There are three main sources in your home; the first being air leaks.
Air can leak into the home through walls, roofs, and floors and have damaging effects on a house. Uncontrolled airflow through the envelope of the home not only carries moisture into framing cavities, causing mold and rot, but it can also account for a huge portion of a home’s energy use and can cause indoor air quality problems. In a leaky house, large volumes of air – driven by exhaust fans, stack effect, and the wind – can blow through the floor, walls, and ceiling.
The second source of moisture is diffusion through materials.
This is a process by which vapor spreads or moves through permeable materials caused by a difference in water vapor pressure. An example of this is when the soil becomes saturated and moisture enters the crawl space through the walls by vapor diffusion. Installing a vapor barrier can help reduce the amount of moisture that makes its way into the crawl space and into the rest of the home.
The final source is internally generated moisture.
A family of four produces on average two pints of water an hour, or up to 25 pints of water a day, simply by washing dishes, taking showers, cooking, and breathing.