Picking Panes That Save Energy
In the bygone days of single-glazed windows, significant energy loss often occurred through the glazing. But advances in building codes, which require double-glazed windows, as well as great strides in window and glass production, have made today’s windows quite capable of meeting energy conservation and comfort needs as well as bringing a bit of the outdoors inside.
Improved thermal performance begins with insulated glass, which combines two or more layers of glass, separated by an air- or gas-filled barrier. When gas is used to fill the space between layers, it typically is argon or krypton, both inert gases that cause few problems should they leak from the space. Tinted glass or glazing adds another conservation element by reducing heat gain and glare. Reflective coatings cut heat and light penetration even further but might cause building occupants to feel as if there isn’t enough light in the room. To address this potential drawback, low-e (low-emissivity) coatings reject solar heat while allowing daylight to pass.
By itself, glass is a poor insulator. However, multiple layers of glass separated by air spaces improve the window’s energy efficiency significantly. Basically, insulated glass is two or more layers of glass separated by air- or gas-filled spaces.
To understand how different forms of insulated glass are compared and contrasted requires facility executives to understand two characteristics used by window experts — solar heat gain coefficient (SHGC) and visible transmittance (VT). The former represents the amount of solar heat that is transmitted through the glass. The latter is the amount of visible sunlight that reaches the inside.
As the name implies, insulation is an important property of insulating glass. Insulation value is rated on a scale called the U-factor: The lower the number, the less heat that is transmitted through the material and the better the energy performance.
“When it comes to the windows in a building envelope, the primary concern is not the U-factor, but rather the amount of solar heat gain,” says John Carmody, director of the Center for Sustainable Building Research at the University of Minnesota. “Windows allow significant amounts of solar heat into the building.”
Typical single-glazed windows can have an SHGC of 0.82 (82 percent of the sun’s heat enters) and a VT of 0.88 (88 percent of visible light transmitted). Clear double-glazed units with air gaps generally have an SHGC around 0.70 with a VT of about 0.78, while clear triple glazing would have an SHGC around 0.61 with VT at 0.70.
Varying the visible transmittance might seem restrictive, but Adrian Tuluca, principal of Steven Winter Associates, says that a window with a VT of 0.70 would seem just as clear as a single-glazed unit to most people. “It’s very difficult to see the difference.” Nevertheless, there are times where optimum visual factors are essential, as in planetariums where even a minute difference in visibility would be detrimental.
Today’s building codes in most areas of the country generally require insulated glazing. But not all insulated glazing is created equal. The Insulating Glass Manufacturers Alliance is attempting to have 50 percent of North American insulating glass fabricators certified by the end of this year. Certified fabricators must meet IGMA’s voluntary performance standards, ratings and recommendations.
Double-glazed windows are the most common form of insulating glass. However, there are times when triple-glazed windows are a better choice. “Triple glazing does a better job in controlling solar heat gain, but there is a loss in visible transmittance,” says David Eijadi, vice president, the Weidt Group. “When we do energy simulation modeling, we often find it has a much longer payback period. It’s unlikely that triple glazing would be the best option for buildings with high internal HVAC loads. But smaller buildings in cold climates may find that triple glazing pencils out well. The specific climate and actual utility rate structure will determine that payback.”
In fact, that is what Cambria Office Facility in Edensburg, Pa., discovered. Studies found that the facility more than recouped triple glazing’s initial price tag by no longer needing a perimeter heating system. Perimeter heating in colder regions of the country often is required to compensate for the window’s cold surface, even in double-glazing applications.
The 34,500-square-foot facility was constructed for and leased by the state. Though triple-glazed windows cost about $15,000 more than double-glazed units, the perimeter heating system that would have been required had triple-glazed windows not been specified was $25,000.
According to “Window Systems for High-Performance Buildings” by Carmody, Stephen Selkowitz, Eleanor S. Lee, Dariush Arasteh and Todd Willmert, the Cambria project’s air-conditioning system was downsized from 120 tons to 60 tons. Project architects and engineers estimate that about 25 percent of this reduction — about $10,000 — can be credited to more energy-efficient windows.
Insulating glass windows rely on a space between the individual lites of glass. This space normally is about 1/2 inch and is an air space. However, some windows use argon or krypton in this space to improve thermal performance. Krypton performs better than argon in thermal performance tests but is more expensive.
“You can get argon windows with U-factors of 0.25 or 0.26 and krypton units with U-factors of 0.22 to 0.24, says Tuluca. Clear double glazing, by comparison, will have a U-factor in the 0.48 range. “The problem is that we don’t know if the gases will stay there.”
Tinted glass — also referred to as heat-absorbing glass — was the first technology developed for controlling solar heat gain. The outer pane is tinted so that it absorbs the heat and then releases it to the outside. Other advantages are reduced glare from outdoor brightness and increased privacy.
Tinted glass takes two forms. Traditional tinting reduces heat gain but also visible light. Spectrally selective tints, generally offered in light blue or light green, transmit the daylight portion of sunlight but absorb the near-infrared heat produced by the sun. As spectrally selective tints are absorptive, they generally are used on the outside lite. During daylight hours, tinted glass increases privacy. At night, the effect is reversed, making it more difficult to see out than to see in.
The National Glass Association defines reflective glass as “clear or tinted glass that has a very thin layer of metal or metallic oxide on the surface. The reflective coating reduces heat gain and glare from the outside while allowing visible light to enter.” Reflective glass gives the building a mirrored appearance. Typical coatings are silver, copper, gold and earth tones that can be combined with tinted glazing to create unique exteriors. Because reflective glass repels and absorbs the sun’s rays, it reduces the solar heat gain that enters the building, thereby reducing air-conditioning loads. Some coatings are durable and are applied to the outside surface while others are applied on the sealed side of the double-glazed window for protection.
Reflective glazing is popular for large windows and in hot climates. Because conventional reflective glazings are essentially mirrors, consideration also must be given to their effect on surrounding buildings. Again, reflective glazings only act as mirrors during daylight. At night, when interior lights are on, they do not provide privacy.
The surface on which a reflective coating is placed is important. Each surface of insulating glass is described by a number, beginning from the outside glass surface and proceeding inward. So, in a double-glazed window, there are four surfaces:
- No. 1 represents the exterior surface;
- No. 2 is the inside of that exterior surface;
- No. 3 is the enclosed side of the interior surface; and
- No. 4 is the surface inside the building.
If a conventional reflective coating is on the No. 1 surface, it will create a mirror effect. On Nos. 2 and 3, that same coating will create a tinted effect.
“We used to think that energy-efficient windows needed to keep the heat out, which tints and reflective glazing do,” says Carmody. “But now we realize that if we bring some of that daylight into the building, we can turn off or dim interior lights. So when we look holistically at the issues involved with windows, we often find conventional reflective glazing performs poorly compared to low-e glass.”
Improving test methods for SHGC ratings as well as developing new technical procedures for rating window coatings are goals of the National Fenestration Rating Council (NFRC). Tony Rygg, NFRC chairman, says NFRC is committed to improving rating procedures. “We appreciate the effort made by window film manufacturers to work with us to develop technical procedures that will eventually allow them to rate and certify their film products through NFRC,” Rygg says.
Low-e glass is a form of reflective glass with a thin metallic coating that reduces VT about 10 percent compared to uncoated glass. Low-e glass reduces heat loss through windows and reduces glare. It also reradiates heat absorbed from sunlight back inside the room, allows daylight to enter without letting heat escape and protects the interior against ultraviolet radiation. The UV resistance can be particularly beneficial for facility executives concerned with UV damage to carpets, draperies and other furnishings.
“Low-e coatings can be placed on clear as well as tinted glass,” says Carmody. “There is a full range of coatings with different characteristics. When combined with the various tints, low-e coatings present a matrix of possibilities.”
The low-e coating is put on different window surfaces, depending on the climate conditions. “In warm sunny climates such as Arizona, the low-e coating would be on surface No. 2 to help keep the heat out,” says Eijadi. “But in northern climates, the low-e coating would be on surface No. 3.”
Low-e glass reflects sensible heat, which is generated by the heating system inside the building. “In northern areas, low-e coatings let in the heat from the winter sun while retaining the heat generated from inside the building,” according to the National Glass Association. “In southern areas, low-e coatings . . . reduce the glare and reflect the sun’s rays away from the structure.”
“When you are interested in harvesting daylight for your building, a good choice is a low-e, moderately tinted insulating glass window,” Eijadi says. “Remember that lighting energy consumption in commercial buildings often is more than the HVAC energy needed. By using an integrated design approach to the windows selected, you often can use natural daylight to mitigate your lighting energy needs.”
Low-e coatings do add to the cost of the window, but the premium is minimal, Tuluca says. “The premium for low-e on insulated glass is 50 cents per square foot. When you weigh all the potential energy savings, there’s no reason not to include a low-e coating requirement on new window installations.”
Other glazing treatments for insulated glass include surface coatings and treatments such as ceramic frit, acid etching and sandblasting. These techniques reject heat, diffuse light and offer unique aesthetics for certain applications.
Ceramic frit is composed of glass particles, paint pigment and a mixture medium to blend the two. It can reduce solar heat gain but often is used for its aesthetic appeal.
Acid etching and sandblasting allow light to pass through but diminish transparency. These treatments diffuse light but may increase glare as surface brightness is increased.
Under development are transparent insulation-filled glazings and evacuated windows in which the space between lites is a vacuum. Also coming are smart glazings, notably electrochromics, which use low-voltage power to modulate from clear to full color. Switchable glazings can change properties to control solar heat gain, daylight, glare and view. Photochromics change transparency as a reaction to changes in light intensity while thermochromics react to temperature changes.
With myriad options for selecting insulated glass windows, facility executives are bound to find one combination that is perfect for their specific application.
Rita Tatum has covered facility management and technology issues for more than 25 years. She is a contributing editor to Building Operating Management.