The source of condensation, or “sweating” on windows and mirrors in a home is humidity, or visible water vapor, which is present in virtually all air. When this vapor meets a surface that is at a cooler temperature than the “dew point”, the vapor turns into visible droplets of liquid called condensation. You have often seen this happen to bathroom mirrors and walls after someone has taken a hot shower. Condensation can also occur on windows during the winter, if the inside air contains enough water vapor. A little condensation or “fogging” now and then is to be expected and causes no problems.
We tend to notice condensation only on windows, mirrors, etc. because excess moisture does not penetrate glass and is simply more visible there. Of more concern, however, is the less visible condensation that can penetrate and collect in walls and ceilings. Excessive condensation can cause damage. The excess moisture can eventually deteriorate the drywall, wood window sills, and window treatments.
When you see condensation on glass surfaces, take it as a warning that you have an excessive humidity level in your home.
Balancing humidity for comfort and condensation prevention:
Seasonal changes can cause humidity levels in your home to constantly fluctuate with the exterior weather conditions. Since temperatures may fluctuate drastically within a day, adjustment of the humidistat may be required daily. Controlling the amount of moisture in the air is the most effective action you can take to avoid condensation.
The amount of moisture in the air is measured by the “relative humidity” of the air. Relative humidity is the percentage of moisture in the air compared to the maximum amount of moisture it can hold (when it is saturated). When it’s raining or very foggy outside, the outdoor relative humidity level would be at 100%.
Cold air cannot hold as much moisture as warm air. At 100% relative humidity, air at 60 degrees will hold three times as much water vapor when saturated than air at 30 degrees. As temperatures drop during the winter, outdoor water vapor pressure drops as well. Most homes will naturally have lower interior relative humidity levels because interior water vapor diffuses to the outdoors.
Controlled ventilation and elimination of excessive indoor moisture can keep humidity within bounds.
Here are some steps:
- Turn off or set back furnace humidifiers until the sweating stops.
- Use exhaust fans or open windows slightly in kitchen, bathrooms, and laundry room during or after cooking, taking showers, and washing/drying clothes. Never hang up clothes to dry indoors I houses with excessive humidity problems.
- If there are many plants inside the house, concentrate them in a sunlit room where the door can be kept closed, and avoid over watering.
- Keep basements as dry as possible by waterproofing floors and walls. Run a de-humidifier if necessary.
- Make sure that attic vents are open and unobstructed.
- Opening windows slightly throughout the house for a short time each day will allow humid air to escape and drier air to enter. The heat loss will be minimal.