Case Study: Large HVAC plenum over interior space with exterior-grade waterproofing
This case study provides a mostly noncombustible exterior-grade waterproofing design approach for a large HVAC “walk-in” plenum on the top floor of a building that is over interior space (Figure 2, page 28). The owner has a low tolerance for water leakage. Rain and snow can be blown and drawn through the louver during certain weather conditions, resulting in significant water accumulation on the plenum floor.
The walls are covered with standing seam steel panels over metal framing with mineral wool insulation between the framing, air- and WRB, and gypsum sheathing. The floor is covered with flat-seam soldered copper sheet metal that is sloped to drain over roofing underlayment, gypsum sheathing, and metal framing with mineral wool insulation between the framing.
The metal, insulation, and gypsum board materials are noncombustible as defined by the IBC.
The underlayment has flame spread and smoke developed indexes within permissible limits; the authors are aware of at least one underlayment product that has been tested to provide the required fire response properties necessary to establish code compliance. The metal floor framing is designed to support maintenance personnel and align with roofing attachment clip locations. The flat-seam soldered copper and standing seem steel panels have expansion joints for thermal expansion and contraction.
The IMC includes provisions that could allow the use of combustible exterior-grade waterproofing if it is fully enclosed with noncombustible materials. A project-specific analysis would be required to determine if this is acceptable.
Scott A. Tomlinson, PE, is an associate principal with Simpson Gumpertz & Heger Inc. in Waltham, Massachusetts. His broad experience includes the design, investigation, and construction of a variety of building-enclosure systems. Tomlinson specializes in addressing construction and constructability issues and leading complex investigations. He can be reached at SATomlinson@sgh.com.
Makoto S. Weinstein, PE, is a consulting engineer with Simpson Gumpertz & Heger Inc. in Waltham, Massachusetts. He is experienced in the design, investigation, and rehabilitation of building-enclosure systems. He has more than 10 years of experience working on building wall cladding and roofing construction, investigating water infiltration, and code review for various aspects of construction. He can be reached at MSWeinstein@sgh.com.