You actually stated the answer/problem to your own question. You have made the house airtight, and now the moisture has now where to go, but to condensate on the coolest point... your windows. You need to remove the moisture from the air. Fans help as hot moist air rises and can be expelled through venting, but apparently it is not enough. A/C also removes moisture, but unless you live where it is warm in the winter, you probably do not run one much... especially if trying to conserve energy. Dehumidifying will definitely help as will opening windows when the weather permits. Reducing the source of moisture is a good place to start. As in, when taking showers/hot baths, open the bathroom window or utilize the exhaust fan... same as when cooking... or crack a window nearby.