Date Added: September 03, 2008 01:38:18 AM
Author: Ryan Madson
Category: Energy Saving Improvements

All normal air contains some water vapor or moisture.  The ratio of moisture present in the air to the amount it could potentially hold at its temperature is its relative humidity.  When its relative humidity reaches one hundred percent air is said to be saturated, i.e. can hold no more moisture. The vapor in it will condense as liquid water if the temperature is reduced.

If air with a relative humidity less than one hundred percent is cooled, its relative humidity increases. Since warm air can hold more moisture than cold air, the same amount of moisture in the air will produce a higher relative humidity at lower temperatures and a lower relative humidity at higher temperatures.

As the temperature of airdrops and the relative humidity increases, it can eventually reach saturation. The temperature at which this occurs and condensation is produced is called the dew point.

The combination of the movement of water through a material and the condensation of the vapor into a liquid can create problems. If condensation occurs within the wall, the moisture may damage insulation and /or cause wood to rot.
Condensation can be reduced by an effective ventilation system. The ventilation system of a home can consist of either passive ventilation or active ventilation or both. Natural ventilation occurs from the same forces, which cause infiltration. Wind pressure on the home forces air in, and the lower pressure on the leeward side draws out air. In addition, convection currents within the home aid in natural ventilation. For natural ventilation to function adequately, there must be sufficiently porous material on the outside of the home and sufficiently large vent openings placed outside the structure.

Active or forced ventilation may be necessary in areas of high moisture or air contaminant levels, such as the bathroom and the kitchen. A variety of exhaust fans are available for this purpose. While many have these already installed, they are not effective unless they are used regularly.

In addition exhaust fans may be necessary in the attic if there are insufficient vent openings there to allow for natural ventilation. The ventilation system can use a combination of natural and forced ventilation

In the winter, air inside generally has a higher vapor pressure than that outside. Therefore the water vapor tends to pass from the inside of the home, through the envelope, to the outside of the home. As it passes from the warm inside to the cold outside, the temperature drops and the relative humidity increases and may reach the dew point. Condensation then begins to form. If this occurs within the wall, the liquid water accumulates in the wood, rot may develop or the wood itself may swell and buckle. If it accumulates within the insulation, the effective r-value of that insulation is significantly reduced. If the vapor condenses around nails just under the surface of the paint or paper, staining may be seen on the interior surfaces. In very hot, humid areas where the home is cooled and dehumidified, the moisture flows in the opposite direction. The high vapor pressure is on the outside, and the low pressure on the inside. It is possible for condensation to form as the water vapor moves from the outside through the shell toward the inside. The problems, which result from the condensation, are the same. Summer cooling does not usually cause serious problems and will not be addressed here.

There are two major methods, often used in combination for reducing the potential for condensation, which occurs as water vapor passes between warm and cold surfaces. The first is to tighten the warm side surface making it impermeable to water vapor. The second is to install a means for ventilation.

If water vapor is restricted from penetrating the surface, it is not available to condense as it reached cold objects. Some
materials are resistant to the passage of water vapor. Such materials called vapor barriers should be used as near to the surface as possible on the side with the higher vapor pressure, usually the warm side.

Proper ventilation is essential in residences: first, because of the presence of a vapor barrier and the elimination of cracks or penetrations of the envelope, creating a tight inside surface, will all increase the humidity in the home; and second, because some moisture will still pass into the walls and attic despite such barriers. The outside surface of the walls should allow water vapor to pass through easily, and should not be covered with any material resistant to the passage of vapor, unless adequate venting and drainage behind the covering is provided.  Attics should be sufficiently vented to the outdoors, so that moist air can be removed

Areas in which high levels of moisture production exist such as bathrooms and kitchens should be adequately ventilated.


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