I’m sure there is a very advanced, scientific answer as to what causes condensation. However, as it is in most cases involving scientific definitions, I prefer a much simpler route. In layman’s terms, condensation occurs when moist air comes in contact with a cooler surface. Condensation can appear as a light coating of water, water droplets or, depending on the air temperature, even as frost or ice. Condensation happens all around you, but in it’s most obvious state, as a homeowner, you see it quite consistently this time of the year as water forms, collects and drips off some (or all) of your exterior windows.
Basic 6th grade science class taught us that warmer air will retain more moisture, and that it will always seek out (or move) towards items, or areas that are of a colder temperature. That same principal can be applied to your windows. The warm moist air inside the house moves toward the coldest object it can find. In most cases, that will be the glass portion of a window. As the warm, moist air comes in contact with the glass, it descends downward until it reaches the coldest part of the window, and it is at that point that you will typically see condensation or water droplets on the glass. The air movement process is normal and takes the same route in any house, however the differences are basically two separate items that the homeowner has control over…… the temperature of the glass, and the amount of humidity in the air.
The temperature of the glass can be controlled through the installation of windows that have, at a minimum, a double glazed base. (This would be two panes of glass separated by a spacer.) The space between the two panes of glass is very important as it acts as an insulator to the cold outside air. Additional warmth can be generated by replacing the air in between the panes of glass with a colorless, odorless gas, that since it is heavier than air, is a better insulator. Lastly, there are also clear coatings that can be placed on the glass to help allow the sunlight to penetrate and help heat the glass.
Controlling the humidity in the air is accomplished through various steps. If you have a high efficiency furnace the first step should be the installation of an air exchanger. This simple machine will actually pull the cold dry air from the outside into the house to heat it. (Remember 6th grade science again. The colder the air the dryer it is, and thus, the more moisture it can hold.) Secondly, awareness of your environment is also important. Do you shower everyday? Wash an abundance of clothes or dishes? If so, leave the bathroom vent fan on longer during the day, that will pull the warm, moist air out of the building.
For more information on windows and condensation consider this link to the National Fenestration Rating Council. http://www.nfrc.org/documents/condensation.pdf