Windows: An integral part of the building envelope

When open, awning windows create a canopy-like projection, offering protection from the elements.

Framing material
All of the styles of windows reviewed in this article can be manufactured from a variety of materials, so it is important when assessing both existing windows and replacement and repair options to consider not only the operability of the window, but its material composition as well.

Aluminum is the most widely preferred material for commercial applications due to its low cost, versatility, lightweight, ease of maintenance, broad availability, and variety of finishes. Aluminum windows have a poor reputation for thermal integrity, which has been addressed by incorporating thermal barriers.

Although broadly used before the development of its aluminum counterparts, steel’s higher cost, density, and difficult maintenance (due to possible corrosion) have substantially reduced its prevalence. Nevertheless, steel windows have found their role in the industry whenever fire resistance is required.

In the past, wood was used on commercial applications, especially on large institutional or educational buildings. In both replacement and new construction, aluminum windows are now usually specified in place of timber. While wood windows are chosen, it is generally for aesthetic reasons, or
for replacement of historic windows in residential and light commercial applications. While wood windows have a good reputation for thermal and insulating qualities, proper maintenance and care to reduce exposure to moisture are crucial to avoid rapid deterioration. Wood frames can also
be clad with vinyl or aluminum for added protection.

Made from polyvinyl chloride (PVC), vinyl is a rigid and tough material extruded to form the window elements at a reasonable cost. It offers good sound and thermal insulation properties. However, limitations on size and finish colors, vulnerability to fire, sensitivity to extreme cold and ultraviolet (UV) light, and difficulty with refinishing have limited vinyl window use to primarily residential applications. An alternate material to vinyl is fiberglass, which has similar sound and thermal insulation properties, but is more durable. However, fiberglass windows are more costly than vinyl and need to be painted from time to time.

Identifying problems
When an architect or engineer evaluates an existing window, he or she will first establish the style and composition of the window system, then investigate common sources of trouble typical to the type of window assembly and materials.

Water infiltration
Water infiltration can be as minor as small droplets on the interior after a huge storm, or as major as ongoing moisture intrusion leading to structural degradation and mold issues. In both instances, it is critical to determine and address the root cause of the water entry, because even a minor leak can cause serious damage over time.

Moisture-related failures can occur in the window itself, in the rough opening around the perimeter, and sometimes in an adjacent wall assembly. It is good practice to have a building envelope consultant confirm the source of failure, so as to develop the most appropriate repair program for the window or windows in question.

With the arrival of HVAC systems, windows have become a secondary or emergency means of ventilation. Moreover, the windows need to remain airtight to minimize heat and humidity transfer as well as maintain the HVAC system’s efficiency. The same seals protecting against moisture infiltration also guard against unwanted air transfer. Drafts observed near windows, usually accompanied by water entry, can be signs of compromised glazing, perimeter seals, or weather stripping.

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