Liquid water, the most obvious to the homeowner can result from leaky roofs or pipes, overflows in the plumbing sewer system, chronic moisture invasions, or floods.  Water vapor, or humidity is usually less obvious.  Water vapor can accumulate indoors from showers or cooking without proper ventilation to release moisture to the outside.  Insufficient water proofing or vapor barrier use occurs during construction, water vapor can enter through slabs and subterranean walls.  Water vapor can also enter through open windows and doors in humid climates.