evaluate the fire resistance of façade systems. The duration of the test ranges from 20 minutes to several hours (30 minutes in the case of Dubai), depending on the desired rating and the product or assembly being tested. Temperatures inside the furnace reach about 1700°F (~925°C).

The tests used to determine the fire ratings for roofs also provide fire resistance information. In this case, the Class A (highest degree of fire resistance), B, or C rating provides relative information about the ability of the roof covering and assembly to resist the penetration of fire as a result of a standard fire exposure (ASTM E 108).

Summary

Differences in fire performance between different materials can be evaluated by comparing flame spread ratings (Class A is the greatest resistance, followed by B and C) and heat release rate.

  • Noncombustible materials are either defined as such in the building code, or have met the requirements of a standard test.
  • Ignition resistant materials have passed a 30-minute flame spread test.Ignition resistant materials may be combustible.

Fire resistance is typically associated with an assembly construction, and therefore considers the performance of a number of materials that would be incorporated in a wall, floor or roof. The exterior material (i.e., the one exposed to the fire) can be combustible, ignition resistant, or noncombustible since the entire assembly contributes to the rating. Although the fire ratings are in terms of a time (e.g., 20-minute, one-hour, two-hour), they only represent a relative performance (i.e., a two-hour wall is better than a one-hour wall, but they may or may not resist a given fire exposure for those time periods). A nominal “one-hour” wall has been used as one way for a wall having combustible siding to be used in wildfire prone area. Whereas fire resistance information can be used to judge the ability to resist flame penetration into the building, it does not necessarily provide information regarding flame spread. This is especially true since this type of construction is only used when combustible siding is used as the outermost material.

Given the use of these terms, you can rank the expected performance of construction materials as follows:

  • Noncombustible – Best performance for both flame spread and penetration.
  • Fire resistance – Fire Resistant construction – Rely on assembly rating for resistance to fire penetration, and the exterior material (i.e., the one that would be exposed to the fire) for information regarding flame spread.
  • Ignition Resistant – Provides information regarding flame spread. Materials with this classification can be expected to perform better than combustible materials but not as well as noncombustible.
  • Combustible – Materials with this classification will not perform as well as the others discussed in this article, given a comparable fire exposure.