A high-performance, low-environmental footprint building is identified among the biggest megatrends in the design and construction industry. Last month we looked at opportunities aligned with this trend to reduce embodied carbon in metal building, including greater use of renewable energy and renewably produced hydrogen in raw material processing and manufacturing, increased efficiency in closed-loop recycling of metals, and innovation in component manufacturing and distribution. In this issue, we will take a deeper dive into an important facet of this last item, exploring the benefits of off-site construction.
Off-site construction refers to a range of practices that shift construction activities from the construction site to remote manufacturing facilities, including modular construction, panelized construction and component assembly. Modular construction is the fabrication of complete building volumes (e.g., rooms, suites) off-site, usually with interior finishes and mechanical, electrical and plumbing systems installed, that are then trucked to the site and stacked like blocks to create complete buildings. Panelized construction usually involves the off-site fabrication of wall, floor and roof assemblies for transportation to the construction site where they are craned into place and anchored to the building’s structural frame. Off-site fabricated wall panels can come to the site fully insulated and sheathed, in some cases with exterior cladding and even interior finishes already applied. Some estimates put the size of panelized and modular construction to be at least $200 billion by 2030.
In search of construction economies and embodied carbon reduction, mass timber enthusiasts have brought renewed attention to panelized construction, but metal building offers unique opportunities to achieve similar results. Mass timber technologies like cross-laminated timber (CLT) can serve as effective substitutes for composite concrete/steel or stick-framed wood floor and roof decks, and in some cases concrete shear walls. However, the material is not as well suited for exterior walls due to it mass, limited thermal resistance, and lack of voids for electrical and data wiring. By contrast, metal-framed panels offer some unique opportunities for high-performance, low-carbon exterior enclosures.
One of the primary advantages of panelized construction is precision assembly. Traditional on-site construction involves coordination of multiple trades working in less-than-ideal weather conditions, often high above the ground, which can lead to performance defects and schedule delays. Off-site panel assemblies can be fabricated and shipped nearly complete, with sheathing, insulation, weather-resistive barrier (WRB) and windows, and in some cases exterior cladding and even interior gypsum board. The controlled conditions in a fabrication plant allows for tighter joints, careful installation of the WRB over dry substrates, reduced thermal bridging, and other measures to improve building energy performance and reduce operational carbon. Interest in passive house and zero net energy/carbon building is increasing, and these demand reduced air infiltration and thermal bridging.
The efficiency and precision of off-site fabrication can also increase material efficiency and reduce waste. Cold-formed metal framing for example, can be fabricated to the required length from coil, avoiding wasted cut-offs. The controlled conditions in a plant facilitate careful management and optimized usage of materials which reduces waste, and greater ease of recycling of the waste that is generated. The Waste & Resources Action Program (WRAP) in the UK estimates that off-site construction can reduce waste as much as 90%.
Finally, panelized construction offers significant cost advantages, especially related to labor and schedule. The construction industry is facing labor shortages (Association of General Contractors survey revealed that 80% of contractors report difficulty finding qualified workers) and coordination between trades, weather conditions and other factors create many inefficiencies in on-site construction. Off-site assembly allows for significant efficiency with labor allocated over multiple projects under one roof, with further productivity achieved through automation, access to precision tools, and better managed logistics, not to mention safer and more thermally comfortable working conditions. Off-site panel fabrication is estimated to require 50% to 75% less time than traditional site assembly. Reduction in waste, efficient use of materials and elimination of on-site material storage can save an additional 25% in associated costs.
Because code requires inspection by the authority having jurisdiction (AHJ) prior to covering voids in walls, off-site construction necessitates unique consideration. Panelized and modular construction must both be shipped and installed without interior cover (e.g., gypsum board), or manufacturers must work with a qualified/certified third-party inspection firm (or in some states, be approved for self-inspection by the AHJ) to perform pre-cover inspections in the factory. This process can reduce schedule risks by removing some of the coordination and delays associated with on-site inspections by the AHJ.
The construction industry is overdue for disruption. Off-site panelized construction offers quality, performance, cost, schedule and carbon reduction advantages, while retaining creative freedom and design customization for architects and their clients.
Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions in Portland, Ore. To learn more, follow Scott on Twitter @alanscott_faia. Samir Mokashi is a founding principal of Code Unlimited, a minority-owned firm in Beaverton, Ore. IG: @codeunlimited. He has more than 30 years of building code, fire and hazardous materials consulting experience. Learn more at www.codeul.com.