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March 2004 - Moisture

Moisture

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Moisture Sources & Problems

There are many possible moisture sources in a home. Some important sources are water leakage from a leaky roof, deteriorated flashing, plumbing leaks or gutters that don't drain properly. Another source is people. An average family of four can add two to three gallons of water per day into the air just through normal living activities such as bathing, dishwashing, cooking and breathing. Other less obvious culprits are unvented exhaust from ranges, ovens, gas fireplaces, clothes dryers and water heaters; plants and green firewood; and hot tubs, fish tanks and wet laundry.

Problems

The first place you may notice moisture problems is on windows, especially if they are older and inefficient units, such as single-pane or double-pane windows without a gas fill. Some condensation is normal on the coldest days because windows are often the coldest interior surface of the house. However, persistent and severe window condensation is a warning that moisture could also be condensing on other hidden surfaces where it can lead to a variety of problems, such as mold and rot in walls. More obvious problems are wet spots or mold patches on inside walls --- a clear indication to take action.

Leaky houses may not show signs of moisture in the living space. They may, in fact, appear to be too dry because moist air escapes out of the house through gaps and holes in ceilings or walls. This may result in moisture damage within walls or in attic areas as warm, moist house air leaks into cold cavities.

Many modern homes have an adequate amount of insulation and fairly good windows. However, without adequate ventilation these homes can be so tight that moisture builds up from internal sources, such as cooking and bathing, and leads to condensation problems. For instance, condensation can occur inside walls and on the underside of the roof sheathing in the attic --- problems that a properly designed ventilation system can avoid.

Condensation inside walls is not usually a problem as long as it is not persistent. In the winter, the outer walls are so cold that rot and mold growth is inhibited, even if condensation occurs. In the spring, the wall warms up and moisture evaporates back into the air and drains away. However, persistent wall moisture can occur in poorly ventilated, tightly sealed homes.

Attic moisture is a bigger problem. If there's not adequate air sealing between the house and the attic, warm, moist air can be drawn into the attic. When the moist air reaches the cold undersurface of the roof, the moisture condenses, forming frost or wet spots that can drip back on to the insulation and into the ceiling, forming a stain visible from inside. Even worse, roof sheathing can rapidly rot or delaminate and the shingles themselves can deteriorate. Even dry indoor air can cause problems by causing ice dams. Most ice dams form when heated air from the house escapes into the attic and warms the underside of the roof, causing snow to melt and then refreeze when the water hits the cold overhang. Ice dams can force water back into the attic, where it can soak insulation and damage ceilings. Sealing leaks between the house and the attic is important for preventing both types of problems.

Understanding Condensation

Condensation occurs any time warm, moist air contacts a surface that is colder than the air's dew point temperature. At dew point, the air is completely saturated. It is holding all the moisture it can hold for that temperature (the relative humidity is 100 percent). If the temperature of a surface falls below dew point the excess moisture will condense as dew or frost. Window condensation is common in the winter time, as warm, relatively moist air contacts the cold surface of the glass, much like the moisture that forms on a beverage can taken out of the refrigerator.

Thus, all moisture problems depend on temperature and relative humidity. The tighter a home is, the more likely it is that humidity generated inside will remain inside and cause problems unless the home is properly ventilated. But even if moisture levels stay constant, condensation can occur if surfaces get very cold, as they often do in the depths of January nights. Finally, adding moisture to the air will raise indoor humidity levels and make condensation more likely.

Air movement also influences moisture problems. In the winter, the warm air inside the house has a natural tendency to rise. Warm, moist air leaves the house through the attic or the upper story and is replaced by dry, cold air pulled in through the lower level. This is called the stack effect. The stack effect causes moisture problems to be most pronounced in the upper stories of a house. For example, the windows on the lower level may be clear while the second story windows are frosted over.

--- Information provided by Focus On Energy ---

Coming Next Month --- "Solving Moisture Problems."