Condensation as defined in Wikipedia…”is the change of the physical state of matter from a gas phase into a liquid phase. Condensation commonly occurs when a vapor is cooled and/or compressed to its saturation limit when the molecular density in the gas phase reaches its maximum threshold”.
In our northern climate that threshold reduces as the temperature drops which is why you see condensation on the inside of windows in colder weather. When the humidity is too high, it will collect on windows, walls, doors, toilet tanks, electrical outlets or any cool surface. Typically, condensation is not a big problem unless it has been occurring for a long time and is promoting the growth of mold or damaging your walls or wood window jambs.
If you are experiencing condensation on your windows, one way to reduce it is by adjusting your humidifier setting on your furnace as the outside temperature drops. By keeping the temperature of the inner surface of the glass on your windows warmer than the saturation limit or due point condensation will not collect on your windows.
Tight Fitting Window Dressings
Another common cause of condensation collecting on windows is tight-fitting window dressings. Curtains or blinds that inhibit airflow will limit warm air to come in contact with the glass of the window causing it to cool down to a point where condensation starts forming. The solution is simply opening the curtain a bit or lifting the blinds should reduce the amount of condensation.
Recent Home Improvements
If you have recently renovated an older home by replacing the windows and doors or added insulation and siding chances are you’ve made your home more efficient but you may have also sealed many of the places warm humid air used to be able to escape. Although you’ve increased the efficiency you may have also created a situation where you need to vent old stale humid air. Installing exhaust fans in bathrooms and your kitchen may help. When you move air out new fresh air must come in so make sure you have sufficient fresh air supplying your heating system.
External Condensation on Windows
In summer sometimes condensation will form on the exterior surface of your window. This isn’t an indication of a problem with your windows, it’s an indication that they are performing the way they are supposed to.
High-efficiency glass is designed to keep the heat inside in cold weather. As the glass keeps more of the heat inside, the outside surface of the glass stays cooler. When the temperature drops below the dew point condensation may collect on the outside of your windows. This can happen during spring and summer in the evening and early morning but as soon as the sun comes out and the temperature starts to rise the condensation disappears.
For more information about read the City of Edmonton CO2RE document Condensation Concerns.