Picking panes that save energy

Building Operating Management, Nov 2003 by Tatum, Rita

Advances in THERMAL PERFORMANCE give facility executives wider options in selecting energy-efficient windows

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."

TRIPLE GLAZING

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.