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PROBLEM: My window is fogged up.

ANSWER: First, you must determine if the fog is inside your home, between the glasses of the sealed unit, or even in some cases on the outside of the glass! Remove the screen if the window is a casement or awning. Touch the inside glass. If you can touch the moisture, you probably have a condensation problem.

-Condensation will occur on any surface where the surface temperature is lower than what is necessary to support the level of relative humidity in the air. This is not a window problem but is an airflow and humidity issue in your home. Check your relative humidity near the window with a barometer. Compare your humidity and exterior temperature to the following chart. (You may choose to live with a degree of condensation if the high humidity is a more comfortable living environment, while the humidity may be causing hidden damage to other parts of your home, it will not hurt your vinyl windows.)

Outside Air Temp (Celsius) – Indoor Relative Humidity (%)
10 70%
5 60%
0 55%
-5 45%
-10 40%
-15 35%
-20 30%
-25 25%
-30 20%
-35 15%
-40 5%

If the air circulation in the room with the fogging problem is poor, you may have fogging at even lower levels of relative humidity.
To eliminate fogging, there are two approaches and a variety of solutions. Try various combinations to discover what works for you!

  1. Lower the humidity in the room.
    A. -Moisture in your home is created from cooking, washing, watering plants etc. Check to make sure that where ever possible, moist air is immediately directed out of the home. Accomplish this by having and using good venting systems in your washrooms, attached to your clothes dryer, over your stove etc. Move plants that require high amounts of water to a less sensitive location in your home.
    B. -Blinds, curtains (even gauze!) and other window treatments will trap air next to the glass and lower the surface temperature of the glass. Leave treatments as open as possible, particularly at night, in order to maximize air movement st the window.
  2. Raise the air temperature at the window.
    A. -Add heat to the room by redirecting airflow to increase the flow of warm room air across the window. Redirect air from nearby heating sources toward the window. If the air in the room in general is cold, raise a local thermostat or use a space heater. Remove heat deflectors from below windows so heat will flow across the glass surface.
    B. -Turn off any central or room specific humidifier. Add a dehumidifier to the specific location. Opening a window a crack will allow drier outside air to mix with the air in your room. As it warms, it will lower the room’s relative humidity. Since health requires numerous full changes of air in a living space, this is not a waste of energy.

Exterior fogging:
If the moisture is clearly on the outside of the home, you have achieved an extremely efficient home system! This can occur when your glass is cold because heat is not leaking through from your home. It is a goal that thermal engineers strive to achieve!

Moisture between the panes of glass:
This is a seal failure. Consult your warranty for details on receiving service to replace the failed glass unit

What causes condensation?

Condensation will often occur during colder weather when the moist air inside the home comes into contact with cooler surfaces. The moisture in the air can condense to form water droplets or even frost. The risk of condensation increases as the weather gets colder and/or the inside humidity rises.

For windows, doors and skylights, condensation can form:

  • on the glass
  • on the frames and sashes
  • near the weather seals
  • on the walls around a skylight
  • between the glass panes if the seal is broken.

Reducing condensation

There are simple steps you can take to avoid or reduce condensation

1. Buy high performance ENERGY STAR® certified windows, doors and skylights. They are more energy efficient and have less condensation compared to ordinary models.

They are more energy efficient and have less condensation compared to ordinary models.

Features that contribute to better energy efficiency are:

  • triple glazing.
  • low-emissivity (low-e) coatings on the glass
  • inert gas between the glass panes
  • thermally improved spacer bars
  • insulated frames, sashes and doors
  • greater air tightness

Note:  Low-e coatings on high performance windows make the glass more energy efficient by reducing heat loss. However, if the night has been cool and calm and the air is humid, condensation may form on the outside surface of the glass around sunrise. Also, there is a higher risk of condensation if the low-e coating has been applied to the room-side surface of the glass.

2. Proper air circulation near windows is also necessary to minimize condensation.

Air flow across the glass surface helps to keep it warmer, and that is one reason heat sources such as central heating vents and electric baseboards are located beneath windows in many homes. Deep window sills, closed drapes and blinds, and even nearby furniture can block air movement and allow cool air to pool near a window, which can lead to condensation on the glass.

3. Make sure your Heat Recovery Ventilator (HRV) is on if you have one.

Humidity levels—cooling and ventilating equipment

Homes today are more airtight than houses of the past. As a result, moisture can build up inside, especially during winter when windows are kept closed. Dampness, mould growth on walls and heavy condensation on windows are signs of too much moisture.

Over time, this excess wetness can cause poor air quality and damage the structure of a home.

Follow these suggestions to reduce moisture.

Check your humidifier.
If your furnace has a humidifier, set it lower to reduce humidity levels.

Ensure that your clothes dryer vents outside. Install exhaust fans in bathrooms and kitchens, and make sure the fans vent directly to the outside. Consider installing a heat recovery ventilator to increase ventilation and reduce humidity.

Create less moisture.
Use lids on pots when cooking. Keep your showers short. Store wood outdoors.

Keep moisture out.
Cover exposed earth floors in basements and crawl spaces with moisture barriers. Install a sump pump to remove excess moisture under the foundation. Fix water leaks in the basement. Make sure the grading slopes away from your house so water does not flow towards the foundation. Ensure eaves troughs adequately divert water away from your home.