Sub-framing System Selection

by Stacy Rinella | September 17, 2024 10:07 am

[1]

Any architect or engineer knows many details must be taken into consideration during the selection of a rainscreen sub-framing system. This article provides a baseline understanding of the differences between steel, fiber-reinforced polymer (FRP), and composite metal hybrid systems installed on the exterior of a building. It will show how composite metal hybrid sub-framing systems combine the structural integrity of steel with the thermal insulative properties of FRP to benefit building enclosure design. This article will also discuss newer and cutting-edge composite metal hybrid products, continuous insulation (CI) systems, and building enclosure systems offering unique value to architects.

 

Steel sub-framing

Steel is a familiar choice of architects as it stands out for strength and durability. Steel offers exceptional load-bearing capacity, ensuring the structural integrity of a building’s sub-framing. Its inherent toughness makes it resilient against various environmental stresses and helps to deliver long-term stability. As many sub-framing installers are familiar with and experienced in its installation, steel products can often be integrated into projects quickly and
installed efficiently.

The challenge, however, lies in structures seeking more efficient enclosures with high resistance values (R-values), energy code compliance, and healthier buildings. Since steel is a conductive material, steel sub-framing products act as a thermal bridge, allowing the temperature outside a structure to transfer through the CI system into the interior of a building. The high R-value targets many architects seek can be significantly diminished when using steel and other conductive sub-framing products—for example, the thermal performance of a galvanized steel Z-girt can reduce the R-value by more than 50 percent. As steel is susceptible to corrosion, impacts on structural integrity and the need for repair are possible.¹ 

[2]

 

Fiberglass/FPR products

The desire for energy efficient building enclosures and high R-values ushered in an increased demand for FRP sub-framing products. Several manufacturers offer FRP Z-girts and clips to secure CI onto the exterior of a building. Made entirely of fiberglass—a non-conductive and versatile material well-suited for use in both new construction and repair applications—these Z-girt and clip products are efficient at eliminating the thermal bridges which occur in buildings that might otherwise use steel or other conductive materials in their sub-framing system. As such, FRP Z-girts, and clips regularly exhibit energy efficiency percentages well into the upper 90s—meaning that effective R-values on the constructed building are nearly identical to the R-values that are specified.²

Fiberglass products, however, do not have the same structural strength and fastener retention as steel—reducing their longevity, durability, and fastener retention, especially under the extreme service temperatures that can occur inside sub-framing systems and rainscreen assemblies.

[3]

 

Composite metal hybrid (CMH)

Architects seeking the best of both worlds—the load-bearing capacity and structural integrity of steel plus the non-conductive, thermally efficient properties of fiberglass—have an alternative in composite metal hybrid (CMH) systems. CMH is a high-performance Z-girt system that combines the corrosion-resistant and insulative properties of FRP with the structural resilience of a continuous metal component on each of the substrate—and cladding-facing flanges. This unique composition of CMH systems guarantees high structural strength and ensures robust screw retention—allowing screws to permanently fasten into a continuous metallic structural support for effective and reliable rainscreen load distribution. Installers often favor CMH systems for their ease of installation—comparable to installing standard steel Z-girts.

Many details must be considered during the selection of rainscreen sub-framing systems and building enclosure systems.

 

Notes

¹ Finch, Graham, and Higgins, James. The Guide to Cladding Attachment Solutions for Exterior-Insulated Commercial Walls. RDH Building Science, 13 Mar. 2019, rdh.com/blog/guide-cladding-attachment-solutions-for-exterior-insulated-commercial-walls/

² Krause, Gary M.; Krause, Matthew; Walsh, Sean; and Kukkonen, Zach. Short-Term Fastening in FRP vs. Permanent Fastening in Steel & GreenGirt® CMH (Composite Metal Hybrid) for Construction Framing. Advanced Architectural Products, 14 Nov. 2023, greengirt.com/a2p3001-short-term-fastening-in-frp-vs-permanent-fastening-in-steel-greengirt-cmh-composite-metal-hybrid-for-construction-framing/.

 

Zach Kukkonen is a design engineer at Advanced Architectural Products. He received his BA in journalism from the University of Wisconsin in 2007 and his BS in civil engineering from Michigan Technological University in 2012. He has spent 11 years in the composite sub-framing and window industries, specializing in structural analysis, product application, and thermal analysis. Kukkonen has been involved with all facets of composites—from testing and structural analysis to pultrusion and installation.

 

Kevin Mlutkowski joined Advanced Architectural Products in 2023, bringing more than 20 years of experience in expanding market opportunities within the fields of structural engineering, fire protection, and green building. He holds a BS in technical communications with a minor in chemistry from Lawrence Technological University, as well as an MBA from Oakland University. In his current role, Mlutkowski leads efforts to expand the markets and strengthen the brands for GreenGirt continuous insulation systems and SMARTci building enclosure systems.

Source URL: https://www.metalarchitecture.com/articles/sub-framing-system-selection/

---