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Humidity and condensation in the house: A basic guide to reduce and control household humidity.

What is condensation?

Stained ceilings, water streaming from windows and mold on trim and walls are all the common results of excessive water vapor in the air.

If we could only see the water vapor in our homes we would have a better chance of controlling it. We cant see water vapor, but we can see its effects. The most common is condensation on the inside surface of your new windows. The fog, or in severe cases, the frost that forms on the glass or edge of door is a sure tell tale sign that the humidity levels in your house are too high.

Condensation problems arise because air can hold onto limited amount of water vapor at any given temperature. Cold air can hold less water than warm air.

When air at a given temperature contains all the water vapor it can hold, it is said to have relative humidity level of 100%. If it contains only half of the water vapor it can hold at the temperature, then the relative humidity is 50%.

If the temperature changes but no water vapor is added or removed, then the relative humidity will also change. It will increase as the temperature falls and decrease as the temperature rises. Relative humidity will rise as the temperature falls until " Dew point" is reached, that is, the temperature at which the air can hold no more water vapor. Any further decrease in temperature will cause some of the vapor to condensate as a fog when the temperature so above freezing or as frost when it is below freezing.

It should be pointed out that a little fogging at the lower corners of the insulating glass units is nothing to concerned about. Heavy fogging or worse, ice formation on the glass surfaces is a cause of concern. It is symptomatic of invisible moisture damage that could be happening in the other parts of your home. Household humidity is a modern problem. More an more of us living in "air tight" homes, they are more economical to heat and cool and easier to keep clean but we have created another problem, and that is lack or air exchange and moisture traps.

Sources of moisture

The principal sources of water vapor in today's homes vary with lifestyles but the following is just a small list of normal daily activities and the water vapor they introduce per day.

Cooking ( 3 meals per day) 3-4lbs.

Dishwashing 1-2lbs.

Shower or bath 1-2lbs.

Weekly laundry 30lbs.

Occupancy 12-15lbs.

How much is enough?

As stated before, we need some humidity in our homes to be comfortable. A house that is too dry is just as bad as house that is too humid. Symptoms of a dry house are sore throats, itchy eyes, nosebleeds and dry skin to name a few. Other, not so obvious symptoms are static electricity, cracked and dried out furniture and constant feeling that the house is colder than it really is. Your new windows are a great barometer just where your humidity level should be. Not enough to fog the glass but just enough to feel comfortable.

There are naturally more sophisticated means to check the humidity levels in your home from simple, inexpensive hygrometers to more complicated "wet and dry" bulb hygrometers. There are even inexpensive digital units that will measure temperature and humidity levels available at most hardware stores.

Outside air temp. Max. indoor relative humidity .

Here are the recommended safe humidity levels based on outside temperatures for an interior household temperature of 70 degrees F.


-20 DEG F TO-10--> NOT OVER 20%

-10 DEG F TO 10--> NOT OVER 25%

0 DEG F TO 10--> NOT OVER 30%

10 DEG F TO 20--> NOT OVER 35%

20 DEG F TO 40--> NOT OVER 40%

Practical steps to reduce surface condensation on windows

  1. If your furnace is equipped with a humidifier or you run a separate humidifier unit, turn it off until the amount of condensation is reduced. Make sure that the humidifier and the setting switch are really working.

  2. Run kitchen and bathroom exhaust fans longer than you normally would.

  3. Make sure your clothes dryer is vented to the outside.

  4. Avoid hanging wet clothes inside to dry.

  5. If you have a fireplace, open the damper.

  6. The free circulation of air around windows is important. Leave drapes and blinds open as much as possible.

  7. Leave the furnace fan running than set on automatic.

If condensation is still a problem

Remember that windows don't cause condensation but high humidity levels can. If the above 7 simple steps don't solve the problem you might have yo call in an expert. Ask your heating contractor to check the airflow in the house and your humidifier.

You might also want to inquire about installing outside air vents or larger and more exhaust fans. The most important thing to remember is you cannot afford to let high humidity levels go unresolved.

Exterior condensation on S.I.G. units

A relatively recent phenomenon occurring in high performance windows is that of exterior condensation glass units. While occasionally a source of concern for the customers, this condition is normal given certain combinations of glazing systems and environmental conditions.

Exterior condensation is usually observer under the following conditions:

* High outside relative humidity

*High performance glazing (Low "E"/ Argon units)

*Early morning

*Unobstructed exposure to clear sky to the north, west or south- no wind.

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