flexible duct wrap

Steven Thorsell, L.A., CSI

June 1, 2001

The purpose of this article is to address the issues related to the use of flexible duct wrap systems as an alternative to the fire-resistance-rated shaft enclosures required by the International Codes and the efforts of the National Evaluation Service, Inc. (NES) to evaluate their use for enclosing kitchen grease ducts and ducts associated with heating, ventilating and air conditioning (HVAC) systems.


The past several years have seen the development and introduction of flexible duct wrap materials for use as a fire-resistance-rated enclosure for kitchen grease ducts and HVAC ducts, and as a method of reducing clearance to combustible materials. These flexible wrap materials typically consist of a highly insulating lightweight flexible "blanket" composed of nonasbestos, inorganic, fibrous refractory material. Two materials are currently prevalent in fabricating the "blanket," insulating ceramic fiber and Alkaline-earth Silicate (AES) wools. The flexible wraps typically have a nominal density of 6 lb/ft3 (96 kg/m3) and a thickness of 1.5 inches (38 mm) for a two-layer wrap installation (see Figure 1) and 8 lb/ft3 (128 kg/m3) and thickness of 2 inches (51 mm) for a one-layer wrap with an overlap wrap, collar or checkerboard type installation technique (see Figure 2).

The duct wrap blanket may be unfaced, faced on two sides or completely encapsulated in an aluminum foil covering reinforced with a fabric scrim. The key difference between the blankets that are completely encapsulated and those that are faced on two sides is that with the completely encapsulated blanket, both the faces and edges of the blanket are sealed within the facers so that no portion of the blanket core is exposed. This complete encapsulation is intended to protect the blanket core from wicking moisture, grease or condensation, thus minimizing potential fire hazards and increasing the longevity of the blanket. With the two faced blanket, the edges of the blanket are exposed. The use of the wraps that are not completely encapsulated is typically limited to use as the base layer in a two-layer system. Completely encapsulated wraps are typically used on all layers of the enclosure or, in some cases, as the exposed or outer layer. The concern over the potential effect such absorption may have on the wrap has prompted some manufacturers to limit the use of their systems to use of fully encapsulated blankets only in all grease duct applications.

The wrap materials are typically produced in rolls of varying lengths and typical widths of 24 and 48 inches (610 and 1219 mm). The wrap material is installed in direct contact with the exterior surface of the duct in a variety of installation methods. As noted previously, installations may be two complete layers of wrap or a single layer with an overlap, collar wrap or checkerboard method. Each of the three methods used for a single layer installation results in two layers of wrap occurring at all horizontal and longitudinal joints of the system. The number of layers and type of overlap are established by the testing and is discussed later in this article. The most common method for holding the wrap in contact with the surface of the duct is with steel banding placed at regular intervals [typically 10.5 inches (267 mm) on center] along the length of the duct (see Figures 1 and 2). As duct widths increase beyond 24 inches (610 mm), the use of insulation impaling pins and speed clips welded to the underside of horizontal duct runs and on vertical duct runs are used to supplement the banding and prevent the sagging of the wrap (see Figure 3). The elimination of sagging is important due to the fact that when the blanket is not in direct contact with the surface of the duct, there is no means to retard the heat flow from the interior of the duct to the exterior. During an interior fire condition, the resulting air space between the exterior of the duct and the inner face of the blanket quickly becomes extremely hot, and could result in the ignition of any combustible material on the exterior surface of the duct. The oxygen present in the sagged areas can then feed the fire. Some manufacturers also perform testing of the systems to establish the equivalency of the impaling pin method with the performance of the banding method.

To complete the system, a through-penetration firestop assembly is installed at locations where the wrapped duct assembly penetrates a fire-resistance-rated floor/ceiling or wall assembly.

Applicable Code Requirements

The use of these materials are most typically intended for use as part of a system for the enclosure of kitchen grease ducts and HVAC ducts as an alternative to a traditional fire-resistance-rated shaft enclosure. In order to evaluate the use of these systems as an alternative to a shaft enclosure, it is necessary to review the intended use(s) and the requirements governing shafts used for the enclosure of HVAC and grease ducts in the 2000 edition of the International Building Code (IBC) and the International Mechanical Code (IMC).

The primary uses that are of concern in the evaluation of these systems consist of:

  • use as an alternative to a fire-resistance-rated shaft enclosure of a duct penetrating a fire-resistance-rated floor or wall assembly;
  • use as an alternative for enclosure of kitchen exhaust grease ducts, and
  • use as a method of reducing clearance to combustibles for grease ducts.

In addition, since the materials are typically left exposed, the materials must be tested and evaluated for surface-burning characteristics as an interior finish. Additionally, in order for the material to be used in buildings of Type 1 and 2 (noncombustible) construction, it must be evaluated for compliance with noncombustibility requirements.

The following presents an overview of the code requirements of the IBC and IMC, which are applicable to the evaluation of this type of material, as well as the data necessary to determine compliance with those code requirements. It should be noted that while this article specifically discusses the requirements of the 2000 International Codes, the provisions of the BOCA National Building Code/1999, 1999 Standard Building Code®, and 1997 Uniform Building Code are very similar to those of the 2000 International Codes.

Alternative to a Fire-resistance-rated Shaft

For this application, we begin with a review of the requirements of Section 707.2 of the IBC and Section 607.6 of the IMC.

To determine code compliance, the fire-resistance rating of the system must be determined first. Section 703.2 of the IBC states the methods that are permitted for use in determining the rating of any assembly.

To establish the fire-resistance rating of these systems, the ASTM E 119 test method is used. The critical criteria are the ability of the material to resist the penetration of fire and hot gases as well as the ability to limit the transfer of heat under the time-temperature curve established by ASTM E 119. To establish this performance, full-scale fire tests are conducted on a wall assembly constructed of the type(s) of steel in the minimum thickness which are representative of the ducts the material is intended to enclose. The maximum end point temperature criteria of ASTM E 119 [not to exceed an average of 250°F (139°C) above ambient for all thermocouples or 325°F (181°C) above ambient for any single thermocouple] is used to establish the maximum hourly rating of the test specimen.

Once the hourly rating for the test assembly is established, the provisions of Section 707.5 must be reviewed.

To determine compliance with this section requires establishing the ability of the material to maintain the structural integrity of the assembly protected, including the supporting construction. This is done through the full-scale engulfment testing performed in accordance with ASTM E 119 to evidence that the protected assembly does not suffer structural failure or collapse when exposed to the same hourly exposure used for the ASTM E 119 testing discussed previously. The full-engulfment test is the same test procedure that would be used to establish the hourly rating of protected structural framing members. The test assembly consists of a wrapped duct that is suspended by supporting framing in the furnace. (See Photo 1.) The manufacturer must decide whether to perform the test with the supporting framing members (such as threaded rods and angle iron) protected with a layer of wrap material or exposed and thus unprotected. The results of this test serve to establish the minimum requirements for the specific supporting construction and the maximum duct size that can be accepted as an alternative to the code. (See Photos 2 and 3.)

Next, since the protected assembly is intended for installation on ducts which penetrate fire-resistance-rated floors or walls, the performance of the protected duct assembly in conjunction with a through-penetration firestop system must be evaluated. This code requirement is found in Section 302.2 of the IMC and Sections 711.3.1.2 and 711.4.1.2 of the IBC.

The F and T ratings of this testing, in conjunction with the hourly rating determined by the ASTM E 814 testing, establishes the maximum hourly rating of the system, where the system is installed in an application that penetrates a fire-resistance-rated wall or floor assembly. It must be noted that the use of the wrap system will be limited to the minimum thickness of the steel tested in the ASTM E 119 testing, and the maximum size of the duct tested in the full-engulfment and through-penetration firestop testing.

The maximum permitted hourly rating is determined by the lowest of the maximum hourly ratings determined by the ASTM E 119 and ASTM E 814 testing. The use of the material on duct sizes exceeding the maximum sizes tested may be permissible when additional testing and/or an engineering analysis is performed to determine that there will be no adverse structural effects, such as excess deformation or deflection, on ducts larger than those tested.

Finally, since the material is an exposed duct covering, Section 604.3 of the IMC requires testing in accordance with ASTM E 843 for surface-burning characteristics and testing in accordance with ASTM C 4114 for hot-surface performance be performed.

Alternative to a Shaft Enclosure for Kitchen Exhaust Grease Ducts

For this application, a review of the requirements of the exception to Section 506.3.12 of the IMC is in order. Since the issues related to equivalence with a fire-resistance-rated shaft are applicable, the criteria stated in the preceding paragraphs are used as a basis for this evaluation. In addition to these criteria, Section 506.3.11 states that the material is required to be listed and labeled by an approved agency. The requirements for the listing and labeling program requires that an independent third-party agency review and approve the manufacturer’s in-house quality assurance program, perform random inspections and testing of the material at the manufacturer’s plant(s), and label the material as complying with the minimum performance requirements. The NES has conducted evaluations of both testing laboratories and quality assurance agencies to further assist code officials in the task of evaluating and considering these systems for approval.

Alternative Method to Reduce Clearance to Combustible Materials

For this application, the UL 1978 test method is used. In this test, a sample duct is constructed and wrapped with the material in accordance with the manufacturer’s installation instructions and gas burners are installed inside the duct. A piece of plywood is placed in contact with the top surface of the wrapped duct and thermocouples placed between the plywood and wrapped surface, on the exterior of the wrap and between the duct surface and the wrap. The gas burner is lit and the internal temperature of the duct is brought to 500°F (260°C) and held until equilibrium temperatures are attained on the grease duct surfaces and plywood. Next, the internal temperature is increased until it reaches 2000°F (1093°C) and is held at this temperature for 30 minutes. (See Photo 4.) The pass criteria for this test is that the temperature on the exterior surface of the wrap, under the plywood, must not exceed 90°F (32°C) above ambient room temperature during the 500°F (260°C) exposure and 250°F (139°C) above ambient room temperature during the 2000°F (1093°C) exposure. The results of this testing serve to establish the minimum duct thickness and maximum duct size permitted for use in grease duct applications where the enclosure is in contact with adjacent combustible materials (zero-clearance to combustibles). As with the previous uses, engineering analysis may be employed to extend the use of the system to duct sizes other than that tested.

In summary, in order to determine compliance with the requirements of the code, it is essential to identify the intended use of the system, and then review that use against the testing that has been performed. Through this process, it can be established what conditions and limitations apply, based on the available technical data, which can then be compared with the actual use intended.


1ASTM E 119-98 Standard Test Method for Fire Tests of Building Construction and Materials, American Society of Testing and Materials, 1998.

2ASTM E 814-97, Standard Test Method for Fire Tests of Through-penetration Fire Stops, American Society of Testing and Materials, 1997.

3ASTM E 84-98e1, Standard Test Method for Surface Burning Characteristics of Building Materials, American Society of Testing and Materials, 1998.

4ASTM C 411-97, Test Method for Hot-Surface Performance of High-Temperature Insulation, op. cit., 1997.

5UL 1978-95, Standard for Grease Ducts, Underwriters Laboratories Inc., 1995.