In addition to providing light and ventilation, windows play an important role as part of the building envelope. When they are performing well, windows help maintain a comfortable indoor environment by limiting conductive thermal losses and gains, regulating solar heat gain, blocking air infiltration and preventing condensation on interior surfaces.
But when windows are performing poorly, they cause headaches for occupants as well as owners. It costs money to replace old windows, but installing new windows might be one of the best investments available, thanks to the energy-saving features being manufactured in windows today.
When to consider replacing windows
Tenant complaints and high energy bills are the strongest indicators it’s time to replace old windows. In cold areas, for example, the outer perimeter can be cold and drafty—downright uncomfortable, if not unusable—due to poor window performance.
“When you have non-insulated glass, without any kind of coating, it can be extremely cold in the winter, and that cold glass will cause an interior draft,” says Kerry Haglund, executive director of the Efficient Windows Collaborative (EWC), Wyoming, Minnesota.
“Also in the summer, the glare and the heat coming through can be extremely uncomfortable, which tends to pull people away from the window,” she says. “And in a commercial setting, that’s leasable space, so it becomes a money issue.” In both cases, pushing HVAC systems harder to compensate for the loss drives up costs without improving comfort.
Fortunately, the materials and technologies used in today’s windows have come a long way in recent years, both in terms of energy efficiency and visual performance. The biggest challenge today is selecting the combination of performance characteristics that are appropriate for each particular installation.
What makes a window
Aluminum, steel, wood, vinyl and fiberglass are common framing materials. The choice of framing is often based on durability, appearance and performance considerations. Aluminum frames are still the mainstay for commercial replacement windows, according to Bill Hoberg, owner of Glass-Rite, Albuquerque, New Mexico.
But that may be changing with the increasing attention being paid to energy efficiency. Even with the inclusion of thermal breaks in an aluminum frame, “You still lose a lot just through the frames,” Hoberg says. “The thermal break stops the cold from being transmitted through the frame, but it’s not an insulator.”
Fiberglass and vinyl are other framing options that generally are better from an energy perspective, but they are not always suitable depending on window size and strength requirements. Of the two, fiberglass is more expensive, even though it’s likely to require painting after 10 years.
Frame construction also can be a concern. “With an aluminum frame, every corner is coped and notched, then screwed together,” Hoberg says. “That leaves room for water or air infiltration at those joints. It’s the same way with fiberglass, whereas vinyl frames are impervious—both the frames and sashes—because they’re welded together.”
Replacement windows can be residential grade, commercial grade or architectural grade, and all of them consist of a frame, glazing and a sealing system. Residential-grade windows generally are manufactured as complete units. Although manufacturers offer various standard sizes, custom sizes that are appropriate for any given situation are readily available.
Commercial-grade windows generally feature more durable construction and are rated for a longer service life. Architectural-grade windows are typically considered part of the curtain wall and have greater structural performance requirements. Architectural windows also are likely to be specified and purchased as components, then assembled and installed by a specialty contractor.
Selecting the window grade often depends on whether the owner is looking for a quick ROI or is more concerned with the long term. “All windows are rated based on their performance and how well they handle wind load, wind resistance, air infiltration and water penetration,” says Renee Doktorczyk, president of ArchiTech Consulting, Inc., Mt. Prospect, Illinois.
A non-prescriptive, performance-oriented and material-neutral standard for rating windows and doors was initially developed in the late 1990s and has gone through several development cycles. Today, ratings are based on the North American Fenestration Standard/Specification for windows, doors, and skylights. Additional performance factors related to energy efficiency are included in the International Energy Conservation Code and ASHRAE Standard 90.1.
“The windows are integral to the way a building performs,” Doktorcyzk says. “When talking about commercial grade, window selection needs to be a carefully thought-out decision, including things like the finishes and what kind of lifespan you expect of the window.”
Then there is the glass itself. “The glass is really the important part of the insulating value of the window,” says Doktorcyzk. “If the owners are looking at both window replacements and a mechanical upgrade, by picking the right glass they may be able to downsize the mechanical equipment, and the whole building will run more efficiently.”
Sealing systems add to window efficiency and vary depending on whether the window has one, two or three panes of glass. In additional to keeping the elements outside, the seals on double- and triple-pane windows seal off the space between the panes to create an insulating layer. For even greater insulation, the space can be filled with a heavy inert gas, such as argon.
What makes a window better
“The low-e coatings are probably the biggest reason why you would switch,” Hoberg says. “You may have a whole building that’s all double-pane windows, but they were put in 20 years ago and don’t have low-e glass. You’re losing a lot of performance because of how much heat is coming in due to the sun. The low-e coatings do a good job helping with the energy efficiency in both summer and winter. The coatings developed in the last five-to-10 years have made window replacement much more worthwhile.”
Hoberg primarily uses “soft” coatings that are applied during the glass-manufacturing process, but after it has cooled somewhat. Compared to the older-style hard coatings, “The soft coats are optically much better,” he says. The coated surface or surfaces are placed facing the air space, which ensures they remain protected.
Low-e coatings are rated based on their emissivity—essentially how effective they reflect heat. Low-e coatings come in various types and can be optimized for heating, cooling and daylighting.
“Coatings are tweaked for climate,” Haglund says, including combining reflective tints and low-e coatings on different layers of glass. “Blocking the solar gain coming through the glass is particularly important in commercial applications that have a lot of floor-to-ceiling glass because a lot of heat can come through that glass.” The best approach is to block that heat and reflect it back out before it enters the building.
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