Too much moisture in your home can hurt your home's durability and impair your health. Your home has the greatest chance of collecting moisture during the winter and spring. A small amount of moisture in and around your home is OK, but beyond a certain point moisture problems occur. These problems include mold, peeling paint, rust, and rot. Mold, in particular, can cause serious respiratory problems on top of mold damage to the home.
The two common solutions to moisture problems are removing the moisture source and ventilating the home.
Removing the moisture source is always preferable. Rain penetration and ground water seepage are the two of the most serious problems a house can suffer. Leakage through the roof or around windows shouldn't ever be allowed to persist. Penetrations in roofs and the tops of exterior window trim are common leak sites. Fixing these leaks is the first defense against moisture problems.
Besides rain leakage, the ground is the biggest potential moisture source in a home. Watering ornamental plants near a home's foundation and rainwater collecting around the foundation, for example, can cause serious problems. Water may seep through the foundation and into the floor structure. Install rain gutters, and re-landscape, if necessary, to prevent water draining toward the house. Install a sump pump to dry out your wet basement or crawl space. A ground-moisture barrier should also be installed on any unfinished crawl space floor to prevent water vapor from rising out of the ground into the house. A ground-moisture barrier is simply a piece of heavy plastic that covers the ground.
Sump pumps are a necessity for homes, troubled with standing water in crawl spaces and high water table.
Homes with many occupants need ventilation to remove human-caused moisture from sweat, showers, and cooking. Bathrooms and kitchens should have operating ventilation fans vented to the outdoors. If your home doesn't have such fans, installing them would be a very worthwhile home improvement.
Both Residential Energy: Cost Savings and Comfort for Existing Buildings and Saturn Energy Auditor Field Guide contain more information on solving moisture problems.