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By Benjamin Lund
February 2002 -
Windows & Exterior Walls
There is no universal replacement window. Successful window replacement projects require careful consideration of the overall impact new windows will have on the facility. Without a look at the big picture, replacement windows may fail to fully meet all facility needs or create more problems than they solve. Four major elements must be considered: energy, maintenance, security and aesthetics.
The most common reason for replacing windows is to improve energy performance. Facility executives have many options for improving window energy performance. They can choose insulated, low-e, spectrally selective or reflective glazings. Reflective or spectrally selective films can also be applied to existing windows.
The thermal efficiency of insulated glass has resulted in its almost universal use as a replacement glazing. Insulated glass offers a slight improvement in security by providing an additional layer of protection. It can help to reduce building maintenance requirements by eliminating or drastically reducing condensation on the interior surfaces of windows, particularly in cold climates. It has little or no impact on building aesthetics. In most cases, two layers of glazing is the most cost-effective choice. The additional cost of triple glazed units is justified in only far northern climates with high heating loads.
Low-e glazings are also widely accepted for energy efficiency. Low-e glazings use a thin, metallic oxide coating deposited on the glass to reduce radiant heat loss through the window. Virtually invisible, the coating improves the thermal efficiency of the window by 10 to 15 percent.
Low-e glazings are best in applications where annual heating loads are greater than cooling loads. Low-e coatings have no impact on maintenance or building security and will not produce any significant change in the building’s appearance.
Spectrally selective glazings and films have evolved from older tinted glazings that reduced the amount of visible light, infrared light and ultraviolet light coming through the window. While these helped reduce the cooling load, they also increased the need for lighting. Spectrally selective glazings use a thin metalized layer to allow visible light to pass through the window, while blocking a portion of the heat-producing infrared radiation. The materials and the thickness of the layer determine how much infrared radiation and visible light pass through the window.
The warmer the climate and the greater the solar load, the greater the benefit of spectrally selective glazings. They have no significant impact on maintenance or security, but they affect aesthetics, as most have a light blue or light green tint.
Reflective glazings and films produce mirror-like windows. These glazings typically are constructed to reflect between 30 and 70 percent of both heat and visible light.
Reflective glazings are most often used in commercial facilities with large areas of glass, located in warmer climates. While the glazings have no significant impact on building maintenance, they reduce daylight in building perimeter areas, so lighting energy use rises. Since reflective glazings prevent anyone from seeing into a facility during daylight hours, and greatly reduce the ability of occupants to see out at night, they have security implications. They greatly alter the appearance of the facility.
Maintenance factors are often overlooked when evaluating window replacement projects, even though window maintenance costs often equal or exceed energy costs. There are two major maintenance costs associated with windows: painting and cleaning. Other costs include caulking and general repairs, but these typically are small in comparison to painting and cleaning.
Wood windows must be repainted every four to five years in most applications, at an average cost of $100 per window, including scraping and painting. It is this ongoing expense that has led to the widespread use of vinyl- or aluminum-clad windows, which eliminate the need for exterior painting. The use of clad windows does not significantly affect energy efficiency, building security or the aesthetics of the building.
Painting is also a significant maintenance expense for steel windows. Depending on the climate and exposure of the windows, steel windows typically require repainting every five to 10 years to prevent corrosion and deterioration. That ongoing expense has led to the widespread replacement of steel windows with aluminum, vinyl or wood units. Upgrading from steel generally improves the energy efficiency of the windows with little impact on building security. Depending on the style of the old and new windows, there can be a significant change in aesthetics.
Another big expense is window cleaning, a labor-intensive process, particularly in buildings with windows that do not pivot for cleaning from the interior. Muntin bars make the task even more difficult and time-consuming. Many new windows can be pivoted to allow cleaning from the inside; snap-out muntin bars reduce the time required for cleaning. Replacement windows designed to ease the cleaning process will have little or no impact on the energy efficiency of the window, security or appearance.
Until last year, the biggest security issue with windows was how to make it more difficult to break into a facility through the windows. Now those security concerns have been expanded to include protecting occupants, contents and pedestrians from shattered glass. History has shown that window glass is the single most deadly source of injury in bomb detonations. Facility executives have several options that can help reduce the risk of injury from shattered glass.
One option is specially designed window films applied to the interior surfaces of existing windows. These films are thicker and stronger and use a stronger adhesive than window films for energy conservation. Although the films will not prevent the glass from breaking, they will hold the broken pieces in place, preventing them from flying into the room. Keeping the broken pieces of glass in place will also make it more difficult for an intruder to enter the building through a broken window. The effectiveness of the films is determined in part by the characteristics of the window glass.
Window security film can be clear or tinted to limit transmission of infrared or visible light. Most security films have expected service lives of 15 to 20 years. They do not affect window maintenance, and their impact on aesthetics is similar to that of energy-efficient films.
Another option is laminated glass. When laminated glass is stressed beyond its breaking point, it will crack and break, but the pieces will remain in place, still attached to the window frame. Even if the force is sufficient to cause the glass to separate from the frame, in most instances it will separate in one or a limited number of pieces, minimizing injury or damage.
Laminated glass also offers excellent protection against breakage from forced entry and high winds, including hurricanes. It does not affect energy use or maintenance. It is available as a clear or tinted glazing so it can be installed without altering the appearance of the facility.
(For more about window security, including the use of ballistic glass at the Pentagon, see “Threat of Terror.”)
The number of options available for window replacements or upgrades makes it important for facility executives to plan carefully. At least three issues should be reviewed before a window replacement program gets underway.
1. What are the replacement program’s goals? Typical goals include improving energy efficiency, enhancing comfort, reducing maintenance, increasing security and improving appearance. Too often, though, the goal becomes to replace the windows without a clear understanding of why. Clearly defined objectives make it easier to meet all of the goals.
2. When should windows be replaced? Changes made to the windows will affect other building systems and components. Window replacement may give facility executives the opportunity to modify building lighting systems to take advantage of daylight. New windows can reduce cooling loads, allowing smaller systems to be installed. Replacing windows early in a renovation program will allow facility executives to take advantage of these and other opportunities.
3. How secure must the windows be? Security issues must be integrated into the window selection process, not simply added as an after-the-fact patch. A risk assessment should identify, quantify, and rank the real threats — both natural and man-made — to which the facility is exposed. Replacement window selection must be based in part on the findings of this risk assessment.
James Piper, PE, PhD, is a consultant and writer with more than 25 years of experience in the facilities field.