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Clear-cut Criteria for New Windows



Understanding the most common pitfalls with window selection, design and installation is the key to preventing problems


By David A. Deress  


Windows are a key element in virtually every existing building, as well as in new building design and construction. Selection, design and installation of windows for a building is an important process that can lead to satisfaction or disappointment in the appearance and usefulness of the building.

The features required of windows are numerous. Among the most important aspects that should be considered when selecting a window system are:

  • Resistance to wind loads
  • Resistance to water infiltration
  • Air tightness to prevent drafts
  • Resistance to condensation on the interior of the windows
  • Security and emergency egress
  • Light and ventilation
  • Appearance that enhances the overall design

Whether constructing a new building or replacing existing windows, building owners should consider these aspects in the selection, design and installation of windows. In many cases, other features or functions will be required.

The performance features listed above and others that apply to specific conditions of the project must be prioritized and carefully evaluated. Ignoring any one area that is deemed critical to the needs of building occupants can lead to unsatisfactory results. A simple air or water leak has led to many frustrated occupants and owners, not to mention associated building damage and even lawsuits. Selection should start by consulting the owners and occupants of the building to assess needs based on the identified requirements. Then, designers and manufacturers can address those needs.

New state-of-the-art products or components can fail, go out of style or simply go out of production because of poor sales or poor function. Products or components that have not experienced the test of time under field conditions may result in failures that were not demonstrated in laboratory tests. The simplest window design that meets all of the established criteria is usually best. To obtain a more objective opinion, temper the advice and sales pitches of product representatives with recommendations from designers and users of the product.

Cost should not be the primary factor in choosing windows. Settling on the cheapest window product can lead to low durability, poor performance, water leakage and increased or difficult maintenance. This also holds true for the associated materials and work to provide the proper interface between the window and the wall. If a selected window has all the right features, but the proper flashings are not provided to create a proper interface with the adjacent wall materials, serious water leakage problems could occur that ultimately will cost far more to repair in the long run.

Design Criteria

The top three features in any window selection process are resistance to wind load, water and air infiltration, and the interface of the window system with the walls and exterior cladding. The most visible and common problem that occurs is water infiltration.

Selecting the window with the appropriate design rating should provide proper performance of the window unit. If the interface of the wall opening to the window and of the window to the cladding system is not designed properly, any one of these can be adversely affected even with a window unit that performs adequately. The designer must know the limitations of the window as well as the limitations of the wall and cladding system. For example, a window meant for a punched opening in a masonry wall should not be used in a stud wall with sheathing.

Detailed construction drawings illustrating the various conditions are needed to portray to the contractor what is to be installed and what materials are to be used. The primary point is to express the design intent. If a window product from a specific manufacturer has been chosen in the design stage, many construction details can be worked out early, taking into consideration the specific features of the selected product. This can be helpful in minimizing unanticipated costs and making sure that all of the desired features have been included.

Drawings Crucial

In many cases, however, the manufacturer has not been selected during design. In this case, the designer’s details can only be generic because they will likely differ slightly to significantly depending on the product chosen. In this case, certain details will need to be developed in the construction process. This can sometimes lead to extra installation costs for work that was not shown or anticipated on the construction drawings.

In either case, shop drawings, and sometimes erection drawings, should be prepared by the contractor with the assistance of the manufacturer to illustrate the specific details of window construction and installation required for the project. Preparation by the contractor or manufacturer of drawings should be required in the specifications. These drawings allow the designer to review whether the products being shown are going to achieve the design intent. It also allows those involved to solve design and installation issues, such as how the windows will be attached to the wall and how flashings are to be fitted to control water infiltration. Costly delays have been known to happen on projects where the fit-up of the window system with adjacent materials was not fully considered in the shop drawing phase.

Specifications prepared by the designer must supplement the drawings to list various requirements, such as structural performance, water and air infiltration, and other features and components to be provided. Without complete information provided in the drawings and specifications, the project could be headed toward extra costs and compromises in details that are not as durable or effective.

In addition to shop drawings, field mock-ups and testing should be required in the specifications. This requires the window manufacturer and the contractor to prove that the product as installed will function as designed. It also allows an opportunity to adjust the design and installation procedure in the field to get the proper results before many or all windows are installed.

Lack of mock-up installation and testing has led to major setbacks in time, additional cost and delayed occupancy because the problems were found too late in the installation process. It is less costly to remove and rework a single mock-up window than it is to remove and rework every window in the building. A mock-up installation adds cost to the project, but it is money well spent to assure properly functioning windows.

The designer and manufacturer’s representative should be requested to make site visits during construction to observe the installation work as well as the mock-up installation and testing. Their input can be invaluable during the mock-up work when adjustments or clarifications of the design drawings or shop drawings are needed.

The responsibilities of creating the interface of windows with adjacent cladding system are not always clear. This requires careful coordination among numerous building trades that must consider every other element adjacent to but beyond their scope of work. For example, if the sheet metal subcontractor is not given the proper dimensions or configuration for a sill flashing, the windows or the cladding may not be able to adjoin properly, resulting in leakage, or the flashing may not be able to collect and control water as the designer intended.

Installation Considerations

The workers installing the mock-up and observing the testing should be the actual people installing the windows because they are the ones that learned from the mock-up test experience. Deviations from the final, approved mock-up installation can lead to water and air leakage that can be devastating in terms of damage to the building and loss of use.

If unanticipated conditions arise during the installation work, the contractor must communicate and coordinate with the designer and manufacturer to resolve the issue. As-built conditions that get ignored can result in failures of various types, such as water leakage, air infiltration or excessive deflection. It is always less costly to resolve such conditions during construction than waiting until later.

As simple as it sounds, the manufacturer’s installation recommendations are not always followed — usually to save time to meet the project schedule. If a contractor decides on its own to make a change or address an unanticipated condition without seeking the proper advice, it is possible that the installation instructions could be violated, which ultimately could mean that the warranty could be voided. This consideration alone is reason enough to have the designer and manufacturer’s representative on site during the installation.

If choices of materials for certain items are left to the contractor or if the contractor changes the type of materials for some reason, it is critical that the contractor verify that the materials used are compatible with each other. Consider metals and sealants. Contact of differing metals can cause a galvanic reaction that produces rust and deterioration. Sealants that are not able to adhere properly to the substrates or other sealants are going to fail and allow leakage. These problems also lead to higher maintenance costs, reduced durability and shortened window life.

In a typical building project, windows represent a sizeable portion of the building’s envelope and a significant portion of the project cost. The selection of windows for a project can be fraught with confusion for the architect, contractor and building owner. Literally hundreds of window manufacturers offer different combinations of these features at widely varying prices.

It takes teamwork and communication among the owner, building users, designers, manufacturers, contractors and others to make sure that each feature is identified and prioritized. By assembling the best project team as early in the planning stages as possible, and making sure that all parties remain involved throughout the project, facility executives can avoid most common problems and failures.

David A. Deress is a consultant and branch manager of the Seattle office of Wiss, Janney, Elstner Associates. Anthony D. Cinnamon is a senior architect and engineer with the firm.




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  posted on 2/1/2003   Article Use Policy




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