The exterior wall portion of the building enclosure is a multifunctional part of the built environment. It keeps the majority of exterior environmental loads from getting in, it keeps the majority of building-generated heat from getting out, it is the aesthetic facade, it supports the facade structurally, it manages moisture flow through the exterior wall, and it controls the spread of fire throughout the building envelope; just to name a few of the performance requirements. While building enclosures range from elaborate and cutting edge to basic and cost conscious, every building enclosure must be code compliant.

Photo courtesy of Laminators Incorporated

Exterior wall assemblies must be designed to meet code-required structural support, continuous insulation, moisture management, and fire resistance performance criteria, but available solutions often interfere with one another.

Surprisingly, a code-compliant building enclosure is difficult to design, in large part, because there are conflicts within the building codes that make the path to compliance unclear. This article separates the potential code conflicts into four types:

1. Structural Requirements and Moisture Management
2. Structural Requirements and Energy Code/Continuous Insulation
3. Structural Requirements, Moisture Management, and Energy Code/Continuous Insulation
4. Structural Requirements, Moisture Management, Energy Code/Continuous Insulation, and Fire Requirements

While there is no magic bullet, this article will identify potential solutions for these four important conflicts and discuss the advantages and disadvantages of each. Pragmatic application of best practices can help a designer avoid the difficulties associated with these code conflicts and create a functionally compliant enclosure.

BASIC ELEMENTS OF AN EXTERIOR WALL SYSTEM

The exterior wall system is tasked with managing and controlling the movement of heat, air, and moisture into and out of the building enclosure, while providing the requisite structural support for the exterior facade. In order to accomplish this extensive list of interrelated, but diverse, design objectives, the exterior wall assembly includes a number of stand-alone components and systems that must work simultaneously and in close proximity with one another without interfering. A few of the basic components and systems commonly found in an exterior wall assembly include: structural elements, drainage plane (typically the air and water barrier), vapor retarder (optional as required), insulating elements, and exterior cladding.

Image courtesy of Laminators Incorporated

An exterior wall assembly often includes structural elements, a drainage plane, a vapor retarder, insulating elements, and exterior cladding.

Structural Elements

The structural elements inside of the exterior wall system include the structural supports that attach the veneer, or other types of exterior cladding, to the structure. Building codes require that these structural elements be designed to support the self-weight of the veneer and to withstand appropriate environmental loads, such as wind loads. Structural elements include, but are not limited to, the primary wall structure, sheathing, secondary framing system elements (e.g. cold-formed metal furring), and other structural members that are specific to certain proprietary exterior wall systems.

Drainage Plane

The drainage plane in an exterior wall assembly exists to effectively manage moisture but is regularly called upon to manage airflow as well. Most often, the drainage plane is defined with an air and water barrier, an element designed to both manage water and airflow at the same plane within the exterior wall construction. The water barrier is designed to limit exposure of bulk rainwater and condensation to the managed portions of the exterior wall cavity, to enable the exterior wall cavity to dry, and to prevent uncontrolled water from penetrating further into the interior. The air barrier in the exterior wall assembly is intended to control the airflow between the outdoors and the interior, conditioned space. Airflow control is important because airflow carries moisture, spreads smoke, impacts indoor air quality, and influences the movement of heat, which impacts the thermal performance of the building.

Vapor Retarder

Moisture, in any form, needs to be effectively managed in the built environment. While the drainage plane is designed to control the movement of liquid water; water vapor, which refers to water in its gaseous state, is controlled by a vapor retarder. A vapor retarder impedes the flow of water vapor between the exterior assembly and interior walls. The code defines when a vapor retarder is required, and it is oftentimes incorporated into the drainage plane by using a product that can function as a water barrier, an air barrier, and a vapor retarder.

Insulating Elements

The expansion of insulation requirements in the exterior wall system is a relatively new addition to the energy code—the result of a growing demand for improved systems efficiency and increasing interest in satisfying sustainable design initiatives. Insulating elements in the exterior wall system manage the flow of heat in and out of the building. This reduces the heat lost or gained through the exterior wall and improves the overall performance of the HVAC system because less energy is required to keep the building at its preferred temperature.

Exterior Cladding

Exterior cladding refers to the protective layer or finish affixed to the exterior side of the building envelope. The exterior cladding makes an important contribution to the overall aesthetics of the building, but also provides the first layer of protection against bulk rainwater penetration.

There are a variety of materials available to provide exterior cladding that can meet the specific needs of the building and aesthetic preferences of the design team, including different metals and metal composites, stone, wood, and concrete.