As the cost of heating/cooling increases every year, you should spend some up front $ on good windows/doors and plenty of insulation.My walls are R19 and the cealling is R38 so it takes very little energy to keep it comfy.Upright Freezer running and lighting seems to keep it livable in Winter,but if Humidity is High (Wash.DC) then I turn on the wall unit for cooling.The 6" walls ,2x6 on 24" centers are cheaper to build ,so it makes perfect sense to plan it that way.I took a solar energy course in college and about half the time was devoted to Insulation Tech.Good Luck on Your expansion!