Condensation or "sweating" on windows is caused by excess humidity or moisture in the air in a home. When the moisture in the air comes into contact with cooler surfaces, such as a mirror or window, the air condenses into water droplets called condensation. Condensation increases as the weather gets colder and the humidity in the home increases. Condensation can even occur on the outside of windows.
All air contains a certain amount of moisture. Indoor humidity is generated by many household activities including cooking, heating system, humidifiers, showers and moping floors. Every activity that involves water adds to the humidity level in your home.
Climates are also a factor in condensation. In homes where the January temperature drops below 2 C there is greater temperature differences affecting the glass in your home, thus more likely for condensation to occur. It is typical to experience a bit more condensation to occur at the start of each heating (winter) season, compared to the rest of the season.
With homes becoming more energy efficient through high-performance insulation, moisture-barrier materials and airtight construction it is another contributor to household condensation. As homes are more tightly sealed keeping the cold out it also keeps the moisture in creating higher humidity levels and more condensation.