July 28, 2019
by Jonathan Dickson, P.Eng., BSS
Windows are a critical component of buildings. While they are arguably less energy efficient than the opaque wall alternative, the benefits of windows to building occupants cannot be ignored. Windows are necessary to connect occupants with the outdoors. They bring in light and provide a potential for natural ventilation—both have positive health impacts and improve worker efficiency in office settings. Unfortunately, the installation of a window requires the creation of a hole in an otherwise watertight building envelope, thereby increasing the potential for leakage.
Much has been written about the performance attributes of various window systems, but the practical considerations of a window replacement project are rarely discussed. This article explores some of them, and how they impact building occupants, property managers, consultants, and contractors. While windows are the focus of the article, the recommendations presented are applicable to all glazing systems.
Why are window replacements required?
Window systems are a highly manufactured product as the insulated glazing units’ (IGUs’), gaskets and framing sections are assembled and sealed in a controlled factory setting. The factory-assembled nature of windows is a positive attribute, as human error and weather-related factors are less likely to impact the product’s quality, resulting in a system with a less variable service life than those assembled onsite. A consequence of this manufacturing process is the difficulty encountered during rehabilitation if the primary seals of the window system (those installed between framing sections at the time of assembly) fail (Figure 1). Conversely, site-built assemblies (curtain wall excluded) generally provide easier access to primary seals without the same level of deconstruction required for a window system.
A common approach to addressing a failed window seal would be application of sealant on top of the joint, while the failed internal sealant remains in the system in lieu of deconstruction and reassembly of the window frame. Elastomeric sealants are suitable products but can only be used to address leaks for a certain period of time. This surface application of sealant is a short-term approach to repairing the leak and can only be repeated so many times before building occupants are inconvenienced and potential for damage to adjacent wall elements prevail (Figure 2).
In addition to water leaks, window replacements may also be warranted to address non-water related issues such as thermal comfort, condensation, esthetics, or functionality.
What should be considered during window system selection?
Window systems come in a variety of framing materials, finishes, and IGU compositions and operation types. Many of the options for selection are related to their performance, which is not the focus of this article.
How can a building team ensure the window replacement project runs smoothly?
In an ideal world, occupants would be asked to vacate the building when windows are being replaced. However, building teams are rarely lucky enough to replace the windows in an empty building. Windows are usually replaced in fully occupied and operational offices or residential spaces with a long list of restrictions on how the work is to be carried out. These constraints include, but are not limited to:
These constraints are not mutually exclusive and play a large role in project schedule and cost. How the project is planned and communicated is of equal, if not greater importance, to success than the window system selection itself. Below are some recommendations to improve the planning and subsequent success of a window replacement project in an occupied building.
Communication is critical
Building occupants tend to get frustrated if they are uninformed of an ongoing project’s impact on their lives. Clear, concise communication of the schedule and required preparation of the occupants is a must for project success. While simple in principle, this can prove difficult at a practical level. Specifiers need to pay greater attention to the front-end documentation to outline site-specific contractual obligations to establish the required communication framework.
A proper window replacement requires access to the exterior, which is highly weather sensitive. A few days of wind or rain can set the whole project back several days. The potential (and likelihood) of weather delays needs to be established as an expectation for building occupants. Once weather delays are experienced, a revised schedule must be communicated to residents to re-establish expectations for the execution of work. The process is repeated during the entirety of the project. This required updating of the project schedule should be incorporated into the specifications based on an interval agreed on with the owner.
The preparations required by the occupants prior to the window replacement include, but are not restricted to:
Failure of the owner to notify building occupants of these required preparations may result in schedule delays, impacting all stakeholders in the window replacement project.
Overall, clear communication is required throughout the process to establish and update expectations of the building occupants to ensure the project is successful. This communication should not only be preferred by specifiers, but also recommended to become a contractual obligation within the front-end documents.
How will the work be completed?
Execution of the window replacement can generally be completed in one of two ways: floor-by-floor or drop-by-drop. A hybrid of these options can also be utilized. There is no right answer for the ideal execution method at all properties. Selection of the execution method needs to be based on the type and orientation of the building, window system selection, and overall scope of the project. This is an important practical consideration that should be discussed with all stakeholders prior to tendering of the project as both cost and schedule could be greatly affected.
The floor-by-floor method would see all glazing systems on a single level (or large sections of a floor) being replaced at a single time (Figure 4). This option has multiple advantages such as:
There are also disadvantages to this method of execution. The primary disadvantage would be that this method is not always practical. If multiple elevated platforms (e.g. swing stages) are used in parallel to complete work sequentially, floor-by-floor requires all crews to operate at the same efficiency. If a crew works slower or a platform is not used due to mechanical or weather delays, the whole process slows down to accommodate everyone. Often this method starts as a floor-by-floor execution but quickly transforms into a hybrid model as one crew carries onto the next floor due to delays by other teams. If it is mandated that all the work must be carried out on the same floor, the project cost increases accordingly to account for foreseeable delays.
The floor-by-floor method can also be carried out by mast climber with its own set of practical considerations (discussed later).
The vast majority of window replacements in occupied buildings are completed using a drop-by-drop approach. In this method, a suspended platform is used to replace all windows sequentially on a specific vertical drop of a building before moving to another location (Figure 5). This procedure is often used with multiple platforms (most commonly swing stages) on different drops at a time. As a result of the use of multiple independent platforms, this execution method is more practical than floor-by-floor. Conversely to the floor-by-floor, this method requires more interior security as crews are on multiple floors at the same time and will also result in more inconvenience to occupants as the same space will need to be accessed multiple times (often weeks apart) to replace windows at different locations.
How should the exterior be accessed?
Accessing the exterior of a mid- or high-rise building makes up a large portion of the overall cost of a window replacement project. Access is an important aspect to consider as it has potentially large implications to project schedule and cost. The exterior of tall buildings can generally be accessed in one of two ways: swing stage or mast climber.
A swing stage is a counterbalanced, suspended work platform tied back to the buildings anchor system at the roof level. The swing stage is often the least expensive means of exterior access to carry out work and offers the most flexibility. However, there are exceptions where a mast climber warrants consideration. A mast climber is expensive and would likely not be cost effective unless a more holistic rehabilitation program is being untaken (e.g. balcony or wall rehabilitation and over cladding), or if the glazing area is a substantial portion of the wall. A mast climber is beneficial when an extensive amount of work is required within the same horizontal line. Horizontal strip windows and curtain wall systems are prime candidates for a mast climber if the building orientation is conducive to the use of one. A swing stage may also result in a longer schedule if a project constraint is to minimize interior transportation of materials. For example, existing windows are to be removed and all new materials are to be brought to the work area via swing stage. Since swing stages typically have a weight capacity of around 454 kg (1000 lb) and require two persons for operation, the glazing system is often brought to the ground before the platform is loaded with the new system, potentially increasing the timeline compared to a mast climber.
A mast climber rests on grade with intermediate lateral tie backs to the building (Figure 6, page 40). Most of the tall buildings facilitate underground parking, meaning the weight of the mast climber rests on a suspended slab (podium deck). This requires shoring of the underground levels to carry the weight of a mast climber that is intended to carry several people and hundreds of pounds of materials. At-grade building features, such as awnings or canopies, and pedestrian and vehicular traffic need to be considered. If the building contains multiple overhangs, changes in plane, balconies, or terraces, the mast climber becomes less practical compared to swing stages that more readily adapt to these conditions. A single mast climber needs to follow a floor-by-floor approach and since all workers are on the same platform, the potential for one crew to work ahead of the others is minimized. The mast climber is also less susceptible to wind-related delays, thus allowing for a less variable schedule.
While the contractor controls means and methods for execution of the work, a specifier is recommended to ask for alternate pricing during the tender phase to allow for an educated decision on means of access and its impact on project time and cost.
When should the window replacement be completed?
Like all exterior rehabilitation work, window replacement in warmer weather is more likely to achieve a durable installation as worker comfort improves and adhesives and sealants are installed within their ideal temperature ranges. Installation during the warmer months also removes potential for membranes being installed on cold or frosty surfaces and reduces the possibility for temperature-related complaints from occupants during the replacement. Finally, any quality assurance (QA) testing, such as ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors and Curtain Walls by Uniform Cyclic Static Air Pressure Difference, is only feasible if the exterior temperature is above 6 C (43 F).
In order to undertake a window replacement project, it must first be designed and tendered with the contract awarded before materials are ordered. It is also recommended to complete a site mockup for performance testing and esthetics. The ideal installation time is between late spring and early fall. It is recommended to award the contract in November-December of the year prior to the planned time for window replacement as it would typically take eight to 12 weeks to assemble IGUs and the frames and an additional four to eight weeks would be required to install, review, and test a mockup. Allowing a three-week tender period and a similar lead time for design would mean the project design should commence in September of the year prior to window replacement. It could be 26 weeks or more between the time the contract for design services is issued and the actual replacement begins. This is an important practical consideration for owners regarding the timing of the window replacement process.
Windows are a very necessary component to the building enclosure and require careful review of types, orientations, and detailing for long-term performance. Equal consideration should be given to the window replacement process. Setting realistic expectations, keeping the stakeholders informed, selecting a means of execution specific to the building, and starting the process within realistic timeframes will increase the potential for success. These can be incorporated into the project documents by specifiers and requested by the owner. With due consideration, window replacements can be executed with minimal impact to building occupants and on time and budget, thereby benefitting all stakeholders.
Jonathan Dickson, M.Eng., P.Eng., BSS, LEED GA, is senior project manager at Pretium Engineering. He has been actively involved in the industry since 2010 with extensive experience in restoration of existing buildings. His experience ranges from localized leak assessments to multimillion dollar rehabilitation projects. Dickson can be reached at email@example.com.
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