Best heaters in this house? Those south-facing, double-glazed windows

My wife and I are building a cottage here in Maine. Only the first floor is completed, but we've moved in anyway, knowing that the temporary roof will keep us dry and that we'll stay warm with the help of a small wood stove and three large, double-glazed windows.

That's right, those windows warm us every bit as much as radiant-heating panels - and at a fraction of the cost. The stove works well because, with a little help from us, it burns wood; and the windows work well as long as we open the draperies each morning and let in the sun. That's the beauty of passive solar heat: It's not only free, its operation is virtually effortless.

In his latest book, ''House Warming'' (Atlantic-Little, Brown; hard cover, $ 24.95; paperback, $16.95), Charlie Wing says this of sunshine: ''(It) is to a house as wind is to a boat - free energy.'' But, just as the boat's sails must fill with the breeze if it is to go anywhere, the home's windows must face the sun if they are to warm up the house. Windows become even more effective if they are double-glazed, and still more so if night insulation, such as thermal draperies, shutters, or both, is added.

On a sunny winter day, an equator-facing (south-facing in this hemisphere, north-facing in the Southern Hemisphere), double-glazed window lets in considerably more heat during the average 8 hours of daylight than it loses by conduction over the entire 24 hours. East- and west-facing windows, while net heat losers if they have only a single pane of glass, come out even once the second layer of glass is added.

Add movable night insulation and the south-facing windows keep fully 80 percent of the heat they let in (which is why they are such effective ''heating panels''); and even the east and west windows become net gainers.

Given the correct siting of the windows (southeast- or southwest-facing windows will absorb some 80 percent of the heat that pours through a true-south window), the next thing is to remove those evergreen trees that block out the winter sun. On my site there are three trees that have to come down.

Whatever you do, don't go to the expense of south-facing glass if you can't bring yourself to remove a tree or two that may stand in the way. Even a deciduous tree, minus its leaves, can cut out as much as 60 percent of the incoming radiation.

The final need, then, is to add night insulation to the windows in our cottage. We could buy insulating shades, but they are expensive (from $5 to $10 a square foot), so we plan to make our own. A kit that has been on the market for a little more than a year makes home fabrication of good-looking insulating panels a snap. Wing's book also contains plans for making insulating shutters at a fraction of the cost of the store-bought variety.

The kit enables a homeowner to turn standard foil-faced insulating board from one to two inches thick into decorative slip-in shutters. The key to it all is a flexible edging strip that, when attached to the board, enables it to fit snugly into any window no matter what its size or how many irregularities there are. ''Goofproof'' is one way to describe it.

The insulating board, available from lumberyards or home centers, is cut to size with a knife, edged with the strip, then covered with a fabric of your choice. It takes about an hour to make a panel.

Tamil Bauch of Kingston, N.Y., who developed the ''Slip-In-Panel,'' as it is called, at first suggested that the fabric covering be glued onto the panels. But many innovators went one better and made slip-over covers that could be readily removed and washed when necessary. Now Mr. Bauch recommends this method, too.

Friction holds the ultralight panels in place. When removed in the morning, they can be slipped behind chairs or book shelves for easy storing. Some folks, such as those who have added decorative designs to the covers, simply hang the panels on the wall during the day ''like any other piece of artwork.''

For details on the panel kit, write to Aerius Design Group, PO Box 394B, Kingston, N.Y. 12401.

The Wing design calls for the use of Thermoply, a tough, foil-faced, one-eighth-inch sheet of cardboard that is used as sheathing in the construction industry. The board is placed on both sides of a simple frame made of strapping.

The reflective foil, combined with the dead-air space between the two sheets, provides the heat-saving insulation. A decorative fabric is used to cover the shutters, which Mr. Wing then hinges to the window frames.