​When I first started practicing architecture, primarily residential, back in the Stone Age in the early 1970’s, about 90 percent of all new homes in my region (Building Climate Zone 4) had heating-only mechanical systems, with only about 10 percent having Central Air Conditioning systems installed as original equipment.  Today these percentages are exactly reversed, with even the “modest” homes built to Habitat for Humanity standards in our region out-fitted with Air Conditioning as part of their original mechanical equipment package.  While some of this is due to the fact that when constructed to EnergyStar or comparable standards exceeding current code requirements tightly sealed buildings may develop mold or mildew issues, most are the result of ever-climbing cultural standards -- the same phenomenon that has made a second bathroom pretty much standard in a three bedroom home as small as 1200 square feet.