Cost considerations
It is difficult to compare window, storefront, and curtain wall products in window wall applications without talking about relative cost. When comparing only the glazing system itself, window walls (especially those utilizing window products) are generally less expensive than curtain wall systems. This is why window wall systems are often proposed as value engineering alternatives on projects.
Curtain wall systems, specifically unitized assemblies, have a high degree of prefabrication. This contributes to the higher initial cost, but also reduces the labor and duration of field installation. Installing curtain walls often requires a crane or specialty hoists and a combination of interior and exterior façade access. The ability to install window wall systems from the interior can provide inherent benefits to the construction schedule and overall installed cost when compared to curtain wall installations. This installation sequence also frees up exterior access so other façade work can be completed, and it allows the building to be ‘dried in’ one floor at a time. This follows the typical sequence of the interior framing and finishes work, rather than proceeding by elevation, which can fall out of sequence with the crane operation, framing installation, and other work.
However, most window wall systems, even those with ‘bypass’ frames, require a considerable amount of slab-edge detailing to complete the exterior enclosure, including installation of a WRB, perimeter flashings and sealant joints, exterior insulation, and cladding. Many of these components require exterior access to complete the installation. These material and installation costs are not always accounted for when comparing the overall installed costs of window wall assemblies to curtain wall assemblies.
Ultimately, it is difficult to predict the final installed cost of window wall applications installed slab to slab versus curtain wall applications installed outboard of the slab edge without performing a detailed cost estimate based on the proposed design and installation. More recently, construction budgets have been determined before the glazing system details are fully developed; these budgets include general square-foot prices that do not account for project-specific slab-edge detailing or installation sequencing constraints, and may not accurately compare the various glazing systems.
Conclusion
All things considered, window walls are becoming more common in the building industry. It is critical to not lose sight of the subtleties in performance, detailing, fabrication, and installation that define these systems in order to avoid potential pitfalls as project teams continue to balance performance, constructability, schedule, and cost. This is why it is important for project teams to review, plan, and set performance expectations appropriate for the type of glazing system(s) selected at the start of a project.
Philip Frederick, PE, is a senior project manager in Simpson Gumpertz & Heger’s (SGH’s) Building Technology group. His primary experience involves building enclosure design consulting and investigation of existing building enclosure systems for owners, architects, and general contractors. He can be reached at pfrederick@sgh.com.
Brian Rose, PE, is a senior staff member in SGH’s Building Technology group. He is experienced in the areas of new design enclosure consulting, rehabilitation, and investigation projects for a variety of owners, architects, and general contractors. Rose can be contacted at bsrose@sgh.com.
Bradford Carpenter, PE, is an associate principal in SGH’s Building Technology group. Experienced in investigating, rehabilitating, and designing building enclosure systems on historic and contemporary structures, he specializes in designing and integrating complex building enclosure systems including waterproofing, air and water barriers, rainscreen cladding, and fenestration systems, with a focus on design efficiency, constructability, and performance. Carpenter can be e-mailed at bscarpenter@sgh.com.
