WINDOW TECHNOLOGIES: Assembly

Frame Types

Manufactured Window Units
There are many commercial buildings with punched openings that use either standard or custom-manufactured units. A series of units can be placed together with intervening mullions to create a window wall with a larger expanse of glazing more commonly associated with curtain walls and storefronts. Manufactured units can be made with many frame materials, including aluminum, wood, vinyl, fiberglass or other composites. Aluminum is the most widely used frame type in the commercial window market.

The material used to manufacture the frame governs the physical characteristics of the window, such as frame thickness, structural concerns, weight, and durability, but it also has an impact on the thermal characteristics of the window. Manufacturers are beginning to produce hybrid or composite sash and frame units, in which multiple materials are selected and combined to best meet the overall required performance parameters.

While it is useful to understand the role that frame type plays in window thermal performance, the frame U-factor is not normally reported by manufacturers. The window U-factor, as given on an NFRC certified rating or label, incorporates the thermal properties of both the frame and the glazing. The remainder of this section describes aluminum, wood, and vinyl frames, and introduces some new frame materials that are commercially available.

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Curtain walls and Storefronts
A curtain wall is an external, nonbearing wall that is intended to separate the exterior and interior environments. Unlike manufactured windows or skylights placed into a wall or roof opening, curtain walls can compose the entire outer skin of the building. They consist of vision glazing as well as opaque spandrel panels. The spandrel panels may be glass as well, forming an exterior envelope made completely of glass and framing materials.

Similar to a glass curtain wall, a storefront is a system of doors and windows mulled together as a composite structure. Storefronts are typically designed for high use and strength. A storefront system is usually installed between floor and ceiling on the street level of a building.

Aluminum is used almost exclusively as the frame material in both curtain walls and storefronts. Some steel is used as well, but is often clad on the exterior with aluminum or stainless steel caps. For larger glazing areas, the structural strength and durability of aluminum and steel cannot be equalled by other materials.

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