THE POTENTIAL CODE CONFLICTS WITHIN THE BUILDING ENCLOSURE

While the individual requirements for structural support, moisture control, continuous insulation, and NFPA 285 compliance are certainly achievable, the inclusion of so many different elements in an exterior wall assembly creates opportunities for one system to interfere with another and cause the whole assembly to be noncompliant. In common practice, there are four potential conflicts that designers should be aware of in order to maintain the integrity and compliance of the enclosure.

1. Structural Requirements and Moisture Management

Entrapped moisture is problematic in a building enclosure because it can break down the insulation, contribute toward mold and mildew growth, and corrode the steel studs. While the outermost layer of the building, the exterior cladding, is responsible for shedding most of the rainwater, the drainage plane, which exists in the enclosed space between the exterior cladding and the exterior surface of the building structure, is tasked with keeping any water that penetrates the cladding from coming into contact with the physical structure.

Image courtesy of Laminators Incorporated

This illustrates the use of a perforated cold-formed Z girt to allow for proper and compliant drainage.

In accordance with building codes, the drainage plane of the exterior wall assembly must be designed to allow the water to drain down and out of the building, without interference and without allowing the water access to the structure of the wall. It must also contain the structural supports necessary to hold the veneer in place and sufficiently withstand the weight of the panels and the wind load associated with the location of the building. Unfortunately, placing the elements of the secondary frame into the drainage plane can impact the ability of the drainage plane to manage moisture. The structural supports can create obstacles to the natural egress path of the water as it drains, which could cause a compliance issue. For example, if a structural support within the wall cavity, such as a cold-formed steel Z girt, impedes drainage out of the wall or interferes with airflow impacting the assembly’s ability to dry, there is a potential compliance issue.

Follow a drop of rainwater as it penetrates the exterior cladding and travels into the drainage plane in the wall cavity. The rainwater should be able to flow unhindered down and out of the building. The base flashing should prevent the water from accessing any existing cracks or gaps that would allow it to seep into contact with the building structure. Gravity should allow water to flow over whatever joints or structures may exist in its path. If the water collects anywhere, in any way, such as pooling behind an incorrectly installed structural support, the building enclosure is at risk for the damage that can be caused by entrapped moisture and is not compliant with building codes.

Potential Solutions

There are a number of solutions to allow water a clear path to exit the wall cavity, while simultaneously allowing the secondary framing to support the code-required cladding loads. As mentioned above, Z girts offer a solution that can be installed in a fairly easy and simple manner and are readily available from most building supply companies. Oftentimes, it is economical for contractors to have the Z bent from flat stock. It is important that the detailing of the Z girts be such that it is possible to fasten the Z girt to the wall structure, which typically requires horizontally oriented members. When the members are installed horizontally, there is a greater potential for water flow paths to be blocked at each Z girt. To allow water and air to flow freely within the cavity, it is required to either leave gaps at regular intervals along the length of the Z girts or to specify that the Z girts be perforated along their horizontal leg.

Many proprietary solutions also exist to allow for sufficient structural capacity, while allowing for clear water and air flow paths within the wall cavity. Some systems use discrete brackets to extend the structural supports away from the sheathing by a predetermined distance. The veneer can then be mounted to the structural supports, leaving a fairly open cavity behind for drainage.

2. Structural Requirements and Energy Code/Continuous Insulation

There are many benefits that can be realized from incorporating continuous insulation into a building enclosure, especially when compared to the performance of an exterior wall assembly filled with cavity (batt) insulation. Continuous insulation dramatically improves the thermal performance of the building by minimizing the heat lost through thermal bridging. Thermal bridging occurs when heat flows through the conductive structural supports, often steel or aluminum, that connect the conditioned interior with the exterior. Traveling through the support elements, the heat easily eludes the cavity insulation in a wall or roof enclosure and is lost to the outdoor environment. In a steel-framed building, thermal bridging can reduce the R-value of cavity-insulated wall systems by more than 50 percent. Continuous insulation also reduces the amount of conditioned air that is lost through gaps and cracks in the enclosure because it has fewer gaps and cracks than are found in discontinuous cavity insulation. In addition, continuous insulation often moves the dew point from inside the stud cavity to within the managed exterior wall drainage cavity, which reduces the potential for condensation within the structural portion of the building enclosure.

Photo courtesy of Laminators Incorporated

ASHRAE 90.1 now requires the use of continuous insulation in almost all exterior wall assemblies, and designers are challenged to find ways to attach the exterior cladding to the assembly without breaching the insulation with anything other than screws, bolts, and nails.

There are many different kinds of continuous insulation found on commercial projects. Regardless of whether the particular insulation is a rigid foam product, a spray foam, or stone wool, the most important characteristic of continuous insulation is its continuous and unbroken nature. It is the very feature that gives this layer of insulation its ability to help buildings manage heat so effectively.

ASHRAE 90.1, which now requires the use of continuous insulation in almost all exterior wall assemblies, defines continuous insulation as insulation that is continuous across all structural members, without thermal bridges other than fasteners and service openings. Although the code does not explicitly define the term fasteners, it is generally interpreted as nails, screws, bolts, and items where the thermal transfer is very small. The definition excludes large connection details, such as furring strips, lintels, and clip angles. In practice, this means that a layer of continuous insulation interrupted or breached by a cold-formed steel Z girt is noncompliant and will expose the building to thermal bridging that will dramatically compromise the thermal performance of the enclosure.

The conflict arises because the area in the exterior wall assembly, where designers are tasked with placing a continuous and unbroken layer of insulation, must also host the structural supports that hold the exterior cladding onto the enclosure. Designers are left with a few options for achieving compliance, each with its own advantages and potential issues.

Install Cladding Over Rigid Insulation

One approach to supporting exterior cladding, without breaching the continuous insulation, is to fasten the supports over the top of rigid insulation. In this scenario, the insulation is sandwiched between the sheathing and the furring elements, to which the exterior cladding will attach. This installation method relies upon the long-term stability of the rigid foam layer and the compressive strength of the system. Unfortunately, the rigid foam board can experience some initial crushing that creates inconsistencies or unevenness across the surface and, over time, the foam undergoes a natural creep that further deforms the insulation layer. This instability results in the eventual misalignment of the sheathing, insulation, and furring elements, which compromises the compressive strength of the system. This can result in a substrate that moves out-of-tolerance over the life of the facade, which can be extremely problematic for the exterior cladding it supports and negatively impact the aesthetic of most veneers.

Proprietary Clip and Bracket Systems

An additional caveat to the challenge of incorporating continuous insulation in a building enclosure is that many building claddings are not approved for attachment through more than 1 inch of non-supporting material. In climates where the minimum R-value of the continuous insulation is 7.5, the thickness of the insulation may reach 1½ inches to achieve the requisite R-value.

Proprietary clip systems have been developed that attach exterior cladding to the exterior wall system in a way that is dimensionally stable over time and enables the assembly to accommodate a thicker layer of continuous insulation, if necessary. Unfortunately, these systems may not be viable solutions for projects that require a non-proprietary specification.

Calculate the Wall’s Equivalent R-Value

The ASHARE 90.1 energy code offers multiple compliance paths to provide designers with some much needed flexibility, especially when faced with requirements that conflict with one another. Typically, there is a prescriptive path, which spells out exactly which materials can be used to achieve compliance, and a performance-based path, which enables designers to use different materials or a methodology outside of the prescriptive path, as long as they can demonstrate that the building achieves the desired level of performance. While the prescriptive path tends to be a simpler and more straightforward approach to satisfying thermal performance requirements, because it defines specific continuous insulation requirements, the performance path can offer a means to achieving a compliant design, although it requires a higher level of documentation and more detailed building simulations. As it relates specifically to designing a code-compliant building enclosure, designers can calculate the equivalent thermal values of the exterior wall or hire an energy specialist to provide the calculations.