The International Code Council states that "interior finish includes interior wall and ceiling finish and interior floor finish," that "interior wall and ceiling finish" is "the exposed interior surfaces of buildings including, but not limited to: fixed or movable walls and partitions; columns; ceilings; and interior wainscotting, paneling, or other finish applied structurally or for decoration, acoustical correction, surface insulation, structural fire resistance, or similar purposes, but not including trim," and that "interior floor finish" is "the exposed floor surfaces of buildings including coverings applied over a finished floor or stair, including risers." ² Thus, when dealing with testing of interior finish, a distinction needs to be drawn between walls (and ceilings) and floors.

The fire performance of interior wall and ceiling finish is critical to the development of a fire: interior finish offers fuel contribution and surfaces through which a fire can spread and transport heat and smoke to other parts of the compartment, or even to other compartments.

Therefore, the fire performance of such materials needs to be controlled.

STEINER TUNNEL TEST
The Steiner tunnel fire test method for surface flame spread and smoke development remains the traditional test used to assess fire performance of interior finish materials. Developed by Al Steiner for testing building materials, such as wood or gypsum board, at Underwriters Laboratories in 1944 (Figure 1), the Steiner tunnel test has been standardized by the major North American standards writing organizations (ASTM E-84, NFPA 255, UL 723, ULC S102) and widely adopted by every North American building and fire code.

In the test, a specimen (7.3 m x 0.56 m, normally up to 0.15 m thick), either in one unbroken length or in separate sections joined end to end, is mounted face downwards so as to form the roof of a horizontal tunnel 305 mm high. The fire source, two gas burners, ignites the sample from below with an 89 kW intensity (Figure 2), and the combustion products are carried away by a controlled linear air velocity of 73 m/min (or, exactly, 240 ft/min). The normal output is a flamespread index (FSI) and a smoke-developed index (SDI). Flame spread is assessed visually by the progression of the flame front, while measurements of optical smoke density at the tunnel outlet determine the smoke obscuration. This information is used to plot time-based graphs of flame-spread distance and of optical density. FSI and SDI are then calculated based on the ratio between the areas under the curves for the material being tested and those for a cementitious board (assigned FSI and SDI values of 0) and for red oak flooring (assigned FSI and SDI values of 100).

The building, fire, and life safety codes (IBC, IFC, NFPA 5000, NFPA 101, and NFPA 1/UFC) all contain requirements that limit interior wall and ceiling finish to Class A (FSI ≤ 25; SDI ≤ 450), Class B (25 ≤ FSI ≤ 75; SDI ≤ 450), or Class C (75 ≤ FSI ≤ 200; SDI ≤ 450). A major flaw in the Steiner test appears in the description of the test method above and the results obtained from this fire test: the Steiner tunnel test does not provide results in engineering units. Consequently, the test results cannot be used for a fire hazard analysis or a fire risk analysis.

This test continued to be popular when plastics started to be used in construction and in spite of the fact that the test is not always appropriate for every material. Samples that cannot be retained in place above the tunnel floor or which melt and continue burning on the tunnel floor (typical behavior for most thermoplastics) are still being tested with this equipment even though the results are not representative of the use of the material in realistic situations. ³ The same can also be said about thin materials, which often give low FSI values mainly due to insufficient material in the test method to permit flame spread to be assessed properly. An understanding of some of these limitations has caused the codes to consider alternatives, either as replacements for the Steiner tunnel or as additional options (see section on heat release).

FLOOR FINISH TEST METHODS
Different challenges face interior floor finish than other interior finish because heat and smoke rise in a fire. Thus, floor finish is involved either as the initial material ignited in a fire or as an additional fuel once a fire has become uncontrolled. Consequently, fire safety requirements typically need to ensure that interior floor finish is relatively difficult to ignite and is not capable of slowly spreading flame from the compartment of fire origin to a different one.

The Steiner tunnel cannot assess ignitability, and its fuel source is not appropriate to assess slow flame spread. Experience has shown that many flooring materials (traditional floor finishes such as wood flooring or resilient materials) will not ignite unless exposed to an ignition source of well over > 1 kW/m2, but that some carpet-like or loose-fill materials may ignite at such low heat fluxes. A study of precision of the flooring radiant panel test method found carpets with critical radiant heat fluxes well under 2 kW/m2.4

Therefore, all carpets and rugs sold in they United States5 must meet the "methenamine pill" test (ASTM D 2859), which ensures that flame spread will be minimal.

Most codes also regulate interior floor finish (in occupancies where fire risk needs to be especially minimized) to be tested with the flooring radiant panel (ASTM E 648, NFPA 253, Figure 3) and require a "critical radiant flux" for ignition in excess of 4.5 kW/m2 (Class I) or 2.2 kW/m2 (Class II). In the flooring radiant panel, the floor finish (such as a carpet) is exposed to an incident heat flux from an angled gas-fired radiant panel, with a maximum heat flux of approximately 11 kW/m2 at the farthest end from the igniter. The test method assesses the critical incident flux (which is measured by comparing the distance between the igniter and the point where flame propagation stops to a calibration curve) required for continued flame propagation.

This approach (even if it is based on old-fashioned tests) is quite suitable for interior floor finish. Some applications, typically in the transportation vehicle arena, also require flooring materials to meet one of a variety of smoke obscuration requirements, often based on a static smoke chamber box, either with a traditional radiant heater (ASTM E 662) or with a conical heater (ISO 5659-2, IMO Fire Test Procedures Code part 2, also known as ASTM E 1995 and NFPA 270).