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Understanding common household condensation

Common household condensation, or “sweating” on windows is caused by excess humidity or water vapor in a home. When this water vapor in the air comes in contact with a cold surface such as a mirror or glass window, it turns to water droplets that is called condensation. All homes have occasional condensation, such as a little fogging on the windows, and 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.

Where does indoor humidity come from?

All air contains a certain amount of moisture, even indoors. There are many house hold items that could generate indoor humidity such as your heating system, humidifiers, cooking, showers, In fact, every activity that involves water, even mopping the floors, contributes moisture to the air.

Condensation is more likely to occur in homes where January temperatures drop below 35oF because there are greater temperature extremes affecting the glass in the home.

It is very normal 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 modern insulation, moisture-barrier materials and air-tight construction, we all enjoy a more thermally efficient home – one that blocks the cold out, yet traps the moisture4 in, producing higher humidity levels and…more condensation.

Reducing humidity is the key.

The best way to reduce condensation is by eliminating excessive humidity. So, how much humidity is too much? The following table illustrates the recommended or comfortable levels of 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.

What should it be?

Studies of personal comfort have shown that relative humidity ranges between 30% and 65-70% can be considered ‘comfortable’ depending on activity.

However, from the standpoint of indoor air quality, upper ranges should be maintained below 50% (dust mite populations increase rapidly at relative humidity levels above 50% and fungal amplification occurs above 65%).

Buy a Hygrometer and keep track of your indoor humidity levels. These instruments are relatively inexpensive and can generally be purchased online or in many hardware or discount stores.

Six simple solutions to controlling indoor humidity:

  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 house plants, try to concentrate them in one area and watch 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.

What about exterior condensation?

The same basic situations that cause condensation on the interior portion of a window can also cause condensation on the exterior portion of a window unit. The following are usually the reason for exterior condensation on your window:

  • High relative humidity in outside air
  • Still air
  • Clear night sky
  • Glass temperature below dew point temperature
  • Well insulated glazing

When exposed to the above-mentioned conditions, the exterior surface of the glass will cool, causing the glass temperature to fall below the dew point of the ambient air. When this occurs, moisture from the air will condense 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 the specific set of atmospheric conditions exist and that the insulating glass is indeed doing its job – that of insulating the building from the environment. In this case, the insulation capability is what retards the flow of building heat through the glass and prevents warming of the exterior above the dew point.

A final word

Condensation can be very difficult to solve. There are many factors that affect condensation, such as, the number and type of windows in your home, the heating system — hot air or water, the type of insulation and vapor barrier and even the type of soil and quality drainage. If you still have condensation problems after following the simple preventative steps mentioned above, you may need to consult a professional heating contractor or a qualified expert.

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