ASTM E84
Standard Test Method for Surface Burning Characteristics of Building Materials
Last updated: April 28, 2026
Contents
Overview
ASTM E84 is the surface burning test that decides whether a building material can be used as an interior wall or ceiling finish. The standard is also called the Steiner Tunnel Test after the apparatus that runs it. It is one of the most widely cited fire-test standards in the IBC, NFPA 101, and product spec sheets across insulation, paneling, acoustical tile, intumescent coatings, and decorative finishes.
Two numbers come out of the test: a Flame Spread Index (FSI) and a Smoke Developed Index (SDI). The IBC and NFPA codes use those numbers to assign a Class A, B, or C rating, which then governs where the material can be installed. Class A is the most stringent. Exit corridors, stairwells, and healthcare exam rooms typically require Class A finishes. Less critical spaces accept Class B or C.
ASTM E84 and UL 723 are harmonized. They use the same apparatus, the same 10-minute burn duration, and the same indices. A product tested to one is accepted under the other by code officials. More on that below.
What ASTM E84 Measures
An E84 test produces two indices. Both are unitless numbers normalized against reference materials, not absolute physical measurements. The reference points matter because they explain why the scale tops out where it does.
Flame Spread Index (FSI)
FSI measures how rapidly flame spreads across the face of the specimen during the 10-minute test. The index is calibrated so that select-grade red oak flooring is assigned an FSI of 100 by definition. Inorganic reinforced cement board is the zero point. A material with an FSI of 25 spreads flame about a quarter as fast as red oak under identical test conditions. Lower is better.
Smoke Developed Index (SDI)
SDI is a photometric measurement of smoke obscuration in the exhaust airstream during the same burn. Like FSI, it is normalized against red oak (SDI of 100) and cement board (SDI of 0). The IBC and NFPA codes cap SDI at 450 across all three classes. A material can have an excellent FSI of 5 but still fail if it produces too much smoke under flame, which is a common failure mode for some flexible foams and certain plastics.
Watch out: A product can pass the FSI threshold for its target class and still fail SDI. Check both indices on the data sheet, not just the flame spread number. The SDI cap of 450 is the same for Class A, B, and C, so smoke is the limiting factor more often than buyers expect.
Class A, B, and C
The IBC and NFPA codes group ASTM E84 results into three classes. Some older documents call them Class I, II, and III. They map one to one. Class A and Class I are the same thing.
| Class | Flame Spread Index | Smoke Developed Index |
|---|---|---|
| Class A (or I) | 0 to 25 | 0 to 450 |
| Class B (or II) | 26 to 75 | 0 to 450 |
| Class C (or III) | 76 to 200 | 0 to 450 |
Class A is required for interior finishes in critical-egress and high-occupancy spaces: exit enclosures, exit passageways, healthcare exam areas, and most vertical exits. Class B is generally acceptable for exit access corridors and rooms in many occupancies, especially when sprinklers are present. Class C is the permissive end and is commonly used in low-risk back-of-house areas.
IBC Table 803.13 lays out the specific class required for each combination of occupancy, sprinkler status, and location within the building (vertical exit, exit access corridor, rooms and enclosed spaces). Always cross-check the table against the building's actual occupancy classification before specifying a finish.
The Steiner Tunnel Test
The apparatus is a horizontal furnace, roughly 25 feet long, with a methane gas burner at one end. The specimen is mounted as the ceiling of the tunnel so that flame from the burner travels along its underside. Air is drawn through the tunnel at a controlled rate. The test runs for 10 minutes after ignition.
During the burn, an observer records the maximum distance the flame travels along the specimen, timestamped against tunnel markings. A photometer in the exhaust duct measures light obscuration to quantify smoke production. Both data series are integrated and normalized against the red-oak and cement-board reference points to produce the FSI and SDI numbers.
The test is reproducible across labs because the apparatus dimensions, gas flow, draft, and ignition source are all tightly specified. The same product tested at Underwriters Laboratories, Intertek, and an in-house ASTM-accredited lab should produce indices within a small tolerance.
Tip: When you see a product data sheet citing "ASTM E84 tested, Class A," the report itself should list the FSI, SDI, and the testing laboratory. Ask for the report if a specifier requires it. Generic claims with no indices on the sheet are a red flag.
Building Code References
ASTM E84 is referenced throughout the IBC, NFPA 101, and many product-specific codes. The most important sections to know:
IBC Section 803.1.2
Establishes that interior wall and ceiling finish materials must be classified per ASTM E84 or UL 723. This is the foundational requirement that brings the test into the building code in the first place. Any finish material that contributes meaningfully to a wall or ceiling surface area falls under this section.
IBC Section 803.5 and Table 803.13
Specifies the required class (A, B, or C) by combination of occupancy, sprinkler status, and location. Vertical exits and exit passageways trip the most stringent Class A threshold across most occupancy types. Exit access corridors generally drop to Class B in sprinklered buildings and rise to Class A unsprinklered. Healthcare and detention occupancies have the tightest restrictions.
NFPA 101 Section 10.2
The Life Safety Code's interior-finish requirements parallel the IBC's. Class A, B, and C designations carry the same FSI and SDI thresholds. Some occupancies governed by NFPA 101 (assembly, healthcare, ambulatory care) have additional restrictions beyond what the IBC requires.
NFPA 255 (Withdrawn)
Older specs and product literature occasionally still reference NFPA 255. That standard was withdrawn years ago and folded into ASTM E84 and UL 723. A product showing only an NFPA 255 result on an old data sheet is functionally tested to the same method, but a current ASTM E84 report is the right thing to request for new specifications.
Tip: Class A is a useful default for any interior finish in a commercial project where the spec is not explicit. The cost premium on a Class A sealant or coating versus Class C is usually small, and it removes one submittal-review back-and-forth with the AHJ.
Common Applications
Any material that forms the visible interior surface of a wall or ceiling falls under ASTM E84. The list below covers the categories where the standard is most commonly cited on product data sheets.
- Wall and ceiling panels (gypsum, wood paneling, plastic laminates, decorative composites)
- Insulation products (spray polyurethane foam, fiberglass batt, mineral wool, rigid foam board)
- Acoustical materials (suspended ceiling tiles, wall-mounted acoustic panels, fabric-wrapped panels)
- Intumescent and firestop coatings, sealants, and putty pads applied to fire-rated assemblies
- Wall coverings and decorative finishes (vinyl wall coverings, fabric wall coverings, applied trim)
- Coated steel or composite ceiling systems
- Flooring (when used as a wall finish, which is rare but covered by E84 when it occurs)
Firestop sealants and putty pads are an interesting case. Their primary fire test is ASTM E814 for the through-penetration system rating. Most also carry an ASTM E84 result so that the surface of the installed sealant inside an exit corridor or stairwell is itself code-compliant as an interior finish. If you see both E84 and E814 cited on a firestop product, that is why.
ASTM E84 vs UL 723
ASTM E84 and UL 723 are the same test method published under two different labels. The Steiner Tunnel apparatus, the 10-minute burn, the FSI and SDI indices, and the reference materials (red oak at 100, cement board at 0) are identical. Code officials accept results from either standard interchangeably.
Manufacturers most often test to UL 723 when they are pursuing UL classification on the broader product (since UL is doing the rest of the testing anyway), and to ASTM E84 when the testing lab is an independent ASTM-accredited facility. In practice, the same test report often cites both standard numbers on the cover page.
Tip: If a project spec calls out "ASTM E84 Class A" and your product data sheet only lists UL 723 Class A, it qualifies. The IBC accepts both in the same sentence (Section 803.1.2). No engineering judgment or substitution request needed.
Frequently Asked Questions
What is the difference between Class A and Class I?
Nothing. Class A and Class I are the same designation used in different documents. Class A is the IBC label, Class I is the older NFPA designation. Both require Flame Spread Index 0 to 25 and Smoke Developed Index 0 to 450. Class B equals Class II, and Class C equals Class III.
Is ASTM E84 the same as UL 723?
Yes. The two standards specify the same Steiner Tunnel apparatus, 10-minute burn, FSI and SDI indices, and red-oak reference. Code officials accept results from either interchangeably. IBC Section 803.1.2 lists them in the same sentence as acceptable test methods.
Do I always need Class A interior finishes?
No. The required class depends on occupancy type, sprinkler status, and location. Class A is mandatory in vertical exits and exit passageways across most occupancies. Exit access corridors often allow Class B in sprinklered buildings. Less critical rooms may accept Class C. IBC Table 803.13 has the full mapping. When in doubt, specifying Class A removes ambiguity at almost no cost premium.
What is the Smoke Developed Index limit for each class?
450 across all three classes. SDI is not what differentiates Class A from B from C. Flame Spread Index is the differentiator (0 to 25 for A, 26 to 75 for B, 76 to 200 for C). A material that exceeds SDI 450 fails E84 outright, regardless of how low its FSI is.
Why does my old spec reference NFPA 255?
NFPA 255 was the original NFPA-published version of the Steiner Tunnel test. It was withdrawn and folded into ASTM E84 and UL 723. Older project specs and product literature sometimes still cite it. A current ASTM E84 result satisfies any spec that calls for NFPA 255.
Does ASTM E84 cover firestop sealants?
Partially. The primary firestop test is ASTM E814 (through-penetration system ratings). Most firestop sealants and putty pads also carry an ASTM E84 result so that the visible surface of the installed sealant qualifies as a Class A interior finish wherever the wall surface itself is governed by E84. The two tests measure different things and most firestop products report both.
