The U-value of the window as a whole should be used, because efficient glazings can be compromised with poor frame designs. Aluminum frames, for instance, conduct heat well and should not be used in cold climates. See the National Fenestration Rating Council for a Certified Products Directory that lists U-values for more than 20,000 certified products. Window size and location are determined by the daylight, passive solar heating, and natural ventilation needs. In hot climates, north-facing windows can provide daylighting without heating the house.
Landscaping has other benefits, including natural cooling and protection from the wind.
In climates where natural ventilation makes sense, windows need to be located on both the windward and leeward sides of the building. Roughly equivalent-sized openings on both sides of the building will maximize the air flow through the building. The windows should be offset so that the air will circulate through the building rather than blowing directly through it. Doors For solid doors, insulated metal or fiberglass doors are the best choice. For heavily trafficked buildings, air infiltration through doors is an important energy consideration.