Back when fuel costs first began to skyrocket, energy experts turned to their calculators and came up with some unsettling statistics, among them:

Every day Americans pour 71 million gallons of oil out of their windows in the form of escaping heat in winter and unwanted incoming heat in summer.

Reducing that loss has been a prime concern of homeowners and businessmen ever since. But of major concern was how to do this without cutting out the light, which is the reason the windows are installed in the first place.

The answer appeared to lie in plastic films that could reflect away outside heat during the summer and retain internal heat during the winter.

Several films came onto the market and have proved effective, but at considerable cost in light transmission.

Now Van Leer Plastics of Woburn, Mass., has come out with a product that insulates as effectively as conventional films, but doesn't darken the room.

The difference between the new film and clear glass is detectable when the two are compared side by side. But go into a room where windows are totally treated with Valvac Insulux, as I did, and light transmission appears normal.

The sun's electromagnetic spectrum contains both visible and invisible portions. The Valvac Insulux film's success apparently lies in its ability to exclude the invisible portions of the spectrum while allowing the light to penetrate.

Two years of testing on commercial and residential buildings shows that the film cuts down on 80 percent of incoming summer heat by reflecting the sun's radiant energy away from the building. In winter it does the same with indoor warmth, retaining 40 percent of what would otherwise slip through the glass.

Conventional reflective films are made by bonding a sheet of aluminum, rolled so thin that it becomes translucent, to plastic. But it is dark - and people, homeowners in particular, like light.

The new product does not use aluminum but rather some other metal - several ultra-thin layers, in fact. What that metal is, the company will not say. It does say that the film has an R-value, when combined with glass, of 1.4. This compares with a 1.7 R-value for a fully insulated window at twice the cost.

Obviously, insulating films are not meant for the south-facing glass on a passive-solar home. But they could be used effectively on all other windows on that same solar home.

At this point the new film can only be installed by accredited contractors. Why? The company says its application is simply too difficult for the untrained amateur.

of 5 stories this month > Get unlimited stories
You've read 5 of 5 free stories

Only $1 for your first month.

Get unlimited Monitor journalism.