Using exterior-grade waterproofing in interior applications

By Makoto S. Weinstein, PE and Scott A. Tomlinson, PE

Photo © iStock

Designers of new construction and renovation projects sometimes prefer to use exterior-grade waterproofing and roofing products (collectively referred to as “exterior-grade waterproofing” in this article) within interior space for improved performance compared to typical interior-grade waterproofing products. For example, improved performance may be desired in a mechanical system intake plenum that will be exposed to rain and snow that infiltrates the louver, or at some other unique interior application.

Unfortunately, the use of exterior-grade waterproofing within interior space has fire- and life-safety code implications that require analysis. If the exterior-grade waterproofing is noncombustible, the fire- and life-safety code analysis is easy. However, if it is combustible, the analysis is more complex and code requirements often constrain the waterproofing design.

A primary constraining factor is the manufacturers of exterior-grade waterproofing generally do not perform the tests required to demonstrate the fire response properties necessary to establish code compliance for interior use. The same complication exists if the designer desires to use interior-grade waterproofing products in some exterior applications, although this is not the subject of this article.

The different fire response properties and test methods required for exterior and interior applications of waterproofing products to aid designers, with a particular focus on the differences between interior (ASTM E84 and ASTM E648) and exterior (ASTM E108) fire test standards, will be explored first. The ASTM standards describe the material behavior when in contact with fire, but they are fundamentally different tests providing dissimilar fire response properties. As a result, they are not readily comparable.

A case study of a plenum design approach to navigate the code requirements to use exterior-grade waterproofing within interior space to meet the owner’s performance expectations will also be presented.

Code requirements relating to fire safety

Schematic sketch of Steiner Tunnel Furnace used to determine ASTM E84 fire response properties. Illustration courtesy Simpson Gumpertz & Heger Inc.

The National Fire Protection Association (NFPA) 101, Life Safety Code, International Building Code (IBC), and International Mechanical Code (IMC) are commonly adopted codes that must be considered for interior waterproofing analysis; the 2021 versions were reviewed for this article. The codes reference the following ASTM standards to determine the code required fire properties; the codes also reference similar NFPA and Underwriters Laboratories (UL) tests that are not reviewed in this article:

  • E84-19A–Standard Test Method for Surface Burning Characteristics of Building Materials (IBC references 18B).
  • E648-17A–Standard Test Method for Critical Radiant Flux of Floor-Covering Systems Using a Radiant Heat Energy Source.
  • E108-17–Standard Test Methods for Fire Tests of Roof Coverings.

NFPA 101 Chapter 10 (Interior Finish, Contents, and Furnishings; Sections and and IBC Chapter 8 (Interior Finishes; Sections 803 and 804) require fire testing and classification of interior wall and floor finishes based on standards ASTM E84 and ASTM E648, respectively.

For interior walls, NFPA and IBC both require classification of the finish materials as Class A, B, or C based on flame spread and smoke developed indexes of the material when tested in accordance with ASTM E84:

  • Class A–Flame spread index 0-25; smoke developed index 0-450.
  • Class B–Flame spread index 26-75; smoke developed index 0-450.
  • Class C–Flame spread index 76-200; smoke developed index 0-450.

For interior floors, NFPA and IBC both require classification of the finish material as Class I or II based on its critical radiant flux when testing in accordance with ASTM E648:

  • Class I–Critical radiant flux of not less than 0.45 watts/cm2.
  • Class II–Critical radiant flux of not less than 0.22 watts/cm2, but less than 0.45 watts/cm2.

IMC Chapter 6 (Duct Systems, Sections 602.2 and 602.2.1) provides the requirements for plenum enclosure construction materials that are exposed to the airflow. These sections generally require “Plenum enclosure construction materials that are exposed to the airflow” to “have a flame spread index of not more than 25 and a smoke-developed index of not more than 50 when tested in accordance with ASTM E84 or UL 723…” Section 602.2.1 provides exceptions for combustible materials that are fully enclosed by noncombustible enclosures and other approved materials.

NFPA 101 (various chapters) and IBC Chapter 15 (Roof Assemblies and Rooftop Structures; Section 1505) require fire testing and classification of roof coverings. Both codes require testing in accordance with ASTM E108 and classification of the roof covering as Class A, B, and C. However, ASTM E108 Classes A, B, and C are different than ASTM E84 Classes A, B, and C, as described below. Unfortunately, the similarities in classification names can lead to a misunderstanding of the intended code requirements.

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