Get a hygrometer and see what the relative humidity level is in the house. If there are any uninsulated walls, just like a cold glass in the summer, it will sweat. You need to have enough insulation to move the dew point out further or lower the moisture levels. A really tight house, with numerous people taking showers, washing clothes, cooking, etc. (and even breathing) will add a fair amount of moisture into the house. You need to run the exhaust fans when showering and cooking. They also may want to consider an air-to-air heat exchanger to bring fresh air and exhaust the old from the house to prevent the sick house syndrom. There's more to making an efficient house than buttoning it up tight...people make pollution, too, and you need to dilute that with some hopefully fresh air from outside.