Condensation is the process by which water molecules change from a gas into a liquid. This process can only happen when two conditions are present, the first being high relative humidity and the second, cold temperature.
Insulating glass is typically made from two or three layers of glass with a spacer between them. The spacer is typically made from aluminum tubing or structural foam, which assures a gap between the sheets of glass that is filled with dry air or another gas. Dry air is rarely used today because it is not as good an insulator as other gases that are commonly available. Most often, Argon gas is used because of its superior resistance to heat transfer, thus providing an insulating barrier between the indoor temperature and the climate outside. The edges of the glass, where the spacer contacts the inner and outer surfaces, had been the “weak link” in insulating glass units in the past. This is because the spacer was an aluminum tube, which is a very good conductor. Using this type of spacer caused the edges of the glass to be significantly colder than the middle. Moisture in the air would condense on the cold edges of the glass long before it was likely to form anywhere else. Today, most glass manufacturers use a structural foam spacer that has very good insulating properties. This, combined with the use of Argon gas fill almost eliminates the condensing problem on insulating glass today.
When condensation does form, solutions include lowering the humidity of the room, raising the glass temperature or replacing your windows. Glass temperature can be raised by removing curtains or blinds and repositioning furniture so that it is not in front of the windows. Windows that are blocked by any of these things have a lower exposure to heated air flow.
You can increase the air flow in your home by regularly using ventilation fans in bathrooms and kitchens (this also decreases the relative humidity in the house) and by opening doors and windows. Make sure the vents in your attic or basements are fully functioning for even more air circulation. Older houses can be draftier than new homes. This is an indication of two different problems. One is that the house is not as “tight” as a newer home, allowing air to move from outside to inside through cracks and other openings. A draft is also an indication that the outside walls, windows and doors are cold. When the heated air in the house touches this cold surface, it cools down. Cool air is heavier than warm air, so it drops down the wall. This air movement is felt as a draft. The leaks in you older home help with ventilation and reduce relative humidity, but can cause high utility bills and personal discomfort.
The other factor in condensation is high relative humidity. This can be addressed as described previously using exhaust fans to remove moisture from kitchens and baths. If you have a lot of house plants, you may want to consider repositioning them or moving them outdoors as plants create humidity. Alternatively, you can also purchase a dehumidifier to remove excess moisture from the air.
Despite employing these methods, you may just need to replace your windows. Condensation and ice can form due to missing or defective weather stripping, faulty installation, poorly fitting or broken window panes, failed insulating glass and more. Even if you have windows with insulating glass, technology has changed a great deal in the past 10 or 15 years. If you notice condensation in your windows, you may want to wait to make sure it is not simply due to unusually cold winter weather, but before making a decision that could cost hundreds if not thousands of dollars, you should definitely contact a professional.