Building Design

Fire-rated Wall Design Problems

A typical architecture student gets no exposure to designing fire-rated walls while in school. Once they get a job working for an architect, they are expected to learn how to look up UL designs for rated walls, and draw wall details based off that information. Instead, many new architects simply copy wall type details from other projects that the office drew rather than look up the information, thinking that if the project was approved, the information must be correct. And some architects still have not looked up any UL fire-rated wall designs after many years on the job.

Many architects think about wall design in terms of exterior finishes – selection of brick or stucco or some type of metal cladding – and determine steel studs or wood studs, with gypsum board on the interior side – but they give very little thought to the insulation, sheathing, house paper or felt, number of layers of gyp board or required thickness of the gyp board – the other components in the actual construction of the wall. Thought also needs to be given to air gaps and vapor barriers; there is condensation that must be drained from inside of the wall to prevent molding. And to consider what order, from outside to inside, do these go. Selection of manufacturers and attention to whether these materials are combustible or not is completely ignored.

In addition to the lack of thought regarding the wall type detail and design of the fire-rated walls, many projects get approved (by the government entity that reviews and approved plans for construction) without any verification that the UL design numbers cited on the wall type details are correct, or that the material components spec’d are in compliance with the UL design . The government employees do not look up the UL information. They see citations and then approve the wall details as is. I have seen wall types for 2 hour fire-wall designs get approved with a simple citation of “rigid insulation” or “cavity insulation” with no specific manufacturer or info about what the insulation is made out of, if it is combustible or not.

Given that combustible insulation products are substantially cheaper than non-combustible products, clients and contractors prefer to purchase the cheaper (and inevitably, combustible) products. NFPA 251 requires wall assemblies to be tested by making a 100 SF (min 9.5 ft length) mock up of the fully constructed assembly. They do not require testing of each component in the wall assembly. Many manufacturers will make mock-up wall assemblies with their products – many will exceed the required size limits, for example, build a two-story wall with a window mock-up – and do testing to determine the limit of the flame and smoke spread, and provide certifications for architects to use when designing walls – but limited flame spread with a certification does not equal non-combustible. It can be quite confusing to determine if a product is non-combustible or just limited flame spread. Manufacturers want to sell their products. The UL wall designs do permit combustible materials when they have test data from specific wall assemblies, thus allowing combustible products to be installed in 2 hour fire-rated exterior walls (bearing and non-bearing).

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