by Cynthia L. Staats, PE, LEED AP, and Michael J. Louis, PE
Nothing gives construction teams heartburn faster than leaks in a newly installed curtain wall or storefront assembly. With the growing desire for glass-clad buildings and shorter construction schedules, there is a rise in easily avoidable leaks in glazing systems due to installation omissions and shortcuts. Although some leaks can be easily remedied, others are systemic and can be persistent for the life of the glazing system.
The details outlined in this article are some of the more consistently overlooked items during a glazing system’s installation. It is a good practice to discuss these items with the construction manager (or general contractor) and the installer at the beginning of the glazing system installation. While the shop drawing and submittal process may be intended to cover some of these items, the individual who usually develops and submits the shop drawings is not always the person who installs the glazing system.
This article explores stick-built and field-glazed installations, but much of the information can also be useful when inspecting some aspects of a unitized wall assembly at a fabrication shop. Correcting these deficiencies after the installation is complete can be costly both in terms of money and schedule.
Starting with the sill receptor
The last line of defense for directing incidental leakage back out of the glazing system, the sill receptor goes by several names, including sub sill, starter sill, and receiver. During the system’s installation, there are two important aspects to ensure—the sill receptor is appropriately sized for the glazing system and its splices are properly detailed. (While the installer is ultimately responsible for this aspect of the job, projects do not always play out in the way they are supposed. It is beneficial for the whole project team to be aware of these issues, but each person has his or her own role with which they are usually preoccupied. The last line of defense would be the building envelope engineer—it would be helpful for the building envelope engineer to bring this to the project team’s attention during construction administration site visits. However, the contractor is ultimately responsible to ensure this.)
A sill receptor is made out of a two-dimensional extrusion; its main function is to provide a means for anchoring the sill of the glazing system product. As a secondary function, the sill receptor is intended to function as a ‘pseudo-flashing,’ containing and draining the infiltrating water back to the exterior. To suit this latter function, the sill receptor must be fitted with end dams so the infiltrating water does not simply flow off the ends of the sill receptor and leak to the building interior.
To create the end dams, flat stock aluminum is fastened to each end of the sill receptor extrusion, and sealant is applied where the flat stock contacts the extrusion to create three-sided containment. We have often observed the installer cutting the sill receptor to the same length as the glazing system frame and then forcing the glazing system onto the sill receptor (which ends up too snug). This results in end dams being forced outward and the sealant between the end dams and the sill receptor tearing apart. The same outcome can also occur when the installer is not careful in inserting the glazing system onto the sill receptor or if the installer omits the fastener securing the end dams on the sill receptor.
It is very difficult to achieve a reliable seal between the sill receptor and the end dam after the glazing system is installed. If the end dam seal is disrupted during installation, the best practice is to go back and create a continuous seal before proceeding with the installation.
Likewise, the splice between two sill receptor extrusions is another critical seal that cannot be reliably sealed after the glazing system is installed. The splice is typically made by bedding an L-shaped piece of aluminum or silicone membrane in sealant at the joint. Therefore, the installer can only access the upturned leg portion of the sill receptor after the glazing system is installed. The horizontal portion of the extrusion cannot be accessed after the glazing system is installed. If this seal is omitted during installation or not properly formed, water collected by the sill receptor can leak into the building.