All Things Gypsum
Why a Duct?
envision the explosive growth of southern, urban America without forced cool air. Just imagine Las Vegas or Houston in July without A/C. It's frightening.
Beyond demographic changes, the ability to develop large air-conditioning systems forever altered the way that non-residential buildings were designed and constructed. Gone were the operable windows and large unoccupied interstitial areas that facilitated the creation of drafts and natural breezes in buildings built before the '20s. In were systems that used pipes and ducts to move coolant and air vertically between building floors and to and from fixed chilling units.
Cool airNot surprisingly, the implementation of air conditioning systems resulted in an increase in the use of metal-fabricated air movement ductwork. This in turn led to the development of other air movement systems, one of the more intriguing of which is the grease duct system. Grease duct systems are primarily used to evacuate heat and fumes from internal cooking and frying units and are commonly attached to grill and fryer hoods in restaurants. They can occasionally be identified from the exterior of
a building by the mushroom-shape cap that is placed immediately on the duct at the point where it vents through the building perimeter to the outside.
Grease duct systems typically evacuate grease-loaded fumes from occupied building areas. Consequently, mechanical codes have historically mandated that grease duct systems be separated from building interiors by encasing them in protective fire-resistant shaft enclosures. For many years, these systems have been almost exclusively composed of multiple layers of gypsum board installed immediately adjacent to but not in contact with the grease ducts.
Unfortunately, there now exists a misconception that gypsum board systems can no longer be used as horizontal grease duct enclosures: Nothing could be further from the truth. Both the International Mechanical Code and the Uniform Mechanical Code still require a grease duct enclosure to be constructed "in accordance with the International Building Code requirements for shaft construction." Since the IBC permits gypsum board systems to be used as a vertical shaft enclosure material, its use as a grease duct shaft enclosure material is also permitted.
Some of the confusion over the use of gypsum board grease duct enclosure systems undoubtedly stems from a misunderstanding of the term "noncombustible" and its application in mechanical codes vs. building codes.
Gypsum board is considered to be a noncombustible building material by all model building codes, for it complies with the standard building code language that defines layered noncombustible building materials. Essentially, all model building codes allow a building material to be labeled as noncombustible if the material passes the ASTM E136 test, "Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 degrees C," or if the core of the material passes the E136 test and the material has a facing that is less than 1/8-inch thick and has a flame spread of less than 50.
The codesGypsum board meets these criteria. Its core complies with the E136 requirements, the paper surfacing is less than 1/8-inch thick, and it has a flame spread that is typically in the 5 to 15 range, considerably less than the maximum of 50 that is required.
Where the confusion comes to play is that the mechanical codes typically don't fully recognize the building code language that allows materials with an E136-compliant core and a thin, low-flame spread facing to be considered noncombustible; they usually are written so as only to allow materials that directly pass the E136 test to be considered noncombustible. The E136 test is extremely difficult to pass; very few materials make the grade and because of its paper facing gypsum board does not meet the stringent requirements of the test.
This differentiation between a mechanical vs. a building code interpretation of noncombustible becomes a factor in very few instances; however, one of those instances is in grease duct enclosure systems where it becomes a determinant in dictating enclosure clearance requirements.
Grease duct shaft enclosure systems must be installed in accordance with the clearance distances mandated in the appropriate mechanical code. Section 506 of the IMC, for example, requires that the "clearance from the duct to the interior surface of combustible construction shall be not less than 18 inches." At first glance, this would seem to preclude the building of a gypsum board enclosure with less than 18 inches of clearance to a grease duct; a distance that might dissuade designers from using a gypsum board system for space reasons. However, closer scrutiny of the IMC reveals language that reduces the clearance distance to 6 inches for "enclosures of ... gypsum board attached to noncombustible structures"; in other words, gypsum board applied to noncombustible (think steel) framing. Gypsum board shaftwall systems are almost always tested using noncombustible framing; thus, they can easily be used to enclose horizontal grease duct systems.
And why the clearance requirement in the first place? Conventional wisdom would point to limitations on the use of gypsum board in immediate proximity to a constant heat source; however, informal tests conducted by the Gypsum Association actually proved that gypsum board performed very well when installed immediately adjacent to a grease duct that had been subjected to an internal fire. Instead, the real reason for the clearance is to maintain the ability to inspect the exterior of the metal portion of the duct subsequent to a suspected internal grease or system fire. To quote from the IMC Commentary: "The minimum clearance is required to allow visual observation of the duct for inspection purposes, especially after a fire event."
Shaft systems that use gypsum board designs as grease duct enclosure systems have an excellent performance record when installed in accordance with applicable code requirements. A number of gypsum board manufacturers maintain code evaluation reports containing systems that have been evaluated for use as grease duct enclosure systems. For information on these systems, contact the gypsum board manufacturer or the GA.