This course is part of the Durability Academy

Designing exterior walls in buildings is a complex process. Beyond the structural system that may be either independent from or integral to the wall system, multiple other criteria must be met. The International Building Code (IBC) requires that the wall include a water-resistant barrier (WRB) in the interest of durability and integrity of the wall. It also indicates the conditions where fire resistance is or is not required in exterior walls according to different aspects of life safety. The International Energy Conservation Code (IECC) requires an air barrier to reduce air infiltration as a mandatory provision. It also requires insulation in the wall with a prescriptive requirement for continuous insulation (ci) outside of the structure in many climate zones in order to address the issue of thermal bridging. There are also project design criteria for general appearance, budget, and reduced maintenance that may impact not only the cladding or outermost surface of the wall but the rest of the assembly too. Altogether, the various materials and products designed and specified into an exterior wall must be able to be constructed as a complete, integrated wall assembly that is functional, code compliant, durable, cost-effective, and meets the design intent of the building.

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Exterior walls can take many forms, but building codes require that they all meet minimum requirements for fire safety either by using noncombustible materials or by demonstrating safety through specific testing.

In light of all of the above, this course will look specifically at ways to solve one of the biggest challenges in many exterior wall designs, namely designing and demonstrating life safety performance during a fire. We will begin with a look at what the building codes require as well as one of the most cited, but often misunderstood, standards referred to in the code, namely NFPA 285. With an understanding of each of these, we will then look at several design approach options to create exterior walls that are safe, fire resistant, and readily able to be constructed.

IBC Requirements

The IBC is widely adopted across the United States by most authorities having jurisdiction (AHJ) as the basis for health, safety, and welfare in buildings. Other than one- or two-family homes and townhouses (which are covered by the International Residential Code), all other new construction or significant renovation of existing buildings need to comply with the relevant provisions of the IBC wherever it is adopted and in place. While there are different versions issued every three years (2012, 2015, and 2018 versions are currently in use around the country), there are some basic things that haven’t changed significantly, if at all, including the following.

Types of Construction

The IBC identifies five fundamental ways to characterize construction as a basis to subsequently refer to specific code requirements applicable (or not) to each type. Chapter 6 of the IBC identifies these five types (using Roman numerals) by listing specific “building elements,” such as primary structural frames, bearing and nonbearing exterior and interior walls, floor systems, and roof construction. In order to qualify as Type I or Type II construction, all of these primarily structural building elements listed in the code “are of noncombustible materials” except in a few cases where some specific exceptions may apply.

Moving on to Type III construction, the code defines this as “that type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of any material permitted by this code.” There is a provision that allows for fire-retardant wood framing and sheathing here if certain criteria are met, but otherwise, combustible materials generally are not allowed on the exterior walls but are allowed on interior assemblies within the parameters of other sections of the code. Type IV construction is similar in that it also requires “the exterior walls are of noncombustible materials” but goes on to indicate more specifically that “the interior building elements are of solid wood, laminated wood, heavy timber (HT), or structural composite lumber (SCL) without concealed spaces.” Finally, Type V construction is “that type of construction in which the structural elements, exterior walls, and interior walls are of any materials permitted by this code.” This type typically means acceptable wood-framed construction that is not Type IV and does not have the same noncombustibility requirements for building elements.

It is significant to note for our purposes that Types I, II, III, and IV all require noncombustible exterior walls as part of their basic description. The clear intent, in the interest of life safety, is to limit the spread of fire whether one originates inside a building or somewhere outside. This the fundamental approach of fire safety in the code: contain a fire if it breaks out through various means, including the use of materials that don’t burn and contribute to growth of the fire. Nonetheless, there are some specific exceptions and variations for different types of exterior walls, and some of those exceptions can be different between different versions (years) of the IBC so it is always a good idea to check the prevailing code for a particular building location. Even so, the noncombustible requirement remains a key point for exterior walls.

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Exterior walls are among the building elements that the International Building Code uses to define and classify different construction types.

Exterior Wall Requirements

In addition to the construction-type descriptions, there are other places in the code that bring additional requirements for exterior walls. For clarity, the code does define exterior walls in Chapter 2 as “a wall, bearing or nonbearing, that is used as an enclosing wall for a building, other than a fire wall, and that has a slope of 60 degrees or greater with the horizontal plane.” This comprehensive definition is the basis for Chapter 14 of the IBC, which is devoted entirely to multiple aspects of such exterior walls.

Beginning with Section 1402: Performance Requirements, the IBC identifies specific criteria for weather protection, including the need for a water-resistive barrier (WRB) behind the cladding or outermost surface. It also lists requirements for structure, fire resistance, flame propagation, and flood resistance. Since some of these things are interrelated, most notably in flame propagation that is directly influenced by the materials used in an exterior wall, it provides a specific clarification. Section 1402.5 indicates that “exterior walls on buildings of Type I, II, III, or IV construction that are greater than 40 feet in height above grade plane and contain a combustible water-resistive barrier shall be tested in accordance with and comply with the acceptance criteria of NFPA 285.”

More simply put, if the required water-resistive barrier is a combustible product (i.e., a plastic or composite membrane or similar) over a stated threshold and it is being used in a wall that is required to be noncombustible (Type I–IV), then there is a standard test required known as NFPA 285. We will delve deeper into that test later in this course, but for now, suffice to say that it is a nationally recognized and IBC-adopted test of the National Fire Protection Association (NFPA) that has been developed to determine the fire safety of entire exterior wall assemblies, not just individual products. Its purpose is to determine whether or not a combustible element is permissible (i.e., safe) for use in an exterior wall based on subjecting a test wall to direct flames and recording the results. There are a few exceptions to requiring this test, mostly involving masonry and concrete construction or low fire risk when the only combustible item in the wall is the water-resistive barrier.

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The International Building Code (IBC) addresses safety in buildings with many specific provisions directly applicable to exterior walls.

Chapter 14 continues to address other aspects of exterior walls, including criteria for wall materials, interior wall coverings, vapor retarders, veneers, and the use of combustible materials on the exterior side of exterior walls. For this last item, those combustible materials must be limited in size, location, type, or comply with other provisions of the code in order to be allowed in Type I–IV construction. There are some specific sections on particular cladding choices, such as metal composite material (MCM) panels, exterior insulation finish systems (EIFS), high-pressure decorative laminates (HPL), and plastic composite decking. In the case of MCM and HPL, there are conditions that can again require that the wall assembly be tested according to NFPA 285 similar to the water-resistive barrier testing requirement.