Why does condensation occur on new high-efficiency windows? 

Interior window condensation occurs on high-efficiency windows because they are designed to be more airtight and energy-efficient. These windows are typically made with multiple panes of glass with insulating gas in between, a low-emissivity (Low-E) coating on the glass, and airtight seals around the frame.

When the indoor temperature is higher than the outdoor temperature, warm, moist air can come into contact with the cold surface of the window. The moisture in the air condenses into water droplets on the glass surface, creating visible condensation. This process is known as the dew point, which is the temperature at which the air becomes saturated with water vapor and starts to condense.

High-efficiency windows are more airtight, which means that they reduce the amount of warm, moist air that can escape from the home. This trapped moisture can then lead to interior window condensation if there is not enough ventilation or if the indoor humidity level is too high.

To prevent interior window condensation on high-efficiency windows, it's essential to maintain proper ventilation and indoor humidity levels. This can be done by using exhaust fans in bathrooms and kitchens, using a dehumidifier, and opening windows periodically to allow for fresh air circulation. Additionally, maintaining a consistent indoor temperature can help to reduce the potential for condensation to occur. 

1. Relative humidity: The amount of moisture in the air relative to its temperature is known as relative humidity (RH). Generally, when the indoor relative humidity is over 50%, it can lead to window condensation. Maintaining a relative humidity level between 30-50% is recommended for preventing interior window condensation.
   
   
2. Indoor sources of moisture: Many activities within the home can generate moisture, such as cooking, showering, and laundry. These activities can increase the indoor humidity level and potentially lead to interior window condensation. Proper ventilation in these areas is crucial to minimize moisture build-up.
   
   
3. Insufficient ventilation: Proper ventilation is essential to maintain good indoor air quality and prevent moisture build-up. A lack of ventilation can lead to the accumulation of moisture in the air and contribute to interior window condensation. It's essential to ensure that the home has adequate ventilation, including bathroom exhaust fans and range hoods in the kitchen.
   
   
4. Temperature differences: Interior window condensation can also occur when there is a significant temperature difference between the indoor and outdoor environments. This can happen during the winter months when the temperature outside is cold, and the inside of the home is warm. To reduce the potential for condensation, it's essential to maintain a consistent indoor temperature throughout the home.
   
   
5. Window coverings: Some window coverings, such as blinds and drapes, can contribute to interior window condensation. These coverings can block the airflow around the window, creating a barrier that can trap moisture against the glass surface. It's essential to ensure that window coverings allow for proper ventilation and air circulation around the window.

In summary, interior window condensation on high-efficiency windows can occur due to a variety of factors, including relative humidity, indoor sources of moisture, insufficient ventilation, temperature differences, and window coverings. Proper ventilation, indoor humidity control, and maintaining a consistent indoor temperature are essential for preventing interior window condensation.