The best window on the market offers an R-3.57, and most do not exceed R-2.5. So you can see that most of the heat loss will occur as heat escapes out through the windows. At the same time, when the sun is out, the windows provide a terrific source of solar heat. The ratio of window area must be balanced to the optimal ratio for your climate.
In Maine, the ratio of floor area to glass area should be roughly 10 percent glass. If the floor area is 1000 square feet, the glass area should be about 100 square feet, which might allow ten windows, including glass doors. Every room in the house needs a window, usually large enough to qualify as a fire escape egress. All the rest of the glass area should be located on the sunniest walls of the house.