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Condensation Info


Common household condensation or “sweating” on windows is caused by excess humidity or water vapour in a home. When water vapour in the air comes in contact with a cold surface such as a mirror or glass window, it turns into water droplets called condensation. All homes have occasional condensation, such as a little fogging on the windows, but this is no cause for concern.On the other hand, excessive window condensation, frost, peeling paint, even moisture spots on ceilings and walls can be signs of excessive condensation and potentially damaging problems in your home. We tend to notice condensation on windows and mirrors first because moisture doesn’t penetrate these surfaces. Yet they are not the problem, simply the indicators that you need to reduce the indoor humidity of your home.



You may be wondering why your new energy-efficient replacement windows show more condensation than your old drafty ones. It’s simple – your old windows were not airtight and allowed humidity to escape. Now that your new windows create a tighter seal, the extra moisture in your home is unable to escape, therefore making you more aware of excess humidity. Windows do not cause condensation; instead they prevent humidity from escaping and provide an easy surface for condensation to collect.



The best way to reduce condensation is by eliminating excessive humidity. The followingtable illustrates the recommended or comfortable levels of indoor humidity during the winter months.By eliminating excessive humidity in your home, you may very well eliminate most, if not all, of your condensation problems.



1. Make sure all sources of ventilation to the outside are functional, and use kitchen, bathroom and laundry room exhaust fans during and after humidity-producing activities to vent excess moisture.
2. Air out your home periodically. Opening windows for just a few minutes a day lets the stale, moist air escape and the fresh, dry air enter without compromising your heating.
3. Check your humidifier settings. Use the humidity comfort levels provided in the table to correctly set and balance the humidity level in your home.
4. Be sure that all louvers in the attic or basement are open and large enough. You can even open your fireplace dampers to allow excess moisture to escape.
5. If you have a large amount of houseplants, try to concentrate them in one area and avoid over-watering.
6. If troublesome condensation persists, see your heating contractor about an outside air intake for your furnace, venting of gas burning heaters and appliances, or installation of ventilating fans.


All air contains a certain amount of moisture, even indoors. Indoor humidity is generated by many household items and activities, such as your heating system, humidifiers, cooking and showers. In fact, every activity that involves water, even mopping the floors, contributes moisture to the air.

Climate can also contribute to condensation. Condensation is more likely to occur in homeswhere January temperatures drop below 2°C because there are greater temperature extremes affecting the glass in the home. It is typical to experience condensation at the start of each heating season. During the humid summer months your home absorbs moisture and then perspires when you turn on the heat. This is only temporary though; after the first few weeks of heating your home should dry out – reducing, if not eliminating, condensation.

You’ll notice the same scenario if you have done some remodeling or building. Due to the high levels of moisture in wood, plaster and other building materials, your home will temporarily sweat during the first few weeks of the heating season. Another factor in the condensation equation is progress. With today’s high-performance insulation, moisture-barrier materials and airtight construction, we all enjoy a more thermally efficient home – one that blocks the cold out, yet traps the moisture in, producing higher humidity levels and more condensation.




Condensation on the outdoor surface of an insulating glass unit is not an indication that the insulating glass unit is defective. This condition usually occurs in late summer and early fall when the night time temperature drops several degrees. The moisture on the outside of the window is condensation. It will soon evaporate when outside temperature rises. As we know, condensation happens on cold surfaces. During the night, if the outside temperature drops below the dew point then condensation forms. The fact that the condensation is forming on the windows means that very little heat from inside the house is escaping through the windows. That’s a good thing and proof that you have energy-efficient windows.
Under the right set of atmospheric conditions, it is possible to get condensation on the exterior glass, specifically under these conditions:

– Glass temperature below dew point temperature
– Clear night sky
– Still air
– High relative humidity
– Well-insulating glazings

Exposed to these conditions, the outdoor surface of the glass can radiate heat away to the night sky such that the glass temperature falls below the dew point of the ambient air. When this occurs, moisture from the air condenses on the glass surface. Only when the glass temperature rises above the dew point will the condensation evaporate back into the air. Dew formation on grass, car hoods and roofs, building roofs and walls is common and accepted as a fact of nature. The presence of moisture indicates that a specific set of atmospheric conditions exists and that the insulating glass unit is indeed doing its job – that of insulating the building from the environment.


Problems arise because air can hold only a limited amount of water vapour, varying with the temperature. Condensation is related to two conditions always present in the atmosphere inside your home. It is likely to form whenever there is an improper balance between relative humidity and temperature.
In addition to the above sources of humidity, such things as gas appliances, dryers vented inside, plants (which put out almost as much water as they receive), pets, humidifiers, damp basements, etc., all increase the humidity level in a home.


-Open window coverings at night to warm up exterior glass.
-Draperies can be opened to allow as much heat transfer through the glass as possible.
-You can also trim shrubbery near windows or doors as this will help promote air circulation.
-Trees or buildings can block the radiation view to the sky reducing the chance for outdoor condensation.
-Raising the temperature setting on your air conditioner may also help keep your window glass at a warmer temperature.