High-Tech Windows


THE DRIVE FOR energy conservation has lost much of its force, observers aften say these days. Solar-energy enthusiasts become dispirited when they see that the most highly touted features of new houses have to do with plush living, not with cutting fuel consumption. Yet the momentum generated by the oil embargoes of 1973 and 1979 is continuing to alter American houses, often through tougher building codes and improved construction techniques. Now that it is routine for houses to be outfitted with thicker insulation and wrapped in plastic to resist drafts, builders are turning to windows as the next place for major improvements.

“R-values"—measures of resistance to heat flow—indicate why windows are important. Typically, the walls of new houses register R-values higher than 12; in cold states like Wisconsin even better ratings—higher than R-15—are standard. Walls with these R-values, which are more than a third higher than the norm of the early 1970s, help houses stay warm in winter and cool in summer. By contrast, a single-glazed window with its alarmingly low rating of approximately R-l is an opening through which heat and cold pass all too easily.

Adding storm windows or switching to insulating glass (two sheets with a sealed air space between them) boosts the Rvalue only to about 2. Triple-layer insulating glass pushes the rating to about 3, but it adds so much weight that the window becomes bulky. Moreover, for every layer of glass added, about one sixth of the sun’s light and warmth is blocked. These drawbacks, along with the higher price, have made triple glazing impractical.

Now builders and designers are embracing a high-tech, or at least highertech, solution: windowpanes containing more than glass. Because of their superior insulating quality, the indoor surfaces of such windows remain at close to room temperature, eliminating condensation and making it possible for people to sit by the window without being chilled.

One of the new methods is to use him rather than extra layers of glass to upgrade a window’s energy efficiency. Since 1981 the 3M Company has been producing SunGain: thin polyester films that can be suspended in the air space between two sheets of glass. A set of two films, pulled taut, creates three separate dead-air pockets between the panes. This “quadpane” window system (manufactured by Weather Shield, of Med-

ford, Wisconsin, and other companies) has a winter R-value of 3.8—nearly double that of standard insulating glass.

A film with more-advanced properties, Heat Mirror, produced by Southwall Technologies, of Palo Alto, California, actually reflects warmth, so that less of the heat generated inside a house radiates out. Heat Mirror is a single film, placed in the air space between two sheets of glass. It has a layer of metal oxide that lets sunlight and near-infrared radiation pass through but that bounces back the radiant energy generated by a furnace or given off by objects within a house. Glass equipped with Heat Mirror has an impressive R-value of 4.3.

Heat Mirror, which went into production in 1981, gave America’s window manufacturers their first opportunity to make “low-E” windows: windows that let in plenty of light and yet have “low emissivity,” a term indicating simply that the window allows less long-wave radiant energy to pass through it than ordinary windows do. The main disadvantage of a low-E window, according to Douglas Balcomb, a leading solar-energy researcher at Los Alamos National Laboratory, is that when it’s installed on a southern exposure, it noticeably reduces the amount of warmth from the sun entering the house in winter. Typically, however, the radiant energy that it keeps from getting out of the house more than makes up for the solar heat that it keeps from getting in. And in places like Miami and Phoenix the reduction of solar heat is actually beneficial. Last year Southwall introduced a variant, Heat Mirror 66, specifically designed for southern and southwestern residential markets: it blocks a still larger portion of solar heat while admitting ample light.

ENERGY-CONSCIOUS builders—for example, Deck House, a manufacturer in Acton, Massachusetts—were quick to adopt Heat Mirror. Lately, however, some have been turning away from film systems to a less complicated innovation: a low-E coating added directly to glass. In some instances the coating is a single layer. In others the coating is composed of three layers—a core layer of a metal such as silver, to reflect long-wave rays, and two additional layers of anti-re - flectant materials, which mask the metal, making it nearly invisible. Even so, under certain light conditions most lowE coatings give a window’ a faint blue or gray cast.

Window-makers find it easier to work with coated glass than with a separate film. Deck House, in fact, switched from Heat Mirror to a coated glass called Sungate 100, which PPG Industries introduced in 1983. Demand for such glass is strong, and now many coated low-E products are available.

So far there are two methods of coating glass. For Sungate 100 (among other products) the materials are “sputtered,” or vacuum-deposited, on the surface. The coating made by this process can be damaged if touched, and it cannot tolerate prolonged exposure to moisture. Consequently, it is installed facing the sealed air space, not the outdoors or the house’s interior. Sputter-coated glass achieves an R-value of about 3— the equivalent of triple glazing—and avoids the extra thickness and weight of a third layer of glass.

Recently another form of low-E glass has come onto the market: a coating that is sprayed on while the glass is hot, resulting in a hard, baked-on surface. “Hard-coat” low-E glass is impervious to moisture, and it can safely be exposed to the atmosphere and to normal handling. For these reasons window-makers like it. Only one such product—Comfort, made by the Belgian company Glaverbel, S.A., and distributed by The Sentinel Group, of Miami — is claimed to have an energy efficiency equal to that of the sputter-coats (though its efficiency is less than that of Heat Mirror or the 3M films). Others improve on the energy efficiency of ordinary glass only half as much as the sputter-coats do. Until the hard-coat products are upgraded, they will remain products oriented as much to the window manufacturer’s convenience as to the homeowner’s benefit. And so you are generally better off getting windows with a sputter-coat or with Heat Mirror or 3M him. Deck House, to its credit, stayed with PPG’s Sungate 100 sputter-coat glass rather than switch to Sungate 200, a hard-coat glass that would have been somewhat easier to work with but is less effective in saving energy.

The one place where hard-coat low-E glass is unquestionably a good choice is on storm windows, since these are not completely sealed from the atmosphere. If I were buying new storm windows for my house, in New Haven, Connecticut, where there’s a good deal of winter sunshine, I‘d consider hard-coat glass for the windows that face east, west, and north, but not for those facing south, where the loss of the sun’s warmth would be appreciable.

Coated glass promises to become one of the “energy upgrades” that builders will offer customers as an option in the years ahead. In the hotter sections of the Sunbelt the option of glass that is both low-E and tinted to deflect summertime heat will be offered. The annual production of low-E glass in the United States is expected to reach 125 million square feet by 1988.

COMPANIES COMPETING intently for the new market in energy-efficient windows have already made further advances. One involves filling the air space in low-E windows with argon, an invisible inert gas. Because argon molecules are large and heavy, they conduct heat less readily than air does. The SNE Corporation, of Wausau, Wisconsin, claims that its sputter-coat low-E window systems filled with argon—sold under the trade names of Crestline and Vetter— can achieve R-values as high as 5.

If you want better energy performance than that, you’ll have to turn to insulating shutters and blinds—movableinsulation, as these devices have been called. In the late 1970s there was considerable excitement over the potential of blinds with air pockets, shutters with rigid foam insulation, fabrics with inner layers of reflective foil, and other such window coverings. And for good reason: movable insulation carries R-values as high as 15.

But the movable-insulation field has fallen on hard times. Some systems have suffered from mechanical problems— the insulating shades have fallen out of their tracks along the window edges, or condensation has formed where the insulation has appreciably reduced the temperature of the glass. Even systems free of malfunctions typically require daily effort on the part of owners to place the insulation over the windows at night and remove it when the sun is shining. Everett Barber, Jr., an authority on solar energy who teaches in the school of architecture at Yule University, says that there’s probably an additional reason why movable insulation has not fared well while low-E glass has come on strong: “Companies like PPG can devote a lot of marketing muscle to promoting glass as the answer to energy problems. Nobody associated with movable insulation commands that kind of marketing and distribution clout, since the major window-shade manufacturers haven‘t chosen to sell insulating shades and those firms that do produce window insulation are small.”

If you go to a lumberyard or a window store, you‘ll probably find displays explaining the advantages of low-E windows: for example, the fact that they block most ultraviolet rays means that they inhibit the fading of carpets, draperies, and other fabrics. You may not find the distinction between sputtercoat and hard-coat glass spelled out, especially if the store deals only in the less-effective hard-coats. However, you should expect to be told the rating of a piece of glass, so that you can judge howwell it will perform. (Sometimes a rating is given as a “l -value”: a number between zero and one that measures how readily the glass transmits heat. Divide one by the U-value to get the R-value.) Many manufacturers provide an energy rating for the entire window assembly— a useful calculation, since any advantage gained from better glass can be lost through conduction of energy through the frame or through air leakage around the sash.

Generally, window assemblies have been getting better. Many have insulating material positioned where it will prevent heat or cold from being conducted through the sash or frame. The edges of some are well flashed to block drafts. Unfortunately, Andersen, of Bayport, Minnesota, and many other manufacturers continue to make double-hung windows that are secured by turning a sash lock where the upper and lower sashes meet. The lock, rather than drawing the two sashes tightly together, may actually push them slightly apart, creating a gap that air can rush through. Pella, a top-ofthe-line window produced by Rolscreen, of Pella, Iowa, is superior in that it uses a spring-loaded sash lock that grips firmly.

Double-hung windows can be awkward to clean. Marvin (a window-maker in Warroad, Minnesota, that wins praise from many architects), Andersen, and other manufacturers have surmounted that problem by designing double-hung windows in which the sash pivots to a horizontal position. Rolscreen and other makers produce casement windows that pivot into the center of the window opening, providing both greater ease of cleaning and better ventilation, although with one drawback: the opened window divides the view in two.

In its windows and doors Rolscreen will also install thin Venetian blinds (Slimshades) between the two panes of glass. Both the blinds and the glass are given a low-E coating, so that a window with the blinds closed can achieve the admirable R-value of 4.35.

A PROMINENT TREND today is toward ornamental windows. Roundarched windows have proliferated, and stained glass is enjoying a revival so widespread that you can find it not just in small specialty shops but also in stores stocking mass-produced windows. Many new stained-giass windows are insulated. The Seagull Stained Glass Company, of Atlantic City, New Jersey, for instance, makes a series of attractive octagonal windows with the stained glass sandwiched between two sheets of clear glass.

The age of the large picture window is over. Though plenty of picture windows continue to be produced, most of them are camouflaged. Dividers that imitate muntins are placed against the glass to give the appearance of many small panes, or “lights.” Rarely is the effect convincing.

In double-hung windows false muntins are just as common and frequently just as fake-looking. Among the worst are the “snap-in” grilles offered by Andersen. If you stand near the window, you can‘t help noticing that instead of resting solidly against the sash the snapin grilles stop short; small extensions fit into little holes in the sash. The impression of falseness is compounded when the grille is white vinyl, clashing with a dark-stained wooden sash. To see this is not to “come home to quality,” as the Andersen commercials sing; it’s to come home to self-delusion.

If you really must have the appearance of traditional muntins in windows made of undivided glass, other manufacturers provide systems that look less disturbingly false. Rolscreen will place a wooden grille between the two sheets, where it won’t get in the way of cleaning. The Woodstone Company, of Westminster, Vermont, one of a number of specialty shops that have sprung up in the past dozen years in response to growing interest in craftsmanship and historic

restoration, makes handsome, well-constructed grilles that can be put on the room side of the glass, on the exterior, or on both the exterior and the interior.

Probably the most ingenious innovation is one that Peachtree, of Norcross, Georgia, introduced last year—a removable solid-pine liner made up not only of muntin bars but also of a large part of the surrounding sash. It’s as if Peachtree had sliced the window vertically; the wood portion facing into the room can be lifted off for cleaning, staining, and painting. In essence the window has been divided into two layers—one functional and permanent, the other ornamental and expendable. The idea is clever, and I would admire it more if I didn’t feel that the search for a better-looking set of false muntins is the architectural equivalent of attempting to make a better clip-on necktie.

The fact is that most systems, including Peachtree’s, are not very convincing from the outside. Anyone driving by can see from the reflections that the window is not what it pretends to be.

It’s possible to avoid all the falseness by buying windows with true divided lights made of double-layer insulating glass. Marvin and a number of other companies make them to the customer’s specifications. (One of the encouraging developments of recent years is the willingness of small companies like Woodstone and of large companies like Marvin to accept custom orders.) As you’d expect, divided-light insulating glass costs considerably more than any of the alternatives—up to twice the price of insulating glass with a grille, for example. What’s more, the larger muntins necessitated by insulating glass affect the aesthetically important ratio of wood to glass. In a modest-sized window, a grid of thick muntins can look ungainly.

Many of those who appreciate historical styling have concluded that the best course is to do what’s been done for decades: buy single-glazed, genuinely multi-light windows and buffer them with storm windows. Windows with true divided lights can be ordered in hardcoat low-E glass from companies with custom shops, and these will perform better than conventional single-glazed windows. They will still fall short of the maximum energy conservation possible today, however. Technology and tradition—two of the major influences on today’s houses—have yet to be fully reconciled. □