Skip to content 
The fire-rating problem in container construction has already been solved.
Fire-Rated Walls for Shipping Containers — Oasis Engineering Engineering Note · Fire Resistance The fire-rating problem in container construction has already been solved. For years, container projects have stalled at plan review on a single question. The answer was sitting in a UL design number the whole time. Here is the framework, the assemblies, and the code basis — published openly. Enrique Lairet, PE · Oasis Engineering April 2026 10 min read Walk into any plan review for a shipping-container building and you’ll hear the same conversation: what is the fire-resistance rating of the wall? The folklore says no UL-listed assembly works for containers. The folklore is wrong. The conversation usually ends one of three ways. The architect submits an alternative-means letter built from first principles — slow, expensive, AHJ-dependent, often rejected. The team strips out the container’s fire-rated function entirely and adds a separate stick-framed wall — wasteful, ugly, and defeats the point of building with containers. Or the project quietly dies. None of that is necessary. UL Design V497 — a one-sided steel-stud partition tested under ASTM E119 — is a direct fit for container construction under IBC §703.2.2(1). No alternative-means submittal required. No engineering-comparison gymnastics. The same design number covers 1-hour and 2-hour ratings. Why this works (the one-page version) Most fire-tested wall assemblies are symmetric — gypsum board on both sides of a stud cavity. When the industry tried to fit those onto containers, the question always came back: do we need to add gypsum to the exterior of the container too? That question has no clean answer, and it’s the wrong question. UL Design V497 is asymmetric. The test specimen has multiple layers of Type X gypsum on one side of a 3-5/8″ steel stud, and nothing on the other side — bare studs facing the furnace. The assembly achieves its rating with one protected face. That’s what was tested. That’s what’s listed. When V497 is applied to the interior face of a container, the corrugated steel container skin replaces what was bare studs in the test. The skin is unambiguously more protective than nothing. The rating holds. The whole industry has been looking for a symmetric tested assembly when an asymmetric one was sufficient. That’s the entire idea. Everything else in this article is execution. The code basis Here’s the relevant section of the IBC, verbatim: IBC §703.2.2 — Analytical MethodsThe fire resistance of building elements, components or assemblies established by an analytical method shall be by any of the methods listed in this section… (1) Fire-resistance designs documented in approved sources. … (4) Engineering analysis based on a comparison of building element, component or assemblies designs having fire-resistance ratings as determined by the test procedures set forth in ASTM E119 or UL 263. UL Design V497 is documented in the UL Product iQ database — an approved source under §703.2.2(1). Direct application. No engineering analysis needed. The 2-layer (1HR), 3-layer (1HR), and 4-layer (2HR) variants are all cataloged under the same V497 design number, and the gypsum manufacturer (National Gypsum) publishes the assembly under GA File WP 1297. The five assemblies We’ve published five canonical assemblies — three direct §703.2.2(1) applications of V497, and two §703.2.2(4) variants that add a UL-listed intumescent coating on the container exterior to give symmetric (both-sides) rated protection. Pick the one that matches your project’s rating requirement and direction. Assembly Rating Direction Code Basis Construction CFW-1A 1 HR Interior §703.2.2(1) 2 layers 5/8″ Type X + ProForm Quick-Set, 3-5/8″ steel studs @ 24″ o.c. CFW-1B 1 HR Interior §703.2.2(1) 3 layers 5/8″ Type X, 3-5/8″ steel studs @ 24″ o.c. CFW-2 2 HR Interior §703.2.2(1) 4 layers 5/8″ Type X, 3-5/8″ steel studs @ 24″ o.c. CFW-1E 1 HR Both sides §703.2.2(4) CFW-1B + UL-listed 1HR intumescent coating on container exterior CFW-2E 2 HR Both sides §703.2.2(4) CFW-2 + UL-listed 2HR intumescent coating on container exterior For typical V-B / R-3 / B occupancies — single-family, ADUs, small commercial, food service — CFW-1A or CFW-1B is what you want. For occupancy separations or 2HR fire walls, use CFW-2. For symmetric ratings (fire on either side), CFW-1E or CFW-2E. What about insulation? The most common follow-up question. Containers are tin cans — they need closed-cell spray polyurethane foam (ccSPF) for thermal performance and air-sealing. Does that break the V497 listing? It does not. Cavity insulation in V497 is optional per the listing. You have two clean options: leave the V497 stud cavity empty (or fill with glass-fiber batt, which is also covered), and apply ccSPF outboard of the studs — directly against the interior face of the container skin. The foam lives in the gap between the container skin and the back of the steel studs. The V497 assembly proper stays exactly as listed. Using this on a real project Read the white paper — especially the Limits and Permit Submittal sections. Pick the assembly that matches your project’s rating requirement and direction. Drop the CFW Detail Sheet into your architectural set, referenced from the wall type schedule. Get a project-specific PE-stamped letter from a PE licensed in the jurisdiction (we issue these directly, or through engineerletters.co for fast-turnaround scopes). Pre-coordinate with the AHJ before submittal. Most reviewers, once shown the V497 listing and the §703.2.2(1) citation, accept the framework on first pass. The hard part — the analysis and the published documentation — is now done. Need a PE letter for your project? Oasis Engineering provides project-specific PE letters and stamping based on these assemblies. We work with container builders, architects, and owners across multiple jurisdictions. Start a project View on GitHub Open-source by design Everything in this article — the white paper, the five assemblies, the detail sheet PDF, the code references, the equivalency analysis — lives in a public GitHub repository under an MIT license. Use it. Adapt it. Reference it in your permit set. If you find an error in the analysis or the citations, open an issue. If a manufacturer would like to
The fire-rating problem in container construction has already been solved. Read More »
