Turning windows into see-through walls

BOSTON Edison's state-of-the-art solar house, now nearing completion here, boasts two radically new types of window. In front of the first, you can feel the warmth of the incoming winter sun; alongside the latter, the incoming light is intense but remarkably cool. On neither is there any need for insulating shutters.

Window technology has come a long way in recent years.

As a demonstration project, the new house has everything the up-to-the-minute solar home should have, including these shutterless windows, where the glazing alone works as effectively in preventing heat loss and unwanted heat gain as conventional double-glazed windows with insulating shades.

The windows might almost be termed see-through walls, because they so effectively restrict heat transfer without impeding the view.

A product known as Heat Mirror has made this possible.

Heat Mirror, very simply, is a double-glazed window with a transparent film sandwiched in between. Depending on the makeup of the film, the windows will do one of two things. They will either maximize solar gain by letting in the sun's heat and then preventing it from flying back outside the moment the sun hides behind a cloud; or they will act as an insulator in both directions, restricting heat transfer either from inside or outside the building.

This means that the owner of a home with this advanced type of glazing will be able to look out at the stars on a crisp winter night without worrying about the size of his heating bill. He will also be able to enjoy the view through a west-facing picture window on a hot summer afternoon without taxing his air conditioner.

As Stephen Strong, architect for the Edison house, sees it, homes incorporating these products will return to using draperies the old-fashioned way - for decoration and privacy.

The two glazing options in the Edison home are used in this manner:

* All vertical windows facing the low-angled winter sun incorporate the glazing that lets in both the sunlight and its accompanying warmth during the day but retains much of the heat at night.

* East- and west-facing windows, and overhead skylights that would otherwise let in too much unwanted heat during summer, incorporate the glazing that insulates both ways.

This latter option is considered particularly useful on the roof of the greenhouse. Plants, while benefiting from high light intensity, can nevertheless do without the excessive heat that comes from a high-riding summer sun.

The two-way insulation is considered particularly appropriate for the South. It is similarly useful in Northern houses lacking an appropriate southern exposure or houses in climates with predominantly overcast winter days.

Its principal use, however, is seen in high-rise commercial buildings, where cooling is generally more costly than heating. Commercial buildings want the visible light from the sun but can do without its heat. In the past, designers of these high-rises have been faced with putting in a larger and much more costly air-conditioning system or going with smaller windows and a somewhat higher lighting load.

Now, with the improved window performance, they can keep the larger windows, gain all the natural light they need, and still not pay the penalty in air-conditioning costs. In effect, they can have their cake and eat it.

In the early 1970s, before the Arab oil embargo, scientists at the Massachusetts Institute of Technology saw that a space-age technology (the ability to apply an ultrathin coating to airplane windshields) could be put to commercial use on the ground. By coating glass or film no more than 100 atoms thick with a specific material, heat could be retained or rejected simply by reflecting it away, without noticeably changing light transmission through the glass.

As the technology was already available, the need at MIT was to develop it to the stage where it would be cost-effective in commercial and residential housing. That goal was met in the late '70s. In 1980 the manufacture of Heat Mirror 88, which maximizes solar gain, began at the Southwall Corporation in Palo Alto, Calif. The two-way insulating products, Heat Mirrors 77 and 55, came A single pane of glass has an R-value of around 0.8; double-panes, 1.75; and a triple-paned window approaches 2.75. Heat Mirror, in contrast, provides a 4.3 to 4.5 R-value at similar cost to triple panes and is a much lighter product - a significant factor in multistory buildings.

Compared with conventional double-glazed windows, Heat Mirror adds some 25 percent to the cost. Payback in the United States varies from 3 to 6 years, depending on the climate. In Boston, with a rigorous winter climate, a short but often intense summer, and high energy costs, the payback is reckoned at three years.

In large new buildings the payback has been called ''instantaneous'' on those occasions where the reduced costs of the central heating and air-conditioning units equal the higher costs of Heat Mirror.

Meanwhile, accelerated sun tests have shown no deterioration in the Heat Mirror film after exposure to the equivalent of 28 years of intense sunlight.

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