Steiner Tunnel Test for Foam Insulation

The International Residential Code (IRC) Chapter 3 requires:

  • Foam insulation to be covered with a 15 minute thermal barrier such as ½” gypsum board.
  • All foam plastic insulation to have a flame spread index less than 75 and a smoke- developed index of less than 450 in accordance with ASTM E84 (Steiner Tunnel) flame spread test.

The codes do not specify that chemicals be added to foam plastic insulation, but usually organohalogen flame retardant compounds are used to meet the Steiner Tunnel test requirements. Thus, since 1976, foam plastic insulations almost universally within U.S. jurisdictions have been required to both meet the Steiner Tunnel Test specifications and also be protected from sources of ignition, heat, or fire by a thermal barrier.

Research has shown that the ASTM E 84 Steiner Tunnel test is unreliable in assessing the hazard of plastic foams.

The most important reason for this is related to the specimen mounting geometry. In the Steiner Tunnel, the specimen is mounted on the ceiling of a long, tunnel-shaped apparatus, and an exposure flame is directed at the specimen from one end as shown below:

Under the test conditions, thermoplastic foams tend to melt and the liquid residue flows onto the floor, out of reach of the burner flame. Since the burner does not apply a flame to the floor, the results are registered as no flame spread having occurred, even though the same product would show extreme fire spread if exposed to a more realistic flame.

This problem is recognized by ASTM, as Section 1.4 of ASTM E 84 states that “testing of materials that melt, drip, or delaminate to such a degree that the continuity of the flame front is destroyed, results in low flame spread indices that do not relate directly to indices obtained by testing materials that remain in place.”

The current edition of the FM Global Property Loss Prevention Data Sheet 1-57 Plastics in Construction says of polyurethane: “Fire retardant additives can markedly improve the performance in comparative fire tests, such as ASTM E84, of foamed plastic. Large-scale corner testing has indicated, however, that the performance of foamed plastic under actual fire conditions is not significantly affected by the use of these additives. Consequently, claims for fire retardancy based on the ASTM E84 tunnel test and similar comparative tests should be disregarded for foamed plastic.” It further states about EPS, “As with polyurethane, additives do not significantly affect the burning characteristics.”

Further Reading


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