I got to wondering just how much heat we lose in winter (or gain in summer) through our windows, as opposed to through the walls.  To do that comparison, I need two things:

1. the wall and window area; heat loss is directly proportional to area.

2. how "lossy" window materials are compared to wall. To simplify things, I'll assume that the entire window is glass.  (I won't be too wrong because I know that wood is pretty lossy, and so is glass.)

I'll pick one typical wall. The wall next to me, for instance, is about 21' wide by 8' high, and has two double-pane glass windows with a glass area of 2.5' by 4' each. That's 21 x 8 or 168 square feet of drywall, less 2.5 x 4, or 20 sq. ft. of window, for a total of 148 sq. ft. of wall.

Now I need to know how "lossy" windows are compared to walls.  Lossiness is measured in something called "u-factor."  Windows are generally given in terms of u-factor.  Walls are measured, not for lossiness, but for resistance to loss.  To make the comparison, I need to choose to work either in u-factor or r-factor.  Conveniently, r-factor is 1/u-factor.  I'll choose u-factor.

EPA ENERGY STAR standards for double-pane glass is a u-factor of .4 or better (lower).  For a reasonably well insulated wall, the r-factor is 21; its u-factor would be 1/21, or about 0.05. 

Heat loss is lossiness x area. We have 20 square feet of glass (small area, very lossy) and 148 square feet of drywall (large area, not very lossy). So the ratio of heat-loss-through-glass to heat-loss-through-wall would be:

(0.4*20) / (.05*148) = 1.08.

In other words, we lose almost exactly as much heat through the windows of that wall as we do through the drywall area!