By Gordon Bock

Hybrid spacers combine materials, such as this dessicant-filled aluminum, to improve efficiency. Photo Courtesy of Andy Olenick

Tintings and Coatings
In the search for performance, one of the first places manufacturers looked for improvements was the glass itself. Tinting glass by coloring it with mineral admixtures reduces the percentage of radiation that it can pass. However, because tinting also reduces visual transmittance (the visible light transmitted through the glass), and the coloring looks subtly unlike traditional glass, it became less popular for residential windows than other applications. So manufacturers shifted their attention to another front: altering the surface of the glass. In the past, these alterations have been in the form of reflective coatings and films that limit heat gain and glare, but since the mid-1990s the trend in residential windows has been toward low-e (low emittance) coatings that improve window performance during both heating and cooling seasons.

Low-e coatings are layers of metal or metallic oxides that are extremely thin (on a molecular level), virtually invisible, and permanently bonded to the glass surface. In double-glazed windows, these coatings face into the gap between the panes of glass and are designed to suppress the heat flow through the window„that is, the radiation from a warm pane of glass to an adjacent cooler pane. Low-e coatings can also be produced to obtain different levels of solar heat gain. Low solar gain coatings, for example, are preferred in regions where keeping the house cool is the main issue. High or moderate solar gain coatings may be desirable where the emphasis is on heating the house, rather than cooling, and the extra warmth from solar gain is welcome in winter. Luckily for old-house lovers, a byproduct of standard low-e coatings (as well as plain glass) is reducing some of the UV rays that cause fading and fabric damage in furnishings. Plus, these coatings can also be designed to be spectrally selective and keep UV transmittance as low as 16 percent. Even better, low-e coatings are relatively inexpensive options to add to a window ($1 to $1.75 per square foot), a cost that may be nearly inconsequential on high-end windows.

Glass and Gas
Another approach is to enhance the construction of the window, and employing multiple layers of glass is the obvious method. Though the most common configuration for American houses is the double-glazed window„that is, two thicknesses of glass separated by an air space that reduces heat and sound transfer„triple-glazed windows are made for commercial applications or super-insulated houses in cold regions such as Canada. In fact, some manufacturers have developed ways of achieving some of the benefits of triple-glazing without the weight or thickness of more glass by incorporating one or more stretched plastic films between double-glazing.

Today many manufacturers also do their best to bolster the thermal performance by filling the void with a low-conductance gas. When a multi-glazed window is made using air alone, the air space is carefully dried and sealed to guard against condensation and to maximize the insulating ability. Even so, sometimes it can travel in currents that conduct heat between indoors and outdoors. Swapping air for a gas that is more viscous or less conductive helps mitigate this problem. Argon, an inert, nontoxic gas, is commonly used because itÍs inexpensive and works best in the same spacing as air„about 1/2" between panes. Krypton is a more expensive gas, but it has better thermal performance than argon, so krypton is often the choice for filling windows that must be kept thin (say 1/4" between glazings)„which is often the case when trying to maintain the look of historic windows. Mixtures of argon and krypton are also employed to balance cost and performance.

The materials that wrap glass and gas influence window performance too. In the 1960s and Í70s manufacturers started using aluminum spacers at the perimeter of the glass to separate the panes at the proper gap„ideal structurally, but a problem, it turned out, thermally. Aluminum is an excellent conductor of heat, and these spacers became an easy path for heat to bypass the glass-and-gas sandwich, compromising the insulating performance of the window and creating cold edges and condensation.

Since then manufacturers have devised a variety of low-conductance edge systems that cut heat loss. Moving to less-conductive metals, such as stainless steel, is one popular approach, often used in combination with thermal breaks„clever cross-sectional designs that make it harder for heat to migrate across the spacer. Some manufacturers eliminate metal altogether in favor of materials with better thermal resistance, such as thermoplastics, fiberglass, or silicone foam. There are even hybrids that combine a metal or plastic spacer with a desiccant, for instance, or add a thin aluminum or stainless shim to a plastic spacer.

Lastly, glass and spacers have to be held in some sort of frame, and this window component can be a major thermal conductor too. Aluminum frames, for example, are very conductive and can affect energy bills as well as draw condensation. Fortunately for old-house owners, wood is a good insulator and can deliver about the same thermal performance as materials such as vinyl, fiberglass, or composites that are employed for energy-efficient widows today. Plus, wood is light and easily maintained, and has a proven track record of weather service and beauty that man-made materials have several centuries to meet.