"Modular buildings have two unique features that may present unforeseen risks to firefighters. You should be aware of and alert to these differences--note the modular buildings in your community during your inspections and prefire planning. First, there may be larger than expected concealed void spaces between each of the modules of the building. In multistory modular buildings, there is typically a void, or interstitial, space between each story and concealed cavities in the walls where two modules fit together. All of these concealed voids are larger that those found in most buildings constructed using traditional construction techniques, and they have the same potential for spreading fire inherent to all concealed spaces. Because of their larger size, they will have both more fuel available for a fire to burn, and more oxygen available to support combustion. 'This condition is not unique to modular construction--any structure where trusses are present has void spaces that are larger than found in structures where dimensional lumber is used to support floors, ceilings, and other surfaces./ Second, modular buildings are constructed using adhesives instead of nails or screws to attach drywall to the ceiling and wall studs. A fire in the concealed space behind the wall or above the ceiling will burn this adhesive, causing it to lose its ability to support the drywall or sheathing. If this happens, the ceiling could collapse progressively into the room. A collapse by itself could injure or trap a firefighter operating in the room; it also could result in a very rapid flashover condition."
Coffee Break Training - Fire Protection Series No. 2008-42
United States Fire Administration: http://www.usfa.fema.gov/