Windows: An integral part of the building envelope

All images courtesy Hoffmann Architects

by Juan Kuriyama, AIA
With reports from clinical research substantiating the importance of natural lighting and ventilation in workplace productivity, health, and well-being, window design has gone beyond building aesthetics and energy efficiency. From the capacity to create dramatic gestures in glazing—afforded by glass curtain wall technology—to improvements in thermal transfer made possible by chemical advances in the industry, and the race to develop carbon-neutral strategies like daylighting IN ORDER TO reduce demand on the power grid, windows have taken on a multifaceted significance in building design.

While it is tempting to give little thought to existing wall openings and not go beyond immediate maintenance demands, periodic repair and replacement projects are opportunities to rethink the window design and to also determine if the existing system meets the needs of the structure as a whole. Although window rehabilitation options may be restricted by many factors—from landmark regulations and structural limitations to aesthetic concerns—it is often possible to improve energy efficiency, optimize light transmittance, and shape building appearance both inside and out. Since deterioration in one building component can adversely affect interrelated elements, addressing distress or inefficiency in the fenestration not only impacts other building systems, but, as new research has touted, also shapes one critical part of the building dynamic: the occupants.

A window components primer
Before considering rehabilitation options for existing windows, it is important to know the composition and properties of the existing wall system to develop an appropriate, compatible repair or replacement design. An architect or engineer experienced in window rehabilitation can determine the structural and material properties of the windows, as well as their condition.

Wall system construction
In contrast to a glass curtain wall—which supports only its own weight (not the weight of the building structure), and is composed mainly of glass units anchored to a frame—a punch opening, or exterior wall opening, is an open space in an exterior wall around which the weight of the wall is directed. The opening can remain unobstructed, as on a balcony or terrace, or it can be filled with a window, louver, door, or storefront. Whether the building is constructed with loadbearing walls and punch openings or glass curtain walls will determine the options available for retrofit or rehabilitation.

Sealant failure at the exterior of a window.

For the purpose of this article, the author will examine the standard punch-opening window, and not the glazed curtain wall, which has its own considerations and options. The basic components of a punch-opening window are:

  • Lintel: It is the horizontal section at the top of the opening, which distributes the load of the wall above the window into the vertical sections on both sides. In order to accomplish this structural task, the lintel employs one of various types of arches, or it manages the load via sturdy materials, such as structural precast or reinforced cast-in-place concrete, stone, reinforced concrete masonry unit (CMU), or steel angles. The lintel system also serves as a water infiltration barrier by means of flashings.
  • Sill: A horizontal section at the bottom of the opening, the sill protects against water infiltration and carries the load of the window into the wall below.
  • Jambs: As the vertical wall sections to each side of the opening, jambs carry the load transferred by the lintel into the wall below. Depending on the size of the opening, they may need to be reinforced.

Window type
The terms “double-hung vinyl windows,” “casement wood windows,” and “pivoted aluminum windows” represent two methods of classifying windows—by operability and framing material. The first two terms refer to the manner in which the operable panes, or framed sheets of glass, move relative to the frame, or fixed portion of the window. The frame structurally secures the window to the perimeter of the wall opening, while the panes support the glazing. Spacers, seals, gaskets, and other accessories employed to fit the glass within the frame are also considered part of the glazing, not just the glass sheet itself. Depending on the style and operability of the window, various hardware elements, such as locks, weights, balances, handles, stops, hinges, or weather stripping, complete the system.

Listed below are some of the common window types, classified by operability:

  • Single- or double-hung: Two sashes sliding vertically in adjacent planes. “Single-hung” refers to one operable sash, and “double-hung” to two. Taller openings may have triple-hung windows. Although familiar to many homeowners, this window type is also widely used on high rises, due to its ease of operation and good wind resistance.
  • Sliding: These windows operate like their single- or double-hung cousins, but slide horizontally. Maintenance of track hardware to remove dust or accumulated particles is critical for proper operation.
  • Casement: One or more sashes swinging outboard or inboard from the vertical frames. Although common on residential applications, consideration must be taken with high-rise commercial use, because the sash and hardware have lower wind pressure tolerance.
  • Awning: One or more sashes swinging outboard from the upper frame. The name is derived from the weather protection offered by the window in the open position, when it creates a canopy-like projection above the opening.
  • Hopper: Operates like the awning style, but this window swings outboard from the lower frame, rather than the upper one.
  • Pivoted: A single sash rotating around a vertical or horizontal axis centered on the frame. Easy to clean, but offers little resistance to wind stress.
  • Fixed: Also known, more familiarly, as “picture windows,” they do not need operable hardware, and so achieve the maximum glass area for a given opening.
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