Modular Concerns: Polyurethane Foam Structural Adhesive, Part 1—Code Problems

By Kevin A. Gallagher

Prior to 2008, my knowledge of manufactured buildings (modular construction) was limited to two things: I needed to drive widely past the convoy of tractor trailers carrying the modular boxes along the highway and, unless told by the homeowner at the time of inspection, I never would have known that the residence was factory made. The modular industry cites several benefits of modular construction including customized, contemporary design. It is a challenge at high noon on a sunny day to distinguish—from the exterior—a modular residence from one built by traditional methods (photo 1). Imagine standing on the front lawn in the early hours of a winter morning with fire or smoke showing on arrival. What information do you need to know as you develop the incident action plan? Are there certain construction techniques used in modular construction that differ from stick-built construction? Do your tactics differ, particularly when the fire moves into the structural elements of the building?

The answers to the three questions above are, in my opinion, 1) a lot, 2) yes, and 3) you bet!

(1) Photos by author.

Following the fire in my community in which a two-story modular home was destroyed, and from which six people were able to escape unharmed, we started asking questions and began receiving some interesting responses. We found that there are significant differences in construction methods, that there are methods used that are code compliant but only because of pretty large loopholes, and that there are methods that challenge our collective interest in protecting occupants and firefighters. [Click HERE for an article on a January 2008 modular home fire which discusses its severity.]

The most glaring difference between modular construction and traditional stick built construction is the use of two-part polyurethane foam structural adhesives. Although there are several structural adhesives on the market that are used in various aspects of construction, this discussion is centered on those that are limited by the companies that make them to prefabricated construction.

If you were to lift the second floor off of a two-story modular home and look down at the ceiling of the first level, or if you pulled back the insulation between floor joists in the attic, you would see a foam-like substance applied to one side of the ceiling joist (photo 2).

(2)

 

That foam-like substance is applied at the factory and is the means by which the gypsum board is attached to the wood joist. There are now different glues on the market and used by the industry. The adhesives with which I am familiar, based on their widespread use in modular homes in my community, are two-part polyurethane foam structural adhesives. Again, these adhesives are limited to the prefabricated construction industry. The adhesive components are stored in large drums and are mixed at the gun applicator at a ratio of 1:1 at the time of application. With the gypsum board laid flat and the ceiling frame placed on top, a worker applies a continuous bead of adhesive using a high-pressure applicator gun along one side of the interface between the wood framing member and the gypsum board (photo 3).

(3)

 

Click HERE to look at the International Code Council (ICC) Evaluation Service’s report on a typical adhesive used by the modular industry and also look under the section titled “Uses.” It states that the product is used as an alternative to mechanical fasteners to attach gypsum board to interior wood-framed partitions and ceilings. Is this an alternative to mechanical fasteners? No one told me that the ceilings in modular homes were held in place by glue only. Surely the building codes must provide clear direction.

Or so you would think. Click HERE to read Section R702.3.5 of the International Residential Code. This table spells out the specifics of applying gypsum board based on the thickness of the board used for ceilings and walls. It addresses the size and spacing of nails or screws (mechanical fasteners) with and without the use of adhesives. Nowhere in this table or in the corresponding text is there an allowance for the use of adhesives as an alternative to mechanical fasteners. Is it just me or are the codes and end use in conflict?

To find out, I sent a letter to the ICC following its very specific format to ask a question, but was offered nothing more than a “please read the text” type of response [click HERE to read the response letter]. The code does not specifically address the use of adhesive as an alternative to mechanical fasteners. However, the “alternative design and methods of construction” section of the administration chapter of the code (R104.11) allows for the use of an ICC Evaluation Report for justifying an alternative method [click HERE to read the follow-up letter). So, the ICC codes allow for deviations if an alternative use is justified and accepts as proof its own evaluation reports—reports paid for by the manufacturer of the product seeking the evaluation.

Digging further, the ICC letter clearly lays the responsibility of a product taking advantage of the “alternative method and design” clause at the feet of the building official. I only say this because that statement is mentioned twice in the letter and underlined both times! So, if I come up with an alternative to mechanical fasteners, pay for an evaluation report by the ICC Evaluation Services, get approval for the products used by a local building official, then I’m code compliant.

However, the construction of modular homes—the only product in which this type of adhesive can be used—occurs outside the community in which it will be finally assembled. The local building official has no oversight, plays no role during construction, and is limited to signing off on inspections preformed on the construction by third party inspectors at the plant. In Massachusetts, like most other states, a state agency provides oversight of the manufactured buildings program. Quality assurance programs are approved, complaints are investigated, and every effort is made to make sure that codes are being complied with. But in this case, who is authorizing the acceptance of the ICC Evaluation Report on the adhesives which, in turn, authorizes the alternative methods clause of the code?

Why is this administrative gobbledygook relevant to the fire service? The answer, I believe, lies in yet another section of the residential code. R702.3.1 states, “adhesives for the installation of gypsum board shall conform to ASTM C557 (Standard for the Installation of Gypsum to Wood Framing).” This standard is also referenced in the product evaluation reports issued by the ICC Evaluation Services [click HERE for the ICC Evaluation Services report] and Progressive Engineering, Inc. (PEI) [click HERE for the PEI report] on commonly used adhesives Remarkably, ASTM C557, the document referenced in evaluation reports which justify the use of adhesives as an alternative to mechanical fasteners, clearly states just the opposite.

“4.3 Although the bonds rendered by these adhesives shall have enough strength by themselves to maintain the bond between adherends, they are not intended as a substitute for the common practice of using mechanical fasteners to maximize integrity of drywall-wood-framing structures.” [Click HERE to read the standard.]

The Gypsum Association, in the document, titled “Specifications for the Application and Finishing of Gypsum Panel Products” (GA-216-2007), states as follows:

“2.2.7.1 Adhesives for attaching gypsum panel products to wood framing members shall comply with Specification for Adhesives for Fastening Gypsum Board to Wood Framing, ASTM C557.”

For those of you, like me, who struggle with code speak, here is a recap:

  1. The ICC does not specifically allow adhesive only installation of gypsum board.
  2. ASTM C557 states that adhesives are not intended as a substitute (alternative) to mechanical fasteners.
  3. The Gypsum Association states if you use adhesives to attach its product to wood framing, follow ASTM C557.
  4. The evaluation criteria that drives the product evaluation report lists the use of the adhesives as an alternative to mechanical fasteners. [Click HERE to read the evaluation criteria.]
  5. The evaluation reports, paid for by the makers of the adhesives, list the use of the adhesives as an alternative to mechanical fasteners.

Our concern simply is this: if glue is the only thing holding up gypsum board in the ceiling assemblies of modular homes, then reassure us that these products have been tested and comply with standards that rate the temperature at which they may fail and result in collapse. Interior structural collapse is high on the list of things firefighters NEVER want to be subjected to.

I had hoped that the ASTM standard would address this concern. Regrettably, it did not. The standard puts the gypsum/adhesive marriage through eight tests. The only test that includes heat is the accelerated adhesive aging component and that test is limited to 158°F for 500 hours.

Look again at the ICC Evaluation Service letter dated May 14, 2010. One of the reasons cited for allowing adhesives without mechanical fasteners reads in part; “…we believe the code requirement is  that interior finishes not become readily detached when subjected to room temperatures of 200 degrees F for not less than 30 minutes.”

200°F!? When was the last time you were in a working fire where the ceiling temperature was only 200°F?

If these products are being used as the sole means to hold up a ceiling, did anyone think of subjecting it to temperatures that come close to what we would expect to find in a room and contents fire? The attached document [click HERE to read the document], from one of the manufacturers of this type of adhesive, sums up our concern with an asterisk: “*This flammability test data is not intended to reflect the hazards presented by this or any other material under actual fire condition.”

Simply, at what temperature does the glue stop doing what it is designed to do? The answer may surprise you and cause you to rethink tactical decisions when confronted with a fire in a modular home.

We will look at the data collected, and the actions taken by code developers in Massachusetts based on that data, in the next installment.

 

Kevin A. Gallagher has served with the Acushnet (MA) Fire & EMS Department since 1986, where he was appointed as chief in 2003. Gallagher has an associate degree in fire science and a bachelor’s degree in political science. He is an adjunct instructor in the Fire Science Program at Bristol Community College. Gallagher serves as the Fire Chiefs Association of Massachusetts representative to the Board of Building Regulations and Standards, the Massachusetts board responsible for overseeing the state’s building code. He has contributed articles to Fire Engineering and has taught several classes at FDIC on the issue of modular construction.

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