Moisture problems in a house may develop when moisture-laden air passes through the random holes, into or out of the structure. Since warm air can hold more moisture than cold air, as the warm air cools, pockets of condensation can form, sometimes leading to hidden mold or mildew growth or rot. This may damage building components and furniture, and contaminate the indoor air that occupants breathe.
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The moisture levels in a house depend on a variety of factors such as lifestyle (e.g., frequent showers), number of occupants (e.g., moisture in exhaled breath), plumbing or roof leaks, ground moisture entering basements, etc. Excess humidity is a frequent problem in under-ventilated houses, and is one of the chief reasons to install a mechanical ventilation system to exhaust the excess moisture to the outdoors.
To better understand how problems with moisture can be avoided, we need to understand how the humidity in the air changes with temperature. Relative humidity (RH) is a measure of humidity in the air, as a percentage of the maximum amount of humidity that the air can hold. Cold air can’t hold as much moisture as warm air. So if the temperature of the air goes down, the RH will go up. If the temperature drops enough, the RH will reach the dew point (100% RH), and condensation could occur hidden within the structure of the house, which is not a good situation. This is why moisture condenses on cold windows in the winter—the temperature right next to the glass is below the dew point of the air in the room. In the winter, closets (especially when they are located adjacent to exterior walls) are often cooler than other parts of a house, so they often have higher relative humidity. This makes them more susceptible to mold growth. The temperature and RH also varies inside building cavities such as roofs, floors, and walls.
The RH of the air can rise quickly, for example, in a bathroom after a shower, but it can also drop just as quickly if you use a local exhaust fan. If a local exhaust fan is not used, and the RH remains high for several days, mold or mildew can quickly start growing.
By controlling the pressure difference and resulting air exchange between the outside and the indoor air, one can minimize the negative effects of the interaction of temperature and humidity. The direction of the air flow is determined by the air pressure inside the house relative to the outdoor air pressure. A house is pressurized if there is greater pressure inside the house relative to the outdoor air pressure, so the overall direction of air flow is outward. As it turns out, hidden pressure-related moisture problems tend to occur in cold climates when a house with humid indoor air is pressurized, and in hot, humid climates when an air-conditioned house is depressurized.
Mechanical ventilation is critical to control the pressures in order to prevent moisture problems and to keep indoor humidities at a comfortable and healthy level. Heat recovery ventilators (HRVs) control the air exchange by bringing in fresh air complete with all the moisture it contained outdoors and by exhausting stale air complete with all of the moisture it contained when it was indoors. Energy-recovery ventilators (ERVs) can also minimize the over-drying effect in the winter, or excessive indoor humidity in the summer, by retaining some of the moisture as needed. From bathroom fans to whole-house ventilation, look for products certified by the Home Ventilating Institute (HVI). Products with this quality certification are tested to ensure that they will provide the proper ventilation according to specification.
Adapted from: Understanding Ventilation: How to design, select, and install residential ventilation systems by John Bower © 2010 The Home Ventilating Institute (HVI).
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