{"id":14976,"date":"2025-12-01T17:30:28","date_gmt":"2025-12-01T17:30:28","guid":{"rendered":"https:\/\/insulation.org\/io\/?post_type=articles&p=14976"},"modified":"2026-01-06T17:35:08","modified_gmt":"2026-01-06T17:35:08","slug":"fire-resistance-rated-air-duct-systems-the-vital-role-insulation-plays","status":"publish","type":"articles","link":"https:\/\/insulation.org\/io\/articles\/fire-resistance-rated-air-duct-systems-the-vital-role-insulation-plays\/","title":{"rendered":"Fire Resistance Rated Air Duct Systems\u2014The Vital Role Insulation Plays"},"content":{"rendered":"\n
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Many ducts in commercial buildings require fire protection in accordance with code prescribed criteria. Life safety ducts\u2014or, in other words, ducts required to remain operational in the event of a fire, such as those supplying fresh air to egress routes, or those extracting smoke during a fire event, to name a couple\u2014are on the list. Building codes, fire test standards, and awareness have evolved over the years, making flexible wrap or \u201cfire wrap\u201d systems a prevalent and widely accepted alternative to gypsum shaft enclosures, providing the required protection for all types of commercial duct systems while saving space compared to the shaft assembly.<\/p>\n\n\n\n

What Is a \u201cFire Wrap System\/Assembly\u201d?<\/h2>\n\n\n\n

Fire wrap products typically consist of alkaline earth silicate (AES) fiber blankets, with maximum use temperatures upwards of 2,000\u00b0F, encapsulated in scrim-reinforced foil. The encapsulation adds handling strength for installation and aids in moisture resistance while providing a canvas to locate product identification. However, fire wrap products themselves do not provide a stand-alone fire resistance rating. The fire wrap is fire tested as one assembly with the duct, resulting in a listed\/labeled fire resistance rated duct system or assembly.<\/p>\n\n\n\n

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Multiple ducts with complex configurations<\/figcaption><\/figure>\n\n\n\n
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Flexible fire wraps use less space than traditional gypsum shaft assembly<\/figcaption><\/figure>\n\n\n\n

Duct Types<\/h2>\n\n\n\n

In addition to commercial kitchen grease ducts, duct types such as stairwell\/elevator pressurization, smoke control, and hazardous (laboratory) exhaust are cited specifically by code as requiring protection. Many other types will inherently require protection simply because they penetrate a fire-rated assembly (wall or floor) separating one fire-rated compartment from another, regardless of function.<\/p>\n\n\n\n

To gain a better understanding of how these systems are utilized, let\u2019s start by outlining the fundamental code requirements for providing a fire resistance rated enclosure for air distribution system (ADS) ducts, beginning with a few definitions in accordance with the 2024 International Building Code (IBC):<\/p>\n\n\n\n

Fire Resistance: \u201cThat property of materials or their assemblies that prevents or retards the passage of excessive heat,
hot gases or flames under conditions of use.\u201d<\/p><\/blockquote><\/figure>\n\n\n\n

Fire Resistance Rating: \u201cThe period of time a building element, component or assembly maintains the ability to confine a fire, continues to perform a given structural function, or both, as determined by the tests, or the methods based on tests, prescribed in Section 703.\u201d<\/p><\/blockquote><\/figure>\n\n\n\n

IBC Enclosure Requirements<\/h2>\n\n\n\n

IBC Chapter 7, Fire and Smoke Protection Features, is fundamentally based on compartmentation\u2014that is, keeping the fire contained to the area of origin. The chapter outlines the fire resistance rating requirements for a building as they pertain to maintaining compartmentation, including walls, floor\/roof assemblies, shaft enclosures, ducts and air transfer openings, penetrations of rated assemblies, and more.<\/p>\n\n\n\n

Regarding the requirements for shaft enclosures, Section 703.2.1, Tested Assemblies, states: \u201cA fire-resistance rating of building elements, components or assemblies shall be determined by the test procedures set forth in ASTM E119 or UL 263.\u201d<\/strong> Fire resistance rated shaft enclosures are constructed as fire barriers (vertical) and floor\/roof assemblies (horizontal), as tested\/listed to ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, and are required to have a fire resistance rating of at least 2 hours when connecting four stories or more, and at least 1 hour if fewer than four stories. ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, and UL 263, Fire Tests of Building Construction and Materials, are nearly identical test methods for evaluating building construction and materials to achieve a fire resistance rating. For conciseness, we\u2019ll focus on ASTM E119.<\/p>\n\n\n\n

Achieving a Fire Rating in Accordance with ASTM E119<\/h2>\n\n\n\n

For a wall or floor\/roof assembly to be assigned a fire resistance rating in accordance with ASTM E119, the assembly must comply with all of conditions 1-4 listed below. The fire resistance rating is based upon the time in minutes (or hours) at which the first occurrence of any of these failure criteria pertaining to structural stability, temperature rise resistance on the unexposed surface of the test assembly (insulation), and prevention of the passage of flame or hot gases and subsequent passage of hose stream (integrity). The specific language defining the pass\/fail criteria are as follows:<\/p>\n\n\n\n

    \n
  1. The wall or partition withstands the fire endurance test without passage of flames or gases hot enough to ignite cotton waste, for a period equal to that for which classification is desired,<\/li>\n\n\n\n
  2. The assembly limits temperature rise to a maximum average temperature rise above ambient of 250\u00b0F by making measurements during the conduct of the fire exposure test,<\/li>\n\n\n\n
  3. The assembly limits the maximum temperature rise over ambient at any individual location to a maximum of 325\u00baF by making measurements during the conduct of the fire exposure test, and<\/li>\n\n\n\n
  4. The wall or partition withstands the impact of the hose stream test. The assembly is considered to have failed the hose stream test if an opening develops that permits a projection of water from the stream beyond the unexposed surface during the application of the hose stream test.<\/li>\n<\/ol>\n\n\n\n

    It is critical (and explicitly required by E119 test method) that all four of these criteria are satisfied, as omission of any one of them could allow fire to spread to another compartment.<\/p>\n\n\n\n

    Unfortunately, code-prescribed ASTM E119 does not have provisions for testing duct enclosure assemblies. However, IBC Section 703.2.3, Approved Alternate Method, permits the establishment of alternative protection methods in accordance with Section 104.2.3, Alternative Materials, Design and Methods of Construction and Equipment. This alternative proposed method complies with the intent of the code and is not less than the equivalent of that required by the code in several categories, including Fire Safety (outlined in IBC Sections 104.2.3.3 and 104.2.3.4).<\/p>\n\n\n\n

    The purpose of a fire separation as required by the IBC is to isolate a fire that may occur on one side of the rated vertical or horizontal assembly from breaching the barrier and spreading to the non-fire side. While the fire separation (tested assembly) is a wall or floor\/roof when tested to ASTM E119, when we introduce a duct penetrating the assembly going from one fire-rated compartment to another, the purpose of the fire separation does not change; it simply becomes a wall or floor with a duct penetrating it and, as such, requires testing as one assembly.<\/p>\n\n\n\n

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    Two parallel wrapped ducts<\/figcaption><\/figure>\n\n\n\n

    Purpose-Built Fire Test Methods<\/h2>\n\n\n\n

    Since a duct system can\u2019t be tested to ASTM E119, we turn to purpose-built duct testing methods. These duct testing methods are specifically designed to apply the basic principles and pass\/fail criteria of ASTM E119 to a duct system to demonstrate equivalent performance to code-required protection while evaluating the duct system\u2019s ability to remain functional. We\u2019ll consider two test methods: ISO 6944, Fire Resistance Tests \u2013 Ventilation Ducts, and ASTM E2816, Standard Test Methods for Fire Resistive Metallic HVAC Duct Systems.<\/p>\n\n\n\n

    ISO 6944<\/h2>\n\n\n\n

    Originally published in 1985 in response to the need for a method of evaluating fire resistance of duct assemblies, the original document scope stated:<\/p>\n\n\n\n

    \u201cThe general purpose of this test is to measure the ability of a representative duct or duct assembly to resist the spread of fire from one fire compartment to another without the aid of fire dampers.\u201d<\/p><\/blockquote><\/figure>\n\n\n\n

    This was the first fire test method of its kind, and it has evolved through several editions since the original 1985 edition, with the most recent being 2024.<\/p>\n\n\n\n

    While the title has changed slightly (Fire Containment \u2013 Elements of Building Construction \u2013 Part 1: Ventilation Ducts), and the method has improved over the years for usability, the fire exposure and pass\/fail criteria have remained fundamentally unchanged and continue to align with that of ASTM E119. Although the time-temperature fire exposure is not exactly that of ASTM E119, ISO 6944 utilizes the ISO 834, Fire Resistance Tests \u2013 Elements of Building Construction, time-temperature exposure. To compare the ISO 834 and ASTM E119 fire exposure conditions, the American Society for Testing and Materials (ASTM) and the National Research Council Canada (NRCC) conducted a study. The results showed that for tests under 1\u00bd hours, the ASTM exposure was \u201cslightly more severe,\u201d while during longer tests, the differences between the two were negligible, thus providing technical justification for acceptance of ISO 6944 tested systems.<\/p>\n\n\n\n

    ISO 6944 tests the ducts in both the vertical and horizontal orientations, penetrating fire-rated floor\/roof and\/or wall assemblies, respectively, for both fire outside the duct (Duct A) and fire inside the duct (Duct B), utilizing three pass\/fail criteria that align directly with those of ASTM E119:<\/p>\n\n\n\n